estes ina mena bier i Nie Bi i stil i i Ah beh ye ia a om ne ele del ne ee ca” sh atta waek oad Seth pele tater telat f fer rif Siti reen a Fe ay i ted cates ay = a7 i 4 6 é ead Rs P Fs in Ki = Thsags eee osiyeaty NPE Py sere? CORNELL UNIVERSITY. THE Rosmell 7. Flower Library THE GIFT OF ROSWELL P. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEGE. 1897 Cornell University Libra Amphioxus and the ancestry of the verteb DATE DUE Columbia Gnibersity Biological Series. EDITED BY HENRY FAIRFIELD OSBORN. - FROM THE GREEKS TO DARWIN. By Henry Fairfield Osborn, Sc.D. Princeton. . AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES. By Arthur Willey, B.Sc. Lond. Univ. . FISHES, LIVING AND FOSSIL. An Introductory Study. By Bashford Dean, Ph.D. Columbia. . THE CELL IN DEVELOPMENT AND INHERITANCE. By Edmund B. Wilson, Ph.D. J.H.U. 20a Pst Oty ‘CVNISSTPT) ONVIT LV ONVINVG SELL NE SAXOINdWY COLUMBIA UNIVERSITY BIOLOGICAL SERIES. TI. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES BY ARTHUR WILLEY, B.Sc. Tutor in Biotocy, CoLtumBiaA CoLLEGE; BALFouR STUDENT OF THE UnIvERSITY OF CAMBRIDGE ACL P™~ LiBRa; =z ae ene WITH A PREFACE Py fF SF I> z OY BY NE VEL HENRY FAIRFIELD OSBORN New Pork MACMILLAN AND CO. AND LONDON 1894 All rights reserved — 2 |} pr 4-y THI oe 'o CopyRIGHT, 1894, By MACMILLAN AND CO. Norwood ress. J. S. Cushing & Co. — Berwick & Smith. Boston, Mass., U.S.A. Dedicated IN GRATITUDE AND ESTEEM TO PROFESSOR E. RAY LANKESTER, F.R.S. BY HIS FORMER PUPIL THE AUTHOR PREFACE, Tuis volume originated in a course of University lec- tures prepared at my suggestion by the author. It seemed important that he should bring within the reach of students and of specialists among other groups, his own extensive observations upon Amphioxus and other remote ancestors of the Vertebrates, as well as the general litera- ture upon this group. While our detailed knowledge of the structure and habits of these animals has been rapidly increasing in recent years, it is still in the main very widely scattered in monographs and special papers. Probably no single group illustrates more beautifully the principles of transformism ; for the Protochordates in their embryonic development exhibit remarkable reminis- cences of past adaptations, and, in their adult develop- ment, the most varied present adaptations to pelagic, deep-sea, littoral, free-swimming, and sessile life. As Lankester has shown, the Ascidians alone give us a whole chapter in Darwinism. But degeneration and change of function constitute only one side of their history. In vii Vill PREFACE. progressive development some of these types have come to so closely resemble, superficially, certain of the larger groups of Invertebrates, such as the Molluscs and Worms, that it is only at a comparatively recent date they have found their way out of these groups into the Protochor- data. Many of these misleading resemblances are now interpreted as parallels of structure springing from parallels in life habit, seen not only in the general body form, but in special organs, such as the breathing apparatus of the Ascidians and Molluscs. By the side of parallelisms are real invertebrate and vertebrate affinities; so that the problem of resolving these various cases of original and acquired likeness in their bearing upon descent has. become one of the most fascinating which modern Zodlogy affords. For example, among the real invertebrate ties of the Protochordates are the ciliated embryos of Balanoglossus and Amphioxus, the Tornaria larva and ciliated ectoderm of Balanoglossus. The nervous system of Balanoglossus presents both ver- tebrate and invertebrate characters; the respiratory sys- tem is identical with that of Amphioxus, while in the embryonic development there are many resemblances 7zzter se. In short, in Balanoglossus and the Ascidians the invertebrate type of structure, whether original or ac- quired, predominates. But in Amphioxus the balance is far on the other or vertebrate side of the scale, and this, with its resemblances to lower forms, gives us the con- PREFACE. 1x necting link between Protochordate and Chordate organ- isation. Before entering into any of these discussions, the author has given a thorough systematic and structural treatment, especially of Amphioxus. This exquisite form, Amphioxus, is of almost world-wide distribution and has enjoyed the attention of every great zoologist for over half a century, yet the most recent studies upon it have been among the most productive of discovery. Its interest and value as an object of biologi- cal education has steadily increased with the knowledge that in contrast with all the related forms, it stands as a persistent specialised but not degenerate type, perhaps not far from the true ancestral line of the Vertebrates. ‘ H.F. O. CONTENTS. S55 PAGE TINTRODUGTION: Baten Ge Si a a STG METAMORPHOSIS OF CIONA INTESTINALIS . «1 ee ee ee 275 Vacuolization of the Notochord. . . . . . . . . . . «6216 Mesenchyme and Body-cavity . . . . . . . . 4. . 2197 Preeoral Body-cavity and Preoral Lobe . . ... . . . 218 Body-cavity of an Ascidian and Ccelom of Amphioxus . . . 220 Fixationof the Ascidian Larva . . 2 .» . + «© 3 + «© « 222 Reopening of Neuropore; Degeneration of Cerebral Vesicle; Formation of Definitive Ganglion . . . 2. 1. 1. 1. 223 Primary Topographical Relations and Change of Axis . . . 226 Formation of Additional Branchial Stigmata . . . . . . 229 First Appearance of Musculature . . . . 1. 2 1. 1. 235 Alimentary Canal and Pyloric Gland . . . . 2 ww 1. 235 Appendicularia . . Win Ree tel tee ces Aueesee tes tar ony £230) Abbreviated Ontogeny of Clavelina. . . . 1. 1. 1. 1 1. 239 IN OTES: = Aieclee Sh SSL ies aS ee MS ee eae a CBD XIV CONTENTS. PAGE V. THE PROTOCHORDATA IN THEIR RELATION TO THE PROBLEM OF VERTEBRATE DESCENT .... . . . 242 BALANOGLOSSUS' Ge cee. Ea? aR wis. Sl ears Moe ee a A External Features, 2. <. a Ger Ge hy a sw, a we AS Nervous System and Gonads . . . . . . . 1 2... 244 Metaiiérism: «gn SAL Ratt inthe » pomcel ‘oN. aK eS lel sa je] | 0 [s t oe ee \welelolo fr) see selvels SK 120 DEVELOPMENT OF AMPHIOXUS. against the ectoderm. Later the dorsal margins of this half-canal meet and fuse in the middle line, and so produce the medullary tube * (Fig. 66). Origin of Mesoderm and Ceclom. In consequence of the flattening and incurving of the medullary plate, pressure is brought to bear on the dorsal wall of the archenteron, and the dorso-lateral bor- ders of the latter acquire the form of two longitudinal grooves (Figs. 65 d and). It is from these grooves that the archenteric pouches are split off. The grooves deepen, and in doing so become divided up into a series of pouches. Eventually the pouches become shut off from the archenteron gradually from before backwards, and then appear as closed cavities on either side of the notochord, which has, in the meantime, been developing (Fig. 65 F). In the higher Vertebrates the mesoderm arises as two solid, lateral, longitudinal bands, which are split off from the primitive endoderm. These mesodermic bands are at first unsegmented, and might be taken to correspond with the longitudinal grooves of the archenteron of Amphioxus, as described above. Later, only the dorsal portion of the mesodermic bands undergoes segmentation, while the ventral portion, which becomes hollowed out to form the general body-cavity, is never segmented in the crani- ate Vertebrates. (Cf. Fig. 33.) In Amphioxus the whole of the mesoderm is contained in the archenteric pouches, and is, therefore, at first entirely segmented. As soon as the pouches have lost their primitive con- * In the Ascidian embryo the formation of the medullary tube takes place after the manner typical of craniate Vertebrates (see below, IV.). EMBRYONIC DEVELOPMENT. 121 nexion with the archenteron, they commence to extend dorsally and ventrally between the ectoderm and the in- ternal organs (Fig. 66). Meanwhile the cells forming the inner or visceral wall of the pouch adjacent to the noto- chord elongate transversely and longitudinally, and begin to form the plate-like muscle-fibres of the myotome. The cells producing these fibres are arranged in such a way that each of them gives rise to a muscle-fibre extending from the anterior to the pos- terior limit of a myotome.* The closely approximated to the muscles are at first notochord and project freely into the cavity of the pouch. The latter gradually grows downwards, until it meets its fellow of the other side ; the two fuse together, and so the cavity is made con- Fig. 66.— Transverse section through the middle of the body of an embryo, with ten pairs of somites, to show the closure of medullary tube and the dorsal and ventral extension of the mesodermic somites. (After HATSCHEK.) a/, Alimentary canal. ch, Notochord, in the cells of which vacuoles have com- menced to form. /.#7, Commencing for- mation of longitudinal muscle-plates from the cells forming the inner wall of the somite. my.c. Myocaelomic cavity. tinuous from side to side, below the intestine. When this occurs, the primarily single cavity of each archenteric pouch becomes divided into two _ portions; namely, a dorsal portion, the somte proper or myocal, and a ventral portion, the ca/om, by a transverse partition, which arises through a fusion between the parietal and * Already in the embryo shown in Fig. 63 C, and even at a somewhat ear- lier stage, the muscles are so far developed that the body can be bent and jerked. By the time the mouth has broken through, muscular locomotion effectually replaces the primitive ez/tary Jocomotion, although the cilia persist to a late stage. 122 DEVELOPMENT OF AMPHIOXUS. visceral walls of the cavity, at about the level of the base of the notochord (Fig. 67). The dissepiments between the myotomes are formed from the contiguous walls of the successive pouches, but ventrally, in the region of the ccelom, they break down, so that the latter then becomes a continuous unseg- mented cavity. On account of the fact that the archen- teric pouches give rise both to the cavity of the somites (myoce?) and to the general body-cavity (ccelom proper or splanchnocel), they are often spoken of as the mzyo- celomic pouches. The cav- ity of the original archen- Fig. 67.— Scheme of a transverse section through the body of a larva with five gill-slits, to show the division between myocoel and splanchnoccel. (After HATSCHEK.) n.c. Spinal cord (medullary tube). ch. Notochord. dm. Muscles. my. Myo- cel. sc. Rudiment of sclerotome. al, Alimentary canal. 5.2.v. Sub-intestinal vein. sp. Splanchnoceel. teric pouches is known as ceéloum, the epithelial which constitute the mesoderm. As differentiation and or- the primitive walls of ganogeny proceed, the meso- derm gives rise to (1) the musculature, (2) the connective tissue, (3) the blood-vessels, (4) the reproductive organs, (5) the celomic epithelium or lining of body-cavity, also called the peritoneum, and (6) the excretory tubules. The development of the last- named structures has, however, not yet been worked out in Amphioxus. The parietal layer of the mesoderm applies itself closely against the ectoderm, and gives rise to the cutis of the body-wall. The connective tissue-sheath of the notochord and EMBRYONIC DEVELOPMENT. 123 nerve-cord, together with the internal sheath or fascza of the muscles of the myotome, arises from the walls of a pouch-like diverticulum of myoccel which grows up be- tween the muscles and the notochord and nerve-cord. (Cf. Figs. 67 and 68.) The myoccel also grows downwards between the somatic layer of the peritoneum and the ecto- derm (Fig. 68). According to Hatschek the dorsal and ventral fin-spaces are also derived from the myoceel.? The diverticulum of the myoccel which has just been described is known as the sclerotomte, since it gives rise to the fibrous sheath of the notochord and_ nerve-cord, which (ze. the sheath) in most of the higher forms becomes replaced by carti- lage, and finally by bone. In the cramiate Vertebrates 44, 68, Scheme of a transverse the sclerotome arises as a Section through region between atriopore and anus, of a young Amphioxus shortly solid proliferation of cells after the metamorphosis. (After HatT- from the visceral wall at the a Peedi Passa gas Myoeal: base of the somite. This 5¢.Sclerotome. ao. Aorta. a, Intestine. zm. Intercoelic membrane. s.2.v. Sub-in- solid proliferation is un- testinal vein. sf. Splanchnoceel. v. fc. doubtedly a modification of Yen"! frspace. a hollow diverticulum, involving, as it does, only the visceral wall of the somite, precisely as we find it in Amphioxus.? (Cf. Fig. 33.) On their outer surface the muscles of the myotomes are not provided with a sheath of connective tissue (fascia), standing, in this respect, in contrast to the condition which obtains in the Craniota. 124 DEVELOPMENT OF AMPHIOXUS. Origin of the Notochord. The notochord is formed from the endodermic cells which lie between the mesodermic pouches and constitute the dorsal wall of the archenteron. The dorsal wall of the archenteron at an early stage becomes converted into a shallow longitudinal groove whose concavity is turned towards the archenteric cavity (Fig. 65 D). This groove gradually deepens (Fig. 65 £), and eventually its walls become closely appressed to one another so as to obliter- ate the lumen (Fig. 65 /). Finally the adjoining cells of the archenteric wall grow across the gap occasioned by the formation of the notochord, and joining together, shut off the latter from any participation in the enteric wall (Fig. 66). In this way is the notochord separated from the endoderm gradually from before backwards. Poste- riorly it remains for a considerable time fused with the endoderm at the point where the anterior wall of the neu- renteric canal becomes continuous with the dorsal wall of the archenteron. We have indicated above that the differentiation of the notochord takes place from before backwards. At its anterior extremity a very noteworthy exception to this rule is presented. In the region of the first myotome the notochord retains an open communication with the archenteron after its lumen has already been obliterated behind this point. Moreover, in the embryo, with eight pairs of myoccelomic pouches (Fig. 68 dzs), the front end of the notochord lies some distance behind the front end af the body, while the anterior portion of the archenteron extends beyond the notochord. Eventually the notochord is continued to the front end of the body by becoming constricted off from the dorsal wall of the anterior sec- EMBRYONIC DEVELOPMENT. 125 tion of the archenteron in the usual way. This retarded growth of the notochord anteriorly indicates that its exten- sion to the tip of the snout is a secondary phenomenon. Ancestrally we are bound to assume it did not extend so far forwards. The forward extension of the notochord Tee THE is, as noted above, obviously useful to Amphioxus in ren- dering its pointed snout sufficiently resistant to en- NACE a STO Looe > able it to burrow in the fe 4 3 sand. When it wants to arugege = SAO) ‘. bury itself in the sand, it STooloyo io) 2. oy TP) has not to take pains to dig a hole, but darts in in the fraction of a second. The histological differen- oe tiation of the notochord 3) L[oJofefof ofeTofoloy oye solelo AN hag commences soon after the SE lo Fig. 68 4:s.— Embryo of Amphioxus, with eight pairs of somites to show the have come together so as to primary relations of the anterior end of . the notochord. From above. After obliterate the lumen. The parscuex.) : cells composing the noto- p.c. Preechordal portion of archen- 5 teron, which becomes converted into the chord are, at the first ap- head-cavities. .p. Neuropore. ch. Noto- a : f th 11 f chord; over which lies the neural tube. proximation or the walls 0 my. Myoccelomic pouches. ze. Neuren- the groove, placed end fo teen. i ? N.B.—In this and other figures of end, but soon begin to inter- Amphioxus embryos here reproduced ] ith th after Hatschek, the so-called mesoder- aCe Wi one another across jie pole cells have been omitted in the middle line (Fig. 65 F), accordance with the observations of WILSON and LWOFF. and finally each cell comes to occupy the whole width of the notochord (Fig. 66). Meanwhile vacuoles begin to appear in the cells (Fig. 66). sides of the chordal groove The vacuolisation of its component cells is an extremely 126 DEVELOPMENT OF AMPHIONUS. characteristic feature of the notochordal tissue throughout the group of the Vertebrates. aN | il 4 | ae At It is carried on to such an extent in Amphioxus as to obscure the original cellular structure of the notochord. The cells anastomose with one another in the longitu- WN) aN! WY WN CUMIN ih AKL | dinal direction, and so pro- WM NAN rh vA IBA 0 Fig. 69.— Median sagittal section of notochord of a young Amphioxus of 8 mm., to show the vacuolar character of the notochordal tissue and the dis- placement of the nuclei to the dorsal and ventral borders. (After LWOFF.) Cc, duce a reticulum the meshes of which represent the vacu- oles whose first origin is shown in Fig. 66. Most of the nuclei become eventually displaced from the centre of the notochord, and are, in the adult, almost exclusively confined to its dorsal and ventral aspects (Fig. 69). The Preoral “ Head-cavities” of Amphioxus. Before leaving the embryonic period of the development it is necessary to consider the origin and fate of what may be called the Aead-cavities of Amphioxus as made known to us by the work of Hatschek. They arise symmetrically as a pair of diverticula from the anterior portion of the archenteron, which lies at first partly in front of the notochord (Fig. 68 ézs) and completely in front of the myoccelomic pouches (Fig. 70). They begin to appear at the stage in which some eight pairs of pouches are already present. Their origin there- fore, in point of time and the subsequent modifications which they undergo, show that they do not belong to the metameric series of the mesodermic pouches, but are structures szz generts. EMBRYONIC DEVELOPMENT. 127 The archenteron extends at first to the front end of the body. Its anterior portion, after the formation of several mesoblastic somites, becomes marked off from the hinder region by a slight constriction, which gradually becomes deeper and deeper (Fig. 70), until finally the whole of this portion of the archenteron is divided into two separate sacs, which eventually lose all connexion with the chenteron (Fig. 71). The ali- ar- mentary canal now no longer reaches to the anterior ex- tremity of the body. Very soon after their separation from the archenteron these sacs enter upon a series of changes by which their origi- nally symmetrical disposi- tion is entirely destroyed. Already in Fig. 71 it can be noticed that the right sac is becoming larger than the left, and the epithelium lining its walls is losing its Fig. '70.— Embryo, with nine pairs of primitive somites, seen in optical section from the ventral surface, to show the origin of the head-cavites. (After Har- SCHEK.) original cubical character, r.a.d. Right head-cavity. /a.d. Left head-cavity. y.c. Myoccelomic pouches the inner ends of the cells bead (primtive somites), arc. Archenteron. are rounding off, and in fact it is being converted from a cubical to a flattened pavement epithelium (Figs. 63 C and 64). trary, retains its original form and dimensions for a long time. The left sac, on the con- During the asymmetrical changes affecting the two sacs, which take place coincidently with the formation of the snout, the left one comes to lie transversely below the notochord, while the right sac becomes greatly enlarged 128 DEVELOPMENT OF AMPHIOXUS. and constitutes the cavity of the snout lying below the notochord (Fig. 63 C). Shortly after the breaking through of the mouth the left sac acquires an opening to the exterior on the left side of the body (Fig. 64). The right sac becomes the pr@oral body-cavity or coelom of the “head,” while the left sac is known as the preoral pit. It is necessary to emphasise the fact that these two structures which are so different in their fully formed con- dition are at first perfectly similar and symmetrical and form a pair of “head-cavi- ties.” Ultimately, as we have seen, only one of them actually persists as a head- Fig. 71. — Anterior portion of em- Cavity; namely, theright one. bryo, with thirteen primitive somites, The entire conversion of from the ventral side in optical section. (After HATSCHEK.) the left sac into the przoral r.a.d. and /.a.d. Right and left head- ioe cavities. c.s..g. Rudiment of club-shaped Plt 1S probably to be regarded ae as a secondary or cenoge- netic phenomenon, but the acquirement of an opening to the exterior is probably not secondary, since a similar opening (the pvoboscis-pore) occurs in Balanoglossus. In addition to the above-described peculiarities which sufficiently distinguish the head-cavities from the myoce- lomic pouches, must be mentioned the fact that at no point of their epithelial walls are muscles developed. It is probable that the preoral head-cavities of Amphi- oxus are homologous with the premandibular cavitics of the higher Vertebrates, from the walls of which the greater number of the eye-muscles are developed.* This view is * This is also the opinion of Kupffer. Singularly enough van Wijhe has advanced the view that only the right head-cavity of Amphioxus is to be EMBRYONIC DEVELOPMENT. 129 strongly confirmed by the mode of development of the premandibular cavities in the Cyclostomes. In these fishes, von KuprF- FER has shown that they actually appear in the form of a pair of diverticula from the anterior extremity of the archenteron (Fig. 72). If a comparison be made between Figs. 70 and 72, it will be at once manifest how close the correspondence is P : ae Fig. 72. — Horizontal projection of between the mode of de- pharynx and preoral endodermic exten- ,, sion of a young dAmmocates planeri of velopment of the head-cavi- 3% mm., reconstructed from a series of ties in Amphioxus and in transverse sections. (After KUPFFER.) p.e. Preeoral endodermic extension Ammoceetes. In the Se- (preorale Endodermtasche). pm. and m. iach: tl imilarit :_ Proemandibular and mandibular portions achians ne simularity 18 of head-cavities. ph. Cavity of pharynx. hardly less striking.® /, 2, 3. First three pairs of gill-pouches. a N.B.— Kupffer considers it probable that the mandibular as well as the pra- . mandibular cavities arise from the single Endostyle and Pigment pair of endodermic diverticula. In the Granules. course of the following pages I have referred chiefly to the praemandibular In Fig. 64 there is to be ea alone so as to avoid complica- noticed a vertically placed structure lying in front of and contiguous with the club- shaped gland. It is atract of very high cylindrical cells forming part of the right wall of the alimentary canal in homologised with the pramandibular cavity (see below, V.). Kupffer regards the preemandibular and mandibular head-cavities as rudimentary or meta- morphosed gill-pouches. This deduction is entirely foreign to the standpoint which I have adopted. The conclusion may seem plausible from the con- ditions observed in Acipenser alone; but when these are regarded from a comparative point of view, the deduction is, to my mind, unjustified. It should be added that Kupffer has shown that the head-cavities (preemandibular and mandibular) of Acipenser also arise as endodermic pouches. 130 DEVELOPMENT OF AMPHIOXUS. this region. (Cf. Figs. 65 Gand 75.) I have shown that this epithelial tract is the rudiment of the evdostyle (vide aufra). It is a curious fact that the first trace of pigment to appear in the nerve-tube is not the eye-spot, but that at a constant point in the region of the fifth somite a black pigment-spot is deposited in a cell in the ventral wall of the medullary tube. This is followed by another smaller pigment granule slightly posterior to the first (Fig. 63 C). The eye-spot appears at the end of the embryonic period. LARVAL DEVELOPMENT. Formation of Primary Gill-slits, ete. With the establishment of the definite relations oi the head-cavities, the mouth, club-shaped gland, first gill-slit, and anus, the embryo enters upon the larval phase of the development. It is no longer, or only very rarely, to be taken from the surface of the sea, but descends to a depth of several fathoms. When kept in aquaria, the larve can often be observed to be suspended vertically, and apparently quite motionless in the water. This suspension is, no doubt, effected by the movement of the long cilia, or flagella, with which the ectoderm is provided, each cell possessing one flagellum.® The principal changes which take place during the early stages of this phase of the development are the addition of new myotomes, the formation of new gill-slits, in meta- meric order, in an unpaired series on the right side of the larva, to the number of from twelve to fifteen, or even sixteen (the more usual number being fourteen), and the origin of the atrial cavity. LARVAL DEVELOPMENT. 13! Each gill-slit breaks through in, or slightly to the right of, the mid-ventral line, and then grows well up on the right side of the body. A larva with three gill-slits and the indication of a fourth is represented in Fig. 73. The originally circular mouth has grown to a much larger size, and extends on the left side anterior to the endostylar Fig. 73. — Larva of Amphioxus, with three gill-slits and the rudiment of a fourth; from the left side. (After LANKESTER and WILLEY.) p.p. Preoral pit. ed, Endostyle lying on right side, seen through the wide lateral mouth. .g/. Position of external aperture of club-shaped gland. 4.5’. First primary gill-slit. av. Anus. N.B.— Actual length of larva, nearly 144 mm. tract (which is on the right wall of the pharynx) and posterior to the first gill-slit. The oral opening later attains to relatively gigantic dimensions, and forms one of the most striking features of the larva. The anus is now displaced from its original ventral position to the left side in consequence of the increased development of the provisional caudal fin. The latter consists of elongated ectodermal cells, in which a certain amount of brown pigment is deposited. Later, when the dermal expansion, which has been described above as the definitive caudal fin, begins to grow out, it pushes the cells composing the provisional fin before it, so that they forma fringe round its border. Eventually the provisional fin disappears entirely. The gill-slits now go on adding to their number, one after the other, until the larva reaches the stage shown in Fig. 74. In this larva there are fourteen primary unpaired gill-slits, lying, for the most part, on the right side of the 132 DEVELOPMENT OF AMPHIOXUS. pharynx, although the more posterior slits bend under the pharynx, while the most posterior have a median ventral position. In front the gill-slits still open directly to the exterior, but the right metapleural fold is seen to be hanging over the tops of them; while the hinder slits now open into the partially formed atrium, which has already closed in se aes Ze Fig. 74.— Anterior portion of larva, with fourteen primary gill-slits and rudi- ments of the secondary gill-slits; viewed as a transparent object from the right side. (After WILLEY.) so. Sense-organ of przeoral pit (groove of Hatschek). e. Endostyle. gi. In- ternal opening of club-shaped gland. s.s. Rudiments of secondary gill-slits. 7.518, ps4, Thirteenth and fourteenth primary gill-slits. The lower margin of the mouth is seen through the anterior gill-slits. Total length of larva, nearly 34% mm. Ts oe pen: piste as posteriorly, as described above. The larva is remarkably transparent, so that its internal organisation can be seen as clearly as possible through the outer body-wall. The long axis of the primary gill-slits is approximately at right angles to the long axis of the body. They are not more numerous than the myotomes in the correspond- ing region of the body, so that the branchiomery at this stage coincides with the muscular metamery. In Fig. 73 the first gill-slit was somewhat larger than the second and third. At about that stage, however, its further growth became arrested, and now it is seen to be considerably smaller than those which immediately follow it. In addition to its external opening on the left side, be- LARVAL DEVELOPMENT. 133 i pe LER LE: gf! Fig. 75. — Transverse sections through the region of the mouth of larvze of Amphioxus, to show the endostyle and the external and internal openings of club- shaped gland. (After LANKESTER and WILLEY.) A. Section passing through the anterior corner of the mouth of a larva, with eleven gill-slits. 4. Section passing through the middle of the mouth of a larva, with twelve gill-slits. al, Pharyngeal cavity. 4.c. Coelom (splanchnoceel). 6”. Branchial epithelium. e.a, Branchial artery. evd. Endostyle. ex.o. External opening of club-shaped gland. fc. Dorsal fin-space. g/. Lower portion of club-shaped gland. .g.s’. First gill-slit. daz. Intercoelic membrane. 7.0. Internal opening of club-shaped gland. Za, Left aorta; there is no corresponding right aorta in the larva. 1. Mouth. rm, Rudiment of right metapleur; a mere ectodermic thickening in 4; a solid thickening of the cutis in 4, in which two of the original enlarged ectoderm cells have become imbedded. These curious cells occur over a long stretch of the metapleural folds during this phase of the development, disappearing eventually. In &, the left metapleur is indicated by an ectodermic thickening immediately below the gill-slit. 2. So-called nephridium of Hatschek. 134 DEVELOPMENT OF AMPHIOXUS. low the mouth (see Fig. 64), the club-shaped gland has now acquired an opening at its upper extremity, on the right side, into the pharynx.’ The gland hes, as usual, behind, and closely approximated to, the endostylar tract, which is bent forwards on itself at its upper end (Figs. 75 A and £). Pigment-spots are present in great numbers at the base of the neural canal. The pigment is deposited in special @.77Te Fig. '76.— Transverse sections through the region of the preoral pit. (After LANKESTER and WILLEY.) A, Through a larva, with twelve gill-slits and no atrium. &. Through a larva, in which the atrium was closed in over all the gill-slits except the first two. (Cf. Fig. 38 C.) a a.m. Anterior median portion of right metapleur. .0.c. Preeoral body-cavity (right head-cavity) ; this cavity becomes much reduced after the metamorphosis, and is largely filled up by gelatinous tissue. 7.2. Preoral pit. 5.0. Sense-organ of preeoral pit (groove of Hatschek). 20.4. Rudiment of left half of oral hood. my'. Sclerotome (diverticulum of myoccel my), Other letters as above. Section Z is taken through a plane slightly posterior to section 4. LARVAL DEVELOPMENT. 135 cells, the pzgment-cells, which arise as modified epithelial cells of the central canal. These cells send out several branching processes, which lose themselves in the fibrous tract of the spinal cord. Already in the youngest larva — namely, that shown in Fig. 64 — the preoral pit had become subdivided into two portions, which, however, retained a free communication with one another. In the course of the changes which the left head-cavity had to undergo in its conversion into the przoral pit it had come to lie transversely below the notochord. Sub- sequently it extended itself, in the form of an offshoot, dorsally to the right of the base of the notochord. This offshoot from the przoral pit appears to serve as a special sense-organ lying ultimately, as mentioned above, in the roof of the oral hood, whose function is possibly to test the water as it enters the mouth (Figs. 76 4 and 3B, and Fig. 74, etc.). Formation of Secondary Gill-slits. Above the primary gill-slits in Fig. 74, and like them, on the right side of the body, is to be observed a longitudinal ridge provided with a series of nodal enlargements which alternate with the primary gill-openings, the first of them lying above and between the third and fourth primary slits. Each of these enlargements represents a thickening in the wall of the pharynx, which has undergone fusion with the bedy-wall beneath the right metapleural fold, in the angle formed by the latter with the body-wall. These metameric fusions of the pharyngeal wall with the body-wall are the forecast of a second row of gill-slits, whose relation to the primary row will become clear as we pro- DEVELOPMENT OF AMPHIOXUS. [ony 153 ceed. With their appearance, the larva enters upon that phase of its development which has been called the later larval period. It is the period of the metamorphosis of the larva, during which the pronounced asymmetrical arrangement of the parts is exchanged for the partial, but not absolute, symmetry which we have noted in the adult. The metamorphosis, therefore, consists largely in the sym- metrisation of the larva. The simultaneous appearance of the six nodal thicken- ings in the exact position, shown in Fig. 74, is very constant. Shortly afterwards a minute perforation appears in the centre of each thickening almost simultaneously, except in the case of the first, which usually becomes perforated rather later than the others. The originally small circular openings of the secondary gill-clefts gradually increase in size and become oval in shape, their long axes being parallel to the long axis of the body, instead of at right angles to it as in the case of the primary slits. Next, the upper borders of the secondary slits begin to flatten, and later to show signs of curving downwards. The changes in shape, which affect the secondary slits at the stages now under consideration, may be expressed by saying that they are at first shaped like a biconvex lens, then like a plano-convex lens with the flat surface directed upwards and the convex surface downwards, and finally like a concavo-convex lens with the concavity directed upwards (Fig. 77). During these changes, which do not take place in all the secondary slits at the same time, the last one especially retaining for a long time its primitive shape, the walls of the successive slits become sharply rounded off and distinct from one another, and anew perforation makes its appear- ance in front, above, and between the second and third LARVAL DEI pnmary slits. This new slit constitutes the definitive first ° ms Ww =r the secondary series (Fig. 77). 4d The larva shown in Fig. 77 presents a very different 4d 1 from one aspect from that shown in Fig. 74; ual, and all intermediate ge which we are now Ti) the aa cavity has become com- at now none of the open directly to the exterior. None of the primary slits now lie entirely on the under the pharynx, and 22aD ae 5 Sy eae : s extend round to the left side. This bodily migration of the primary slits from one side to the other occ in a correlation with the increase in size of the nie He which, as they continue to grow, push, as it were, the See bena primary slits mere them, and so cause the latter to under the pharynx in the way described. The peculiar growth by which the primary gill-slits are gradually carried from the right to the left side, may be described as a trans- verse or rotatory growth affecting the pharynx 7 ‘ovo in 138 DEVELOPMENT OF AMPHIOXUS. the region of the secondary slits. Such of the primary slits as occur behind this region are not affected by the rotatory method of growth, and retain their original position in the mid-ventral line of the pharynx. It is to be noted also that there are only twelve primary gill-slits at this stage. Assuming that in the particular larva here figured there were originally fourteen primary slits, the fourteenth has closed up and vanished without leaving a trace, while a vestige of the thirteenth can still be recognised. The actual process involved in the closure and disappearance of a certain number of the primary slits can, as we shall see, be readily observed in the living larva. Club-shaped Gland and Endostyle. The internal aperture of the club-shaped gland into the pharynx is exceptionally plain at this stage, and its refring- ent walls and relatively large size give it a curiously slit- like appearance. We shall find that the gland subsequently atrophies, but the most persistent part of it — that is to say, the last part of it to disappear —is precisely the internal opening with its refringent border. The endostyle, whose primary position, as we have seen, was immediately in front of the club-shaped gland, now presents a remarkable condition. It has begun to grow backwards and downwards, being probably pulled down, so to speak, by the general rotatory growth of which we have spoken above; and so the club-shaped gland no longer lies behind it, but upon it. The gland itself being disconnected with the wall of the pharynx, except at its upper end where it opens into the latter, is not affected by the complicated changes to which the pharyngeal wall, including gill-slits, mouth, and endostyle, is subjected, so LARVAL DEVELOPMENT. 1390 that it forms a convenient punctiwmn fxn with relation to which the growth of neighbouring structures, particularly that of the endostyle, can be determined. The upper and lower limbs of the endostyle are inclined to one another at an acute angle, and may be said to form two unequal sides of a triangle, the apex of which is directed backwards between the rows of secondary and the primary gill-clefts (Fig. 77). Between the two rows of slits on the right side of the body there is a blood-vessel, representing the anterior continuation of the sub-intestimal vessel, which ends blindly in front above the first primary shit. This is the future ventral branchial artery, with which we are already ac- quainted. When its final situation in the mid-ventral line below the endostyle is remembered, its position in the larva high up on the right side, as in Fig. 74, will appear very striking. Continued Migration of Primary Gill-slits. The secondary slits now go on growing in size, and the primary slits gradually tend to disappear entirely from the right side until, as in Fig. 78, only the original upper por- a growth of the WILLEY.) 140 DEVELOPMENT OF AMPHIOXUS. tions of them are visible from this side. In some of the secondary slits the dorsal margin, which had previously begun to curve downwards, has now reached the ventral margin and fused with it (Fig. 78, third secondary slit). In this way is the tongue-bar formed, and the primitively simple gill-opening is divided into two distinct halves. The formation of the tongue-bars occurs in the secondary slits considerably in advance of the primary, both actually and relatively, since the latter have existed all through the earlier period of the larval development without a trace of tongue-bars. Peripharyngeal Bands. The endostyle has now grown a long distance behind the club-shaped gland, and extends backwards between the two rows of slits as far as the middle of the second secondary slit. From the anterior part of the upper half of the endostyle, which is now nearly equal in length to the lower half, arises an epithelial tract in the wall of the pharynx, which appears in the form of a band of ciliated cells, and proceeds backwards below the notochord to the end of the pharynx. a Fig. 95. — Diagrammatic transverse section through the middle of the body of Ascidia. (After HERDMAN.) The muscular mantle is indicated by the black shading. a. Peribranchial cavity traversed by numerous vascular trabeculze, through which the blood flows into the branchial bars. 67.5. Branchial sac. é.v. ‘ Blood-vessels.” @.Z. Dorsallamina, e. Endo- style. ec. Ectoderm. g. Gonad. gd. Double genital duct. 7. Intestine, with tvphlosole. 7. Rec- tum. 7.0. Renal vesicles. 7. Test. length of the endo- style, it is attached to the muscular mantle. In other words, the right and left halves of the atrial cavity are continuous round the dorsal the are side of but separated from one pharynx, another ventrally by the concrescence of the endostyle with the mantle. (Cf. Fig. 95.) In Amphioxus, as we have seen, the opposite condition ob- tains. There, the dor- sal wall of the pharynx is closely applied to the notochord, while the endostylar tract * Compare the above with the description of the course of the ciliated tracts in the of given on p. 168. pharynx Ammocecetes, ANATOMY AND DEVELOPMENT. 187 is free, so that the right and left halves of the atrial cavity are continuous ventrally, instead of dorsally. In order to see the stomach and intestine, it is necessary to cut through the left wall of the pharynx, since the vis- cera lie, at least in the genus Ascidia (or Phallusia), on the left side of the pharynx. It should be pointed out that the topographical arrangements vary considerably among the different genera of Tunicates. In Clavelina, for example, the viscera lie behind the pharynx, as shown in Fig. 96. On the left side of the pharynx (Fig. 94) the short cesophagus leads into the dilated stomach, which again narrows down to the looped intestine, and finally the lat- ter bends sharply forwards into the rectum, which opens by the anus into the atrial cavity, the excrement being carried to the exterior by the constant stream of water which flows out through the atrial or cloacal aperture. Instead of being straight, as in Amphioxus, the aliment- ary canal is here doubled round upon itself. This U-shaped character of the alimentary canal of Ascidians is shown with great clearness in the case of Clavelina (Fig. 96), where there are no secondary convolutions in the course of the intestine. The Ascidians are one and all hermaphrodite, and the reproductive glands frequently lie between the loops of the intestine, while two ducts, ovzduct and vas deferens, which often present the appearance of a single duct with a double lumen, proceed forwards by the side of the rec- tum, to open into the cloacal region of the atrial cavity near the anus (Fig. 94, g and gd). The ovary and testis, though quite separate in the adult, originate, according to the account given by the Belgian zoologists, EpouarD vAN BENEDEN and CHARLES JuLIn, 188 THE ASCIDIANS. from a common centre of formation, which subsequently undergoes a division into two portions, one of which be- comes the ovary, and the other the testis. Similarly the oviduct and vas deferens are derived by division of a primarily single structure, which arises in continuity with, and in fact as an outgrowth from, the primitive sexual gland. In spite of their hermaphroditism, it would appear that not all the Ascidians are self-fertilising, although many, if not most of them, are. In some cases it is supposed that in different individuals the male and female organs attain maturity at different times, so that in a given individual, when the ovary is ripe the testis is unripe, so that it must be fertilised from another individual, in which the testis is ripe, but the ovary unripe, and so on. Nervous System and Hypophysis. (Neurohypophysial System.) The central nervous system of an Ascidian usually bears a ridiculously small proportion to the bulk of the organ- ism. Its main constituent is a ganglion which lies im-. bedded in the thickness of the mantle, between the oral and the atrial siphons, the two latter structures being innervated by nerves proceeding from the ganglion. As belonging to the central nervous system must also be mentioned a solid nerve-cord which runs along the dorsal border of the branchial sac from the cerebral ganglion to the visceral region (Fig. 96). This was discovered by van Beneden and Julin, and is derived from a persistent portion of the central nervous system of the larva. Beneath the cerebral ganglion is a lobulated glandular organ known as the swdneural gland. It is provided with ANATOMY AND DEVELOPMENT. 189 a duct which runs forward and opens at the end of a ciliated funnel-shaped dilatation into the branchial sac at the base of the buccal tube (Figs. 94, 96, and 97) in front of the peripharyngeal band. The branchial opening of the duct of the subneural gland appears primarily as a simple circular orifice, but it does not usually retain this character in the adult. Generally it assumes a crescentic form by the in- curving of its anterior or posterior lip, and then in many cases the horns of the crescent so formed become coiled over and over con- centrically, and usually in approximately the plane, so that the lips of the aperture assume a very same complicated appearance and constitute the so-called dor- sal tubercle (Fig. 97). It has taken a long time and the work of a great many zodlogists to achieve our (which present knowledge is by no means Fig. 96. — Young Clavelina, shortly after the metamorphosis, from the right side. (After VAN BENEDEN and JULIN.) at. Atrial opening. af.c. Atrial cav- ity. 4.5, Blood-sinus. evd. Endostyle. ep. Epicardium; outgrowth from bran- chial sac behind endostyle, which grows down into the creeping stolon, forming a septum in the latter, and being the chief element in the production of buds. JF Lobes of the fixing organ, which give rise to the creeping stolon. .. Ganglion. gs. Stigmata. 2. Heart. Ay. Hypophysis (dorsal tubercle). 7#/, Intestine. m2. Mouth. oes.Esophagus. £.d. Periphar- yngeal band. fc. Pericardium. 4 Re- mains of tail, withdrawn into the body. v.n. Visceral nerve. complete) of the subneural gland of Ascidians and its duct. 190 Fig. 97. — Hypophysis of Phallusia mentula, prepared out and seen from the inside. (After JULIN.) g. Subneural gland, above which may be seen the outline of the ganglion and its nerves. d. Duct of the subneural gland. z. Dorsal tubercle, the opening of the hypophysis into the branchial sac. The actual opening is indicated in black. pc. Peripharyngeal groove. ef. Epi- branchial groove. d@./. Dorsal lamina, slightly displaced, to show the duct of the subneural gland above it. N.B.—In this species, the atrial and buccal siphons are widely separated, and the duct of the subneural gland is very long. THE ASCIDIANS. The dorsal tubercle was discovered by the celebrated SaviIGNY in 1816, and was for a long time supposed to be an independent sense- organ of an olfactory nature. The subneural gland was detected not as a gland, but as an enigmatical structure lying below the brain by the English naturalist Han- cock in 1868. Its glandular character was demonstrated by NassonorF and Ussow in 1874-75, the last-named author showing its connex- ion by means of the duct with the dorsal tubercle. 1881 JULIN produced admirable memoir on the subneural gland and its duct, and strongly urged its ho- mology with the pituitary body or hypophysis cerebri of the higher Vertebrates. The same suggestion was In an made in a more tentative form in the same year by BaLFour. We shall have to consider this later. question Suffice it to say at present that Julin’s sugges- tion has been accepted to ANATOMY AND DEVELOPMENT. IQI the extent that the subneural organ of the Ascidians is frequently spoken of as the Aypophysis. Circulatory System. With regard to the circulatory system the Ascidians differ markedly from Amphioxus in the possession of a well-defined feart which lies in a distinct pericardium. The heart lies ventrally and usually in the neighbourhood of the stomach. (Cf. Fig. 96.) Its wall is muscular, but consists only of a single layer of cells whose deeper portions (z.e. towards the cavity of the heart) are drawn out into striated muscular fibres, while the outer portions of the cells containing the nuclei project into the cavity of the pericardium. There is therefore no true endothelial lining to the heart, and the cells which build up its wall offer a most interest- ing example of epithelio-muscular tissue, as was first pointed out by Edouard van Beneden. This type of muscular tis- sue, in which the muscle-fibres occur as basal prolonga- tions of cells which still retain their epithelial character, is found, as is well known, in the case of the body-muscles of the Nematode or thread-worms, and is above all character- istic of the Coelenterata (Hydroids and Medusz). There are no true blood-vessels in Ascidians, but the passages along which the blood percolates are merely lacunze in the connective tissue and musculature of the body and between the viscera. They are not lined by an endothelium, and are more correctly described as d/o0d- sinuses. They are often irregular in their outline, as shown in the transverse section represented in Fig. 95, but often again they simulate the appearance of true blood-vessels, as in the case of those branches which pass from the mantle into the substance of the test, as well as the tubes 192 THE ASCIDIANS. which traverse the wall of the branchial sac in every direction. In the second chapter it was pointed out that the Vertebrate heart arose as a specialisation of a portion of the primitive sub-intestinal blood-vessel whose calibre was originally uniform throughout, and that in Amphioxus the cardiac region of the vascular system retains its primitive tubular character. Very different is the actual origin of the Ascidian heart ; although it is simply a dilated tubular structure, yet it arises entirely independently of and prior to the rest of the vascular system at a time, in fact, before the formation of the muscular mantle and before the atrial cavity has so far extended itself as to almost entirely replace the original body-cavity. The blood-sinuses of the Ascidians are rem- nants of the latter. With the formation and growth of the atrial cavity, the perforation of the stigmata, and the development of the muscular mantle, the original body-cavity becomes reduced to a system of narrow canal-like spaces which constitute the above-mentioned blood-sinuses. The general distribu- tion of the blood-sinuses can be made out from Fig. 95. There are two main longitudinal sinuses, one below the endostyle and another above the dorsal lamina, while others are scattered irregularly in the muscular mantle; others again lie in amongst the viscera forming the inter- spaces between the various parts; and finally the bran- chial bars between the stigmata are all hollow, and their cavities are placed in communication with the system of sinuses at intervals as shown in Fig. 95. The periodic contraction of the heart of Ascidians takes place on a highly characteristic and unique plan. Each systole occurs as a peristaltic wave of contraction passing ANATOMY AND DEVELOPMENT. 193 from one end of the heart to the other; but the chief peculiarity in connexion with it is, that after a certain number of contractions in one direction the heart makes a brief pause and then commences to contract again in the opposite direction, and so it goes on contracting now in one direction and now in the other. This phenomenon of the periodic reversal of the direction of contraction of the Tunicate heart is known as the recurrent action of the heart, and was discovered in 1824 by van HAsseELt. The discovery was first made in the case of Salpa, but it has since been found to hold good for all Tunicates. When the heart contracts from its posterior to its an- terior extremity, —that is to say, in the postero-anterior direction, —the blood is thereby propelled forwards into the blood-sinus which lies below the endostyle, and from this it passes into sinuses which run transversely into the bran- chial bars. In the basket-work formed by the intercross- ing of the branchial bars, the blood has a complicated and irregular course, and is finally collected into the dorsal sinus which lies above the dorsal lamina. Here it flows backwards, and after passing in amongst the viscera arrives back to the heart. (Other branches of the sinuses pass into the test, where they end in curious knob-like dilata- tions. ) On the contrary, when the heart contracts in the reversed or antero-posterior direction, the blood which has already been oxygenated in its passage through the branchial bars is sent to the viscera direct, and from there it collects into the dorsal sinus, from which it is distributed over the branchial sac, and so into the sub-endostylar or ventral sinus, in which it flows backwards to the heart. On account of the above peculiarities relating to its independent origin, the histological structure of its wall, 194 THE ASCIDIANS. and its recurrent action, the Tunicate heart would appear to be a unique organ peculiar to the group of the Ascid- ians and analogous but not homologous, or only incom- pletely so, with the heart of the Vertebrates. Again, the vascular system of an Ascidian is only func- tionally comparable to that of Amphioxus, since true vessels provided with an endothelial lining are entirely absent, their place being taken by sinuses which arose by reduction from the original body-cavity. Renal Organs. The renal organs of the Ascidians have no apparent morphological relation to those of Amphioxus, and therefore need not detain us. They consist of a group of bladder-like vesicles with cellular walls lying around the intestine. The products of excretion (uric acid, etc.) are deposited inside the vesicles in the form of solid concretions. There is no excretory duct. In AZolguwla, there is a single large cylin- drical renal sac closed at both ends and lying on the right side of the body, behind the heart, known as the organ of Bojanis. Comparison between an Ascidian and Amphioxus. Having sketched in rough outline the organisation of an adult Ascidian, we are now in a position to consider in what respects it resembles and in what it differs from that of Amphioxus. We shall see that some of the most funda- mental differences will be made good by the structure of the larva,— such as the absence of a dorsal nerve-tube and of a notochord. Let us first consider the resemblances between an adult Ascidian and Amphioxus. ANATOMY AND DEVELOPMENT. 195 In both cases the pharynx is perforated by a great num- ber of gi/l-apertures (gill-slits, stigmata), converting it into a branchial sac and opening into an atrial or peribranchial cavity instead of directly to the exterior. At the base of the pharynx there is a longitudinal gland consisting of a groove open throughout its whole length towards the cavity of the pharynx, and known as the exdostyle, whose histo- logical character is closely similar in the two cases. From the anterior extremity of the endostyle a ciliated band of columnar cells passes round the wall of the pharynx on each side, in front of the gill-openings, and abuts on the dor- sal border of the pharynx, along which it is continued back- wards in connexion with the dorsal famzina in the one case and the hyperpharyngeal groove in the other. This band forms a circlet round the pharynx behind the velum, and is the peripharyngcal band.* We shall find also that the Ascidian hypophysis is essentially homologous with the olfactory pit of Amphioxus. In the Ascidians there are sphincter muscles round the buccal and atrial siphons, and inside the former, in front of the peripharyngeal band (pericoronal groove), there is a circlet of tentacles corresponding perhaps to the velar tentacles of Amphioxus. (Cf. Fig. 94, ¢v.) The differences between the structure of an adult Ascid- ian and of Amphioxus may appear to outweigh the resem- blances, but it must be remembered that they are all correlated with and accessory to the one great difference in the mode of existence of the respective types. An Ascidian is sessile; Amphioxus is free. The former, as it were, builds its house upon a rock and is immovable ; the latter lives in the shifting sands, and is capable of extremely active locomotion. * As mentioned above, this band is usually grooved in the Ascidians. 196 THE ASCIDIANS. In correlation with this sessile habit of existence we find that the Ascidians, in contrast to Amphioxus, are hermaph- rodite, —an almost universal condition among sessile organ- isms of every description. They are unsegmented, the muscles not being divided up into myotomes ; and none of their organs (gonads, renal organs, etc.) are metamerically repeated, unless we regard the successive transverse rows of stigmata in the wall of the branchial sac as evidence of metamerism. It is, however, of a totally different nature from the metamerism of the gill-slits of Amphioxus, and we shall see that only in the earlier stages of their devel- opment can the stigmata of the Ascidians be compared with the former. Another of the most characteristic accompaniments of a sessile mode of life is the U-shaped alimentary canal. Instead of being a straight tube with a posteriorly directed anus as in Amphioxus, the alimentary canal of the Ascid- ians is doubled up upon itself, the rectum is directed for- wards, and the anus opens into the atrial cavity. The absence of a dorsal nerve-tube and notochord in the adult Ascidian has been indicated above. In spite of these great differences, the presence of the endostyle and the perforated wall of the pharynx in the adult, and above all the features in the embryonic and larval development, entitle the Ascidians to be defined as more or less Amphioxus-like creatures which have become adapted to a sessile habit of existence. DEVELOPMENT OF ASCIDIANS. The first accurate and detailed account of the embryonic development of Ascidians was the classical memoir pub- lished in 1867 by KowaLevsky in the Mémoires de lAcadémie impériale des Sciences de St. Pétersbourg. ANATOMY AND DEVELOPMENT. 197 The Ascidian larva was known long before this time, and the external features of its metamorphosis were de- scribed in 1828 jointly by AupourIN and MILne-Epwarps, to whom the discovery of the free-swimming larva is due. Furthermore, the internal structure of the tailed larva, and even the histological structure of the axial rod of the tail, was described with some accuracy by KROHN in 1852, but in ignorance of the details of the embryonic devel- opment, he was unable to give the right morphological interpretation to the various parts, and did not identify the axial rod with the notochord of the higher forms. Segmentation and Gastrulation. The segmentation of the egg, the formation of a hollow one-cell-layered blastula, and the flattening and subse- quent invagination of one side of the blastula to form the two-cell-layered gastrula, take place on a plan so essentially similar to what has been described above for Amphioxus that it is not necessary to dwell at length upon them here. Suffice it to point out that the segmen- tation of the Ascidian egg takes place typically, according to vAN BENEDEN and JULIN, on a strictly bilateral plan. That is to say, when the ovum has divided into two blastomeres, right and left, each blastomere represents and will give rise to the corresponding half of the larval body, and the descendants of the first two blastomeres can be distinguished for a remarkably long time on each side of the middle line of the embryo, —a fact which is highly characteristic of Ascidian development. After the gastrula has begun to elongate, and the blas- topore has been narrowed down by the approximation of its lips to a small aperture situated at the posterior dorsal extremity of the embryo, the formation of the medullary plate occurs. 198 THE ASCIDIANS. Formation of Medullary Tube and Notochord. Here, as in Amphioxus, the dorsal wall of the embryo flattens, while the ventral remains convex, and the ecto- dermic cells on the dorsal side become marked off from the rest by their larger size and columnar shape. The medullary plate extends nearly to the front end of the embryo, while posteriorly its cells form a ring round the blastopore. In the formation of the medullary tube, however, there is an important difference, and the Ascidian embryo con- forms in this point more to the mode of development Fig. 98.— Transverse sections through embryo of Clavelina Rissoana, to show mode of formation of medullary tube and mesoderm. (After DAVIDOFF.) al. Through anterior region of embryo, with medullary groove still open. &, Through posterior region, with closed medullary tube. ch. Rudiment of notochord. ec. Ectoderm. ev, Endoderm. mes. Mesoderm. m.g. Medullary groove. m.t, Medullary tube. which is typical of the higher Vertebrates than does Amphioxus. In the latter the medullary plate sinks bodily below the level of the surrounding ectoderm, which then grows over it. Subsequently while underneath the ectoderm the medullary plate assumes the form of a half-canal open towards the ectoderm, and eventually its margins come together and so form a complete tube. In the Ascidian embryo the overgrowth of the surround- ing ectoderm and the folding up of the margins of the ANATOMY AND DEVELOPMENT. 199 medullary plate occur simultaneously, so that when the latter has the form of a half-canal it is not closed over by a layer of ectoderm, but is open to the exterior (Fig. 98). At a somewhat later stage the two medullary folds meet together and fuse in the middle line (Fig. 98 A), and this, combined with a slight forward growth of the posterior lip of the blastopore, leads to the inclusion of the latter in the medullary tube, so that we arrive at the S25 Ls SG Lo) condition already de- fey ° : scribed for Amphioxus, in which the nerve-tube opens in front to the exterior by the zeuropore and behind into the ar- |S] j Co 1H SEX ge o chenteron by the blasto- pore, which has now become converted into the neurenteric canal. Fig. 99. — 4. Embryo of Phadlusia mam- millata seen in optical section from above, to show notochord. ZB, Section through tail of older embryo of Phallusia mammillata, (After KOWALEV- SKY.) ch, Notochord. derm. mes. Mesoderm, ec. Ectoderm. ez. Endo- nt. Medullary tube. Meanwhile the cells forming the dorsal wall of the archenteron in its posterior two-thirds begin to gather themselves together to form the notochord (Figs. 98 and 99). The cells forming the notochord are at first arranged end to end (Fig. 99), and subsequently interlace in the manner described above for Amphioxus. Origin of Mesoderm. At about the same time in which the formation of the medullary tube and notochord is going on, the mesoderm begins to put in its appearance, and this is the first event in the development in which there is an important dif- 200 THE ASCIDIANS. ference between the Ascidian and Amphioxus. The mesoderm in the Ascidian embryo does not arise as a series of archenteric pouches, but is produced on each side by a solid proliferation of cells from the primitive endoderm which lines the archenteric cavity. This solid proliferation begins in the middle region of the embryo near the an- terior limit of the notochord, and extends backwards (Figs. 98 and 100). It takes place from the dorso-lateral cells of the endoderm, in a posi- tion corresponding to that at which the mesoblastic pouches of Amphioxus grow out from the archenteron. The mesoderm of the As- cidian embryo therefore agrees with that of the em- bryo of Amphioxus in being Fig. 100.— Embryo of Clavelina Ris- soana seen from above, to show the re- lation of parts. (Simplified after VAN BENEDEN and JULIN.) np. Neuropore. ez. Endoderm. evt.c. Enteric cavity. .t. Medullary tube. mes. Mesodermic band. cf’. Notochord. ec. Ectoderm, derived from the primitive endoderm, but differs in be- ing solid and unsegmented.* * For a recent and elaborate discussion of the origin of the mesoderm in the Ascidians see VON DAVIDOFF’s Untersuchungen zur Entwicklungsgeschichte der Distaplia magnilarva, ete., If. Allgemeine Entw. der Keimblaétter. Mitth. Zool. Stat. Neapel, IX. 1891. pp. 533-651. As shown by van Beneden and Julin in Clavelina, the primary mesoderm of the Ascidian embryo can be detected at a much earlier stage of development than in Amphioxus. I have studied the origin of the mesoderm in Cynthia papillosa and found that the primary mesoderm cells are to be distinguished, by their poverty in food-yolk, from the remaining endoderm, at the commencement of gastrula- tion (at the so-called A/aku/a-stage). They occur in the form of a crescent round the posterior margin of the blastopore, and are carried in by the invagi- nation, and then increase in number by mitotic division. In Cynthia, these ANATOMY AND DEVELOPMENT. 201 We thus have two solid longitudinal mesodermic bands inserted between the ectoderm and endoderm. Anteriorly the mesodermic bands consist of several layers of cells super- imposed one above the other (Fig. 98), but farther back they consist of only one layer of cells. Both portions of the mesoderm —namely, the anterior two- or three-layered and the posterior onec-layered portions —arise in continuity with one another, but they have different fates, the former eventually breaking up into loose cells which float about in the body-cavity and constitute the so-called mesenchyme, the latter, on the other hand, becoming converted into the musculature of the tail; whence the former is spoken of as the gastra/ and the latter as the caudal mesoderm. Outgrowth of Tail. In Amphioxus, at the stage corresponding to that of which we have been speaking — namely, when the embryo has an oval or sub-elliptical shape —it bursts through the vitelline membrane inside which it has already been rotat- ing for some time by means of the cilia of the ectoderm, and escapes into the open sea. This is not the case, however, with the Ascidian embryo. The latter is never ciliated externally, and it remains enclosed within the fol- licular membrane throughout the whole of the emdryontc period of development. After the stage in question, the growth in the length of the embryo is accompanied by a ventral curvature, owing to the confined space in which it is contained. Moreover, the increase in length is not due to a simple elongation of the entire body of the embryo, as is the case with Amphi- primary mesoderm cells appear to give rise almost exclusively to the caudal mesoderm, while the gastra/ mesoderm appears to be added in front by prolifera- tion from the primitive endoderm as described above. 202 THE ASCIDIANS. oxus, but it is merely due to the outgrowth of the tail from the body of the embryo (Fig. 101). The structures involved in the outgrowing tail are the dorsal nerve-tube, the notochord, the caudal mesoderm, which lies on each side of the notochord, and will give rise to the muscles of the tail, and finally a solid cord of endo- derm consisting of two rows of cells placed side by side below the notochord (Fig. 99 &). As soon as the tail begins to grow out, the neu- renteric canal becomes ob- literated, and shortly after- wards the anterior neuropore Fig. ror. — Embryo of Phallusia closes up temporarily. Ata mammillata in side view, to show com- mencing outgrowth of tail. (After KOWALEVSKY. : 4 eee ec. Ectoderm. ev, En- si TEODENS » HOt; however, to doderm. mes. Mesoderm; the cells in- the exterior, but into the dicated by dark outlines, beneath which may be seen the notochord and caudal buccal tube. endoderm. 2.g. Neuropore. 2.¢. Medul- As tthe tail grows in lary tube. later period, as we shall see, length, it becomes coiled round about the body of the embryo, attaining two or three times the length of the latter. The cord of endoderm cells in the tail of the Ascidian larva has been supposed to represent a rudimentary intes- tine homologous with the straight intestine of Amphioxus, the larval tail being on this view equivalent to the post-branchial portion of the trunk in Amphioxus. This view, however, is probably not correct, although there is something to be said in favour of it. The probability is that the tail of the Ascidian larva or tadpole, as it is often called, is an organ which has been specially elaborated in the course of its evolution for the particular benefit of the Ascidians, since (exclusive of the pelagic forms) it is their ANATOMY AND DEVELOPMENT. 203 sole organ of locomotion, and hence of transportation from place to place; this only being possible during the larval period. As arule, the larval phase of an Ascidian’s existence is a remarkably brief one, and there is on this account all the more need for an effective propelling organ, which will enable the larva to arrive at a suitable resting-place. In Amphioxus, as described above, locomotion is ef- fected by serpentine movements of the whole trunk in virtue of its muscle-segments, and there is therefore no need for a tail in addition; but there is, nevertheless, a short post-anal extension of the body, which alone can be regarded as the homologue of the tail of the Ascidian larva. In the latter (¢.g. Ciona, Phallusia, etc.) the muscles are entirely confined to the tail, none being formed in the body proper, until after the resorption of its caudal appendage. On the view which I am endeavouring to make clear, it follows that the tail of the Tunicate tadpole is of the same nature as that of the Amphibian tadpole, and, in fact, of the craniate Vertebrates generally, and, as has just been said, is only represented by the short post-anal section of the trunk in Amphioxus. The solid cord of endoderm in the tail is not, therefore, a rudiment of a primitive intestine, but it is analogous to, even if not, as first suggested by BaLrour, homologous with, the so-called post-anal gut which occurs in the em- bryos of the higher Vertebrates, and bears a similar rela- tion to the formation of the tail that the endoderm-cord in the Ascidian embryo does. Thus in the typical Ascidian embryo the elongation of the trunk (body proper) does not take place to any consid- erable extent during the embryonic or even larval period, but only after the metamorphosis. 204 THE ASCIDIANS. With the formation of the tail the enteric cavity be- comes confined as a closed sac to the anterior portion of the embryo. It is bounded dorsally by the nerve-tube, which is somewhat dilated in this region, and in front, at the sides and below, it is in close contiguity with the ectoderm. formation of the Adhesive Papille. At a much later stage than that represented in Fig. tor, the ectoderm bounding the convex anterior extremity of the body becomes raised up into three prominences, whose relations to one another are those of the corners of a tri- angle. They are due to the ectodermic cells at the respec- tive points assuming a high columnar shape. They become eventually raised very much above the adjoining surface of the ectoderm, and become the adhesive papille or fixing glands of the larva. The cells composing them acquire the power of secreting a viscid substance, by which the larva can fix itself to any favourable surface (Fig. 102). Cerebral Vesicle and its Sense-organs. We have spoken above of the dilated anterior portion of the nerve-tube. This is the part of the central nervous system which undergoes the most striking subsequent changes. By a gradual widening of its cavity, accom- panied by a local thinning out of its wall, this portion of the neural tube lying in front of the notochord becomes transformed into a spacious sub-spherical vesicle, known as the cerebral vesicle (Fig. 102). While the anterior portion of the neural tube is enlarg- ing to form the cerebral vesicle, granules of black pigment are deposited by certain cells in the dorsal wall of the vesicle. The granules are at first scattered about in the ANATOMY AND DEVELOPMENT. 205 interior of the cells. The most anterior of the cells con- taining the pigment is at first distinguished from the others solely on account of the fact that the pig- ment-granules which it contains are somewhat larger than those in the succeeding cells. (Cf. Fig. 103.) s Later on, however, pig. 102, Embryo of Ascidia mentula he first pigmented cel] shortly before hatching; from the right side. t Pls (After WILLEY.) is seen to separate itself ch. Notochord, undergoing vacuolisation. e. Eye. ent.c. Enteric cavity. ££ Adhesive from the others, and it papillz. #¢. Anterior portion of nerve-tube becomes gradually trans- (spinal cord). 0. Otocyst, lying on the floor 5 7 . of the cerebral vesicle and projecting up ferred by a differential freely into its cavity. 7a. Right atrial involu- tion. st. St d : prowtl ob ‘the wall. of “eee the vesicle down the right wall to its final position in the ventral wall of the vesicle (Figs. 102, 103). This cell is the ofocyst, and the pigment-granules become consolidated together to form the ofo/7th. The latter is apparently Fig. 103.— Optical sections through cerebral vesicle of embryos of Ascidia mentula, to show mode of origin of eye and otocyst. (After WILLEY.) e. Eye. 0, Otocyst. extruded from the cell (otocyst) in which it was originally formed, and the latter assumes a cup-shape, in the hollow of which the otolith lies. The two structures together form the so-called auditory organ, whose function may be not so much of an auditory nature as that of an equilibrat- ing apparatus. 206 THE ASCIDIANS. The other pigment-cells of the dorsal wall of the cerebral vesicle collect themselves together and form a slight pro- tuberance in the right dorso-lateral corner of the vesicle, while the pigment-granules, which were at first scattered about in the interior of the cells, become concentrated at their converging extremities towards the cavity of the vesicle. And in this way is formed the single eye of the Ascidian tadpole; the original pigment-producing cells constitute the ve¢zza, which retains its primitive position as part of the epithelial wall of the brain.* Subsequently two or three cells from the adjoining wall of the vesicle take up a position, one above the other, in front of the mass of pigment and, having previously, by an alteration in the character of their protoplasmic con- tents, acquired a high refractive index, constitute the /exs of the eye, which projects obliquely downwards into the cavity of the vesicle. (Cf. Fig. 105 4.) The cerebral vesicle of the Ascidian tadpole is the un- doubted homologue of the corresponding, but less pro- nounced, structure in Amphioxus. It differs from the latter in lying wholly in front of the anterior extremity of the notochord, in possessing a more highly organised eye, provided with a cellular lens, and in the presence of an otocyst, which, as we have seen, is evolved from the same group of cells which gave rise to the eye. The eye of the Tunicate tadpole agrees fundamentally with the type of eye peculiar to the Vertebrates, in that the retina is derived from the wall of the brain. On this * The fact that the lens of the Tunicate eye as well as the retina and the otocyst arise by differentiation of one and the same epithelial layer of the primitive cerebral vesicle, has recently been described by SALENSKY for the larva of Distaplia, magnilarva. (W. SALENSKY. for phologische Studien an Tunicaten: I. Ueber das Nervensystem der Larven u. Embryonen vor Distaplia magnilarva. Morph. Jahrb. XX. 1893. pp. 48-74.) ANATOMY AND DEVELOPMENT. 207 account it is called a mzyelonzc eye. In the typical Inverte- brate eye, on the contrary, the retinal cells are differen- tiated from the external ectoderm. Comparison of Tunicate Eye with the Pineal Eye. The Tunicate eye, however, differs essentially from the paired eyes of the craniate Vertebrates in that the lens, as well as the retina, is derived from the wall of the brain. The lens of the lateral eye of the Vertebrates is derived by an invagination of the external ectoderm, which meets and fits in with the retinal cup at the end of the optic vesicle. It is, therefore, an extremely interesting fact which was pointed out by BALDWIN SPENCER, that the Tunicate eye agrees, in respect of the origin of its lens, with the parzetal or pineal eye of the Lacertilia, in which the lens is likewise derived from cells which form part of the wall of the cerebral outgrowth which gives rise to the pineal body. The pineal body is another of those remarkable rudi- mentary structures whose constant presence in all groups of Vertebrates forms such an eminently characteristic feature of their organisation. It develops as a hollow median outgrowth from the dorsal wall of the brain (thalamencephalon), the distal extremity of which dilates into a vesicle and becomes separated from the proximal portion. * For a long time the pineal body was a persistent enigma * According to the most recent work on the subject the distal vesicle be- comes entirely constricted off from the primary epiphysial (pineal) outgrowth of the brain, and the parietal nerve does not represent the primitive connex- ion of the pineal eye with the roof of the brain, but it arises quite inde- pendently of the proximal portion of the epiphysis. See A. Kiinckowstrim, Bettrage sur Kenntniss des Parietalauges. Zoologische Jahrbiicher (Anat. Abth.), VII. 1893. pp. 249-280. 208 THE ASCIDIANS. and the subject of much speculation, one of the most cele- brated hypotheses with regard to its significance being that of Descartes, who regarded it as the seat of the soul. More recently it has been shown to represent a rudi- mentary, unpaired eye. Although in most cases, curiously enough, it exhibits in existing forms no trace of an eye- structure, it has been shown by DE GraaF and SPENCER that, as a matter of fact, in many lizards the distal vesicle does actually become converted into an eye which, though of a rudimentary character, is possessed of a retina, pig- ment, and lens. In these forms the pineal body pierces the roof of the cranium, occasioning the parietal foramen, which is so characteristic of the Lacertilian skull, and the pineal eye lies outside the cranium immediately below the skin, through which it can be distinguished in external view by the presence of a modified scale placed above it. In the animals below the lizards in the scale of organi- sation (Amphibians and Fishes), as well as in those above them, the distal vesicle of the pineal body apparently does not become so far differentiated as to be recognised as an actual eye, except in the case of the Cyclostome fishes, where, as shown by BEARD, it presents the three essential elements of an eye; namely, retina, pigment, and lens, lying, however, inside the cartilaginous cranium. The facts in our possession would seem to indicate that the remote ancestors of the Vertebrates possessed a median, unpaired, myelonic eye, which was subsequently replaced in function by the evolution of the paired eyes. It would, however, be premature either to assert this or to express it as a definite opinion, especially since, in refer- ring to the evolution of the paired eyes of Vertebrates, we are bordering on ground upon which I have no imme- ANATOMY AN > diate intention of treading. The pineal eye may not have been primitively so much an organ of vision as a li percenvie organ, as is no doubt the case with the eve of ; eTu nicate eo abit Mey ete Teel oma gate SMomod@al and Afri By the time that the cerebral vesicle of the Ascidian embryo with its contained sense-organs (eye and otocvst) c 1 approaching the completion of its full development, no wn ; 1 less than three ectodermic invaginations occur in the body of the embryo. One of these is situated immediately i front of and in contact with the anterior wall of the cere bral vesicle, the blind end of the involution pressing e subjacent endoderm. This is the stomodeum, 8 Qu mn er w bh ormation is preliminary to the perforation of the mouth which takes place later, and places the stomodeum in open communication with the portion of the enteric cavity which will become the branchial sac (Fig. 102). It should be emphatically noted that the stomodeeal invagi- nation occurs in the dorsal middle line immediately adja- to the anterior extremity of the central nervous system. cent The other two ectodermic invaginations occur symmetri- cally, one to the right and the other to the left of the dorsal middle line, behind the region of the cerebral vesicle, and constitute the pair of atrial involutions, which, by their wn ubsequent growth and modification, give rise to the atrial or peribranchial cavity. We see, therefore, that the epi- thelium which forms the lining membrane of this cavity is, as in Amphioxus, derived from the external ectoderm. 210 THE ASCIDIANS. For some considerable time after the metamorphosis the young Ascidian possesses two separate atrial cavities, right and left, each opening to the exterior by its own atrial aperture. Eventually the two cavities extend round the branchial sac dorsally, so that their walls come into contact in the dorsal middle line, and finally the dividing line breaks down, and they become continuous one with another dorsally, remaining separated ventrally, as described above. At the same time that the two atrial cavities grow towards one another, their external apertures become in- volved in the same process of growth, and, moving together, finally fuse in the dorsal middle line, and so form the single atrial or cloacal aperture of the adult.* Beyond agreeing in its ectodermal origin, there might appear to be not much in common between the mode of development of the atrial cavity in the Ascidians and in Amphioxus. No morphologist would recognise a fundamental differ- ence in the fact that the right and left halves of the atrial cavity in Amphioxus arise by a single median involution of the ectoderm, instead of from a pair of involutions, and that they are from the first continuous with one another instead of becoming so secondarily (Fig. 104). In like manner, the fact that the two halves of the atrial cavity are continuous with one another ventrally in Amphi- oxus and dorsally in the Ascidians, is easily brought into correlation with the other differences in the organisation of the two types, which have been described above, and is no bar to our regarding the atrial cavity of the one as being homologous with that of the other. * The time at which the atrial cavities fuse together varies very much in different genera. In A/oleula manhattensis, for instance, whose stigmata develop on a similar plan to those of Ciona (see below), there is a single atrial aperture at the moment of the metamorphosis, ANATOMY AND DEVELOPMENT. 211 One feature in connexion with the formation of the atrial cavity, in which the Ascidians stand in marked contrast to Amphioxus, does, however, require a special explanation. Whereas in Amphioxus the atrial involution has the form of a longitudinal groove, in the Ascidians it occurs on each side, as a local inpushing of the ectoderm with a minute circular orifice of invagination.” The fact has already been stated above that the elonga- tion of the body proper of an Ascidian embryo or larva does not, in the main, take place until after the metamorphosis. Fig. 104. — Diagrammatic transverse sections, to illustrate the mode of forma- tion of the atrium in (4) an Ascidian and (8) Amphioxus. (After WILLEY.) The atrial involutions occur at a time when the tail is rapidly increasing its length; the body proper, on the con- trary, remaining stationary so far as increase in size is concerned, and retaining at this stage approximately the dimensions which it possessed when the tail first began to grow out. Moreover, they occur éefore the appearance of any gill-clefts in the wall of the branchial sac, so that in the Ascidians the gill-slits never open directly to the exterior. In Amphioxus, on the other hand, there is no such delay in the elongation of the body of the embryo, but it goes on continuously till the full complement of myotomes has been 212 THE ASCIDIANS. formed. The post-anal portion of the body, which we sup- pose to be the homologue of the tail of the Ascidian tad- pole, does not appear until a somewhat late period in the development. There is very little of it present in the larva with three gill-slits (Fig. 73). The reason of this, as explained above, is that the post- anal section of the trunk is of only minor functional sig- nificance in Amphioxus, but is all-important to the Ascidian larva, and consequently, as is the case with many other structures of great functional importance in the various groups of the animal kingdom, it exhibits a precocious development. Not only, therefore, has the elongation of the body of Amphioxus already taken place before the occurrence of the atrial involution, but the primary gill-slits have also broken through the wall of the pharynx, and open freely to the exterior before the atrium begins to be closed in. In Amphioxus, then, the atrial involution has been drawn out into the form of a longitudinal groove because it occurs subsequently to the elongation of the body and the perforation of the gill-slits. In the Ascidian embryo the (paired) atrial involution has the form of a simple pit with a circular margin, be- cause it arises before the elongation of the body proper of the embryo and before the perforation of the gill-clefts, so that no influence has been at work to draw it out into the form of a groove. We see, therefore, that a great many of the differences between the Ascidian tadpole and the larva of Amphi- oxus can be explained sufficiently to allow of their being brought into genetic relation with one another, by consid- ering the relative time at which corresponding develop- mental processes take place in the two cases. ANATOMY AND DEVELOPMENT. 213 The following table will help to make this matter clearer. ORDER OF ASCIDIAN. AMPHIOXUS. OccuRRENCE. I. Gastrulation. Gastrulation. 2. Oval embryo with medullary | Oval embryo with medullary tube, neurenteric canal, tube, neurenteric canal, notochord, and mesoblast. notochord, and mesoblast. (Last two commencing. ) (Last two commencing.) 33 Outgrowth of tail. Commencing elongation of body of embryo, and escape from vitelline membrane. 4. Continued growth of tail. Continued elongation of em- bryo. 5. Formation of stomodceum and | Formation of mouth, and com- atrial involutions. mencing perforation of gill- clefts. 6. Escape from vitelline mem- | Continued formation of gill- brane. clefts and outgrowth of tail (i.e. post-anal section of trunk). 7 Commencing perforation of | Formation of longitudinal atrial gill-clefts. involution. 8. Metamorphosis and commenc- | Metamorphosis. ing elongation of body proper. Of course the above table has no concern with the actual time (hours and days) from the commencement of the development at which such and such an event occurs. The type of Ascidian referred to in the above description is a simple Ascidian like Czoua or Phallusia. The above table also shows how the development of the Ascidian and of Amphioxus moves along parallel lines up to a certain point, and then at the time of the outgrowth of the tail in the embryo of the former and the hatching of the embryo of the latter, divergences set in. 214 THE ASCIDIANS. It has long been recognised that the development of an Ascidian is much abbreviated in comparison with that of Amphioxus, since in the former it neither comes to the formation of a ciliated embryo nor to the production of archenteric pouches for the mesoderm. One of the chief evidences, however, of abbreviation in the Ascidian devel- opment is the precocious formation of the larval tail. Formation of Alimentary Canal and Hatching of Larva. When the enteric cavity of the Ascidian embryo begins to grow in length so as to give rise to the stomach and intestine, which it does shortly after the appearance of the atrial involutions, there is only one resource open to it on account of the limited space in which it lies, and that is to double round upon itself. This it accordingly does. As the growth progresses, the posterior dorsal angle of the enteric cavity bends sharply downwards on the right side, and then upwards and slightly forwards on the left side, ending at first blindly in the vicinity of the left atrial sac. In this way the four divisions of the alimentary canal become established; namely, pharynx or branchial sac, oesophagus, stomach, and intestine. (Cf. Fig. 105.) By the time these changes have taken place, the embry- onic development is at an end, and the larva is ready to hatch. By spasmodic jerkings of its tail, the larva finally succeeds in bursting the egg-follicle or vitelline membrane in which it has been hitherto enclosed, and so escapes into the open sea. Clavelina and Ciona. While the development of most forms of Tunicata is re- ducible to a common type, yet the details vary within very wide limits in different genera. The tendency here, as ANATOMY AND DEVELOPMENT. 215 elsewhere, is to abbreviate the development by omitting certain ontogenetic processes, and so arriving at the de- sired end, as it were, by a short cut. One of the most impressive instances of such an abbre- viated development, and one which can be demonstrated with the utmost certainty, is afforded by the genus C/ave- ina, in contrasting it with the closely allied genus Czoua. Clavelina (see Fig. 96) is an Ascidian, provided at its base with creeping processes or stolons containing a lumen continuous with the body-cavity, by which it adheres to rocks and weeds. Buds are formed from the stolon, which grow up into new individuals precisely like the parent form which developed from the egg, and soa colony is produced. Ciona also has similar basal processes of the test, con- taining prolongations of the original body-cavity, but no buds are produced. In Clavelina, the embryonic development, up to the time of the hatching of the larva, takes place inside the peri- branchial chamber of the parent, which becomes converted into a kind of brood-pouch. In Ciona, the eggs are extruded into the water, where they are fertilised by the simultaneous extrusion of sper- matozoa from the same individual. Finally, in Clavelina the egg is much larger and contains more food-yolk than that of Ciona. We see, therefore, that in these two genera the egg is at the outset subjected to different sets of conditions, both internally and externally. METAMORPHOSIS OF CIONA INTESTINALIS. Three stages in the metamorphosis of the larva of Czona mtestinalis are shown in Fig. 105. First, there is the free- swimming larva, which, after a pelagic existence of one or 216 THE ASCIDIANS. perhaps two days’ duration, is on the point of fixing itself to a foreign object by means of the sticky secretion of its three adhering papillee. This larva possesses features which we have not yet considered. Let us give our attention in the first place to the tail. Vacuolisation of the Notochord. The vacuolisation of the notochordal tissue, which was described above for Amphioxus, has already proceeded to such an extent that there is no longer any trace of cellu- lar structure in the centre of the notochord. It is entirely filled with a perfectly colourless substance, probably of gelatinous consistency, while the nuclei have been dis- placed entirely from the centre and can be seen to lie closely pressed against the dorsal and ventral sides of the sheathing membrane of the notochord (Fig. 105 A). There is one respect in which the above vacuolisation of the cells of the notochord differs considerably from the corresponding process in Amphioxus and the higher Vertebrates. Whereas in the latter forms the vacuoles appear inside the individual cells, — in other words, are zztracellular, — in the Ascidian tadpole they occur between the cells, and are therefore 7ztercellular. This was first made out by Kowalevsky, and can readily be observed. (Cf. Fig. 102.) The intercellular spaces separate the cells which were previously fitted accurately together, end to end, and, gradually increasing in size, they eventually flow together and so constitute a continuous space, while the cells with their nuclei become thrust aside. Assuming that the vacuoles contain a more or less fluid substance secreted by the protoplasm of the cells, the ANATOMY AND DEVELOPMENT, 217 above difference in the vacuolisation of the notochordal tissue in Amphioxus and the Ascidian larva would resolve itself into saying that the secretion was retained inside the cells in the one case, and deposited outside them in the other. Mesenchyme and Body-cavity. The endoderm cells of the tail, which formed at first a solid cord below the notochord, have now become con- verted into loose corpuscles, which have mostly floated out of the tail into the hinder portion of the body-cavity, and have become indistinguishable from the mesoderm- cells. The latter are beginning to lose their compact dis- position in the form of the two mesodermic bands, espe- cially in the hinder region, and to be scattered about in the body-cavity. The body-cavity of the young Ascidian is not unre- servedly homologous with that of Amphioxus, on account of this remarkable behaviour of the mesoderm. The cavity does not arise in the midst of the mesoderm by a splitting apart of its component cells, but it is simply produced by a separation of the endoderm from the ecto- derm, the two layers being at first in contact at the sides and below; in fact, everywhere, except where the dorsal nerve-tube intervenes. In the cavity thus produced between ectoderm and endoderm the mesodermic bands at first lie freely, and then their component cells break away from their compact association and float about the cavity in the form of scattered corpuscles, known collectively as mesenchyme. This mesenchyme later gives origin to the muscula- ture of the body proper of the Ascidian, and also to the definitive blood-corpuscles, genital organs, and renal 218 THE ASC/DIANS. vesicles.* All these structures are differentiated from the loose mesenchyme cells, all of which at first course round about the body of the young Ascidian like blood, being kept in motion by the beating of the heart. In the stage shown in Fig. 105 A the mesodermic bands are still fairly compact in front, having extended them- selves anteriorly at the sides of the enteron by interstitial growth. Preoral Body-cavity and Preoral Lobe. When the larva first hatches, the endoderm and ecto- derm are in contact with one another at the anterior extremity of the body, just as they are in the earlier stages. (Cf. Fig. 102.) Soon, however, the ectoderm, with the adhering papille, springs away from the endo- derm at this point, leaving a space into which the two lateral mesodermic bands force their way. In this way a special anterior portion of the body-cavity, preoral and przenteric, is produced, and is at first com- pletely filled by a compact mass of rounded cells derived from the mesodermic bands. The end of the body of the larva at which the adhering papilla are placed of course corresponds to the tip of the snout in Amphioxus. Just as Amphioxus burrows into the sand with its snout, so the Ascidian larva fixes itself to the surface of a rock or weed by its snout. The anterior or preoral portion of the body-cavity, of which we have just traced the origin, is, and subsequently becomes in a still more pronounced way, the cavity of the snout, or preoral lobe. * The pericardium arises ventrally from the endodermic wall of the bran- chial sac, and the heart is formed by an infolding of the dorsal wall of the pericardium. ANATOMY AND DEVELOPMENT. 219 Fig. 105.— Metamorphosis of Ciona intestinalis; above is represented the anterior portion of the free-swimming larva from the left side; on the left, the larva, shortly after fixation, from the right side; and on the right, the stage at which the change of axis commences, from the left side. (After WILLEY.) a. Atrial aperture: 4. Branchial sac. ch. Notochord. e¢. Endostyle. # Organ of fixation. g. Ganglion. 4. Neuropore (having reopened into branchial sac). 2. Intestine. /. Pyloric gland. mm. Mouth. x. Nerve-tube. oe, (Esophagus. 0d. Eye. ot. Otocyst. . Pericardium. s. Stomach. s¢ Stigmata. 4 Tail. 220 THE ASCIDIANS. Body-cavity of an Ascidian and Celom of Amphtoxus. We must now endeavour to show how the body-cavity of the Ascidian can be brought into genetic relationship with the ccelom of Amphioxus. The question of the absence of metamerism in connexion with the origin of mesoblast in the Ascidians need not detain us, since it is so obviously correlated with their mode of life. It may safely be asserted that the Ascidian mesoderm, asa whole, is homologous with that of Amphioxus as a whole, but in the details of its origin and fate it is widely different. If we figure to ourselves the coelomic epithelium of Amphioxus losing its character as a membrane and break- ing up into its constituent cells, which would then lie loosely in the body-cavity, we should have essentially the same condition of things as in the Ascidians. There are numer- ous precedents in the animal kingdom for such a disinte- gration of an epithelial membrane. A most perfect instance of it has been described by Dr. R. von ERLANGER * in connexion with the origin of the mesoderm in the fresh-water snail, Paludina vivipara. Here the mesoderm appears at first in the form of a median bilobed archenteric pouch of relatively large dimensions. Soon, however, the cells forming the wall of the pouch begin to assume irregular shapes, and so disturb the contour of the epithelium, and eventually they break apart entirely and fill every nook and corner of the available space with a loose mesenchyme. Similar out-wanderings of cells from an epithelial wall, though not often of such a complete character as the instance above cited, are by no means. infrequent. * Zur Entwicklung der Paludina vivipara. Parts I. and II. Morpholo- gisches Jahrbuch, XVII. 18qr. ANATOMY AND DEVELOPMENT. 221 A striking example is afforded by the body-cavity of the worm-like Balanoglossus, of which we shall speak later. Here, according to Bateson, the cells lining the cavity are continually budding off daughter-cells, which fall into the cavity, and eventually almost entirely fill it up with mesenchymatous tissue. In this case, therefore, mesen- chyme and an epithelial wall coexist. Similarly, the epzthelial sclerotome of Amphioxus is rep- resented by a mesenchymatous sclerotome in the higher Vertebrates. It is not necessary to multiply instances, but many others could be adduced. If, now, this disintegration of parzetal and visceral layers of the mesoderm, which we have imagined above to take place in the ontogeny of an animal like Amphioxus, be supposed to be thrown back in the development, or, in other words, abbreviated to such an extent that the pre- liminary formation of a continuous ccelomic epithelium no longer takes place, we should have precisely those condi- tions which we actually find in existing Ascidians. As in the cases above quoted for purposes of illustra- tion, so in the Ascidians the mesenchymatous condition undoubtedly originated ancestrally from what we may call an epithelial condition. In the Ascidians we may conclude, therefore, that while ontogenetically the mesenchymatous condition is to all intents and purposes primary, from a phylogenetic point of view it is pre-eminently secondary or cenogenetic. Having made the reservations implied in the above statements, we may confidently assert that as a whole the body-cavity of the Ascidians is homologous with the ccelom of Amphioxus, and we may define the former as a ccelom in which the cells, instead of associating together 222 THE ASCIDIANS. to form a lining membrane round the cavity, remain independent of one another and scattered about inside the cavity. Fixation of the Ascidian Larva. When the larva first fixes itself to some available surface, the tail remains for a time stretched straight out and almost motionless, giving perhaps an occasional twitch. Soon the tail is observed to become shorter and to finally disappear, having been drawn within the body proper of the young Ascidian. The entire tail, with the whole of the notochord, musculature, and caudal portion of nerve- tube, becomes thus retracted and invaginated into the posterior region of the body-cavity, wnere it forms a coiled amorphous mass, which goes through a gradual series of histolytic changes, and is finally absorbed by being dissolved in the fluid of the body-cavity (Fig. 105 B). By the time the tail has been completely drawn up into the body, the organ of fixation or snout, as we have called it above, becomes drawn out into a long probosciform structure in a line with the long axis of the body. Its cavity is no longer completely filled with mesoderm-cells as it was at first (Fig. 105 4), but it has become so volu- minous that its contained cells are loosely scattered about (Fig. 105 &). In the concluding chapter we shall endeav- our to show, what has been already implied, namely, that the organ of fixation is seen to the best possible advantage from a morphological point of view in the species now under consideration, viz. Czona intestinalis, and that it is homologous with the preoral lobe (snout) of Amphioxus, including under that term both the przoral body-cavity and the preoral pit, and further that it is homologous with the proboscis of Balanoglossus. ANATOMY AND DEVELOPMENT. 223 At the stage shown in Fig. 105 A, the lumen of the alimentary canal is extremely reduced, and in many places, as in the region of the endostyle, ¢, its opposite walls are in actual apposition, so that the lumen at these points is almost obliterated. This temporary reduction of the lumen of the alimentary canal is due to the narrow space into which it has to be compressed, combined above all with the relatively enor- mous size of the cerebral vesicle, which exercises a great pressure on the subjacent dorsal wall of the branchial sac. It may be added that the larva of Ciona does not take in food independently until after fixation. Reopening of Neuropore; Degeneration of Cerebral Vesicle ; Formation of Definitive Ganglion. One of the most obvious features of the metamorphosis is the rapid expansion undergone by the enteric and body cavities and the no less rapid degeneration of the cerebral vesicle. This expansion, by relieving the crowded char- acter of the various parts, facilitates greatly the study of the changes which take place in the internal organisation. The neuropore, which we have described above as having closed up at an early period, now reopens again and places the neural tube — that is to say, as much of it as remains after the atrophy of the tail—in open communication with the base of the buccal tube (Fig. 105 B, 7). The spacious cavity of the cerebral vesicle has vanished, and its walls have undergone disintegration, and, except for a portion of the dorsal wall which becomes converted into another channel, are now represented by a mass ot histolytic residua filling the original cavity of the vesicle and lying below the anterior portion of the nerve-tube. 224 THE ASCIDIANS. This remnant of the cerebral vesicle of the larva with its sense-organs becomes eventually absorbed, and the eye and otolith may often be found floating about the body-cavity with the ordinary mesenchyme-cells, and occasionally they can be seen actually passing through the heart. The anterior portion of the nerve-tube itself, which now opens into the base of the buccal tube or stomodceum,®* is derived from a portion of the dorsal wall of the original cerebral vesicle which was constricted off from the latter in the form of a narrow tube slightly to the left of the mid- dorsal line (Fig. 105 B, x). Subsequently the cells forming the dorsal wall of this portion of the nerve-tube proliferate and form a solid thickening which becomes the definitive ganglion of the adult (Figs. 105 C, 106, and 107, g). The lumen of the nerve-tube behind the region of the definitive ganglion finally becomes obliterated by the mutual approximation of its constituent cells, and that portion of the primitive nerve-tube which in the larva lay between the cerebral vesicle and the root of the tail is thus represented in the adult by a solid “cordon ganglionnaire viscéral” (van Beneden and Julin) which starts from the posterior end of the adult cerebral ganglion, and, proceed- ing along the dorsal side of the pharynx above the dorsal lamina, becomes lost among the viscera. (Cf. Figs. 96, 105, and 107.) Below and in front of the definitive ganglion, which finally becomes quite separate from the dorsal wall of the neural tube, the lumen of the latter persists and becomes * According to renewed observations on Ciona, I find that the neuropore reopens into the buccal tube precisely in the line of junction of the stomo- dceum with the wall of the branchial sac, so that its upper margin is continu- ous with the (ectodermic) stomodceal epithelium, and its lower margin with the (endodermic) branchial epithelium. (See below, V.) ANATOMY AND DEVELOPMENT. 225 by subsequent extension the lumen of the subneural gland and its duct. Thus the anterior portion of the primitive neural tube, having become constricted off from the cerebral vesicle of the larva, and having given rise by proliferation from its dorsal wall to the definitive ganglion, becomes bodily converted into that structure which we shall call, in agree- ment with JuLIN, the Zypophyszs. The opening of the latter into the base of the buccal tube becomes the dorsal tubercle of the adult. Finally, at a much later stage, the glandular portion of the hypophy- sis arises by proliferation of spongy tissue from the ven- tral wall of that portion of the neuro-hypophystal tube which lies immediately be- low the ganglion. A section through the cerebral vesicle of a larva of Distaplia, a colony-build- ing Ascidian, showing the hypophysis in process of Fig. 106. — Frontal section through being constricted off from the vesicle, is given in Fig. 106. In this genus the con- dition of things generally is very different from obtains in Ciona, but it is introduced the what to show cerebral vesicle of a larva of Diéstapla magnilarva, to show the origin of the ganglion and hypophysis. (After HJORT; combination of two figures.) In the larva of Distaplia, the hypophy- sis opens into the branchial sac _be- hind the stomodceum. cv. Cerebral vesicle. ec. Ectoderm. en. Endoderm. jg. Ganglion. Ay. Hy- pophysis (neuro-hypophysial tube). essential similarity in the mode of origin of the hypophy- sis in this form, as observed by Dr. JoHan Hyorrt. In Distaplia, as is also the case to a less extent in Clavelina, the ganglion begins to develop from the wall 226 THE ASCIDIANS. of the neuro-hypophysial tube while the latter is still in connexion with, and therefore before the atrophy of, the cerebral vesicle, thus indicating a hastening in the devel- opment as compared with Czowa. The convexity caused in the dorsal wall of the branchial sac by the pressure of the cerebral vesicle persists as the anterior portion of the dorsal lamina, and in many or most simple Ascidians becomes grooved, forming the epzbran- chial groove of Juin (Fig. 97). At present it is merely a ridge, the epzbranchial ridge. In Fig. 105 C the proximal (oral) end of the endostyle, é, is seen to be connected with the epibranchial ridge by the peripharyngeal band, which we have already described in the adult. It apparently arises 77 szfw by simple spe- cialisation of the cells forming the epithelial wall of the pharynx at this point. Primary Topographical Relations and Change of Axts. It must be especially noted that the long axis of the young Ciona for some time after fixation is identical with that of the tailed larva, and therefore the primary topo- graphical relations of the various parts are maintained at the stage shown in Fig. 105 5, and we can accordingly make use of this stage in which different structures are much clearer than in the free-swimming larva for the purpose of describing the primary topography, which is of the utmost importance when it is desired to institute a comparison with Amphioxus. Since, as we have seen, the details of the embryogenetic processes differ in many respects widely from what occurs in Amphioxus, we are inevitably compelled to rely to a very large extent on topographical relations in order to estimate the homology of this or that structure in the ANATOMY AND DEVELOPMENT. 227 Ascidians and in Amphioxus. Fortunately there is one structure as to whose complete homology, in the Urochorda (Tunicata), on the one hand, and the Cephalochorda, on the other, no one entertains a doubt, and that is the ezdostyle. We thus have in the endostyle a firm basis upon which to ground our deductions. In the larva and in the young Ascidian before the primary long axis has been disturbed in the way which we shall shortly describe, the endostyle is the most anterior endodermic structure in the body, and lies dorso-ventrally at right angles to the long axis of the body (Fig. 105 A and B, e). As described above in the larvee of Amphioxus, particu- larly in the younger larvee (see Figs. 64 and 73), the endo- style, though lying asymmetrically on the right side, being involved in the general asymmetry of the larva, is quite anterior in position, in front of all the gill-slits and partly in front of, though also partly opposite, the mouth (on account of its asymmetry), and almost at right angles (see especially Fig. 64) to the long axis of the body. As there is only a short stretch of simple endoderm in front of the endostyle in the larva of Amphioxus, we may describe it as the most anterior differentiated endodermic structure in the larva, thus corresponding with remarkable precision to the condition described above in the larval and newly fixed Ascidian. In the middle of the wall of the branchial sac in Fig. 105 B are seen, somewhat in front of and below the atrial aperture, a, of this side, two lens-shaped structures whose slightly concave sides face each other. These are the borders of the two first-formed primary branchial stigmata or gill-clefts. Their actual openings into the atrial chamber are at present so small that they can hardly be seen in 228 THE ASCIDIANS. surface-view, but they are situated at the inner or con- cave sides of the two thickenings. Fig. 107.— Young Crona intestinalis after the completion of the change of axis; from the left side. (After WILLEY.) I, /V. Primary stigmata. a. Anus, situated immediately below the left atrial aperture. end. Endostyle. # Organ of fixation. .g. Ganglion. Ay. Hypophysis. zz¢. Intestine. /.a¢, Left atrial aperture. é. Longitudinal muscle. mm. Mouth. oes, (Esophagus. 7.0. Peripharyngeal band. py. Pyloric gland. st, Stomach. ¢. Coronary tentacles. v.z. Visceral nerve (cordon ganglion- naire viscéral). On either side of the latter can be seen the ordinary cavity of the pharynx proceeding to- wards the oesophagus. At a later stage the openings of the two first-formed stigmata become distinctly visi- ble(Fig. 105 C). Mean- while a change of axis is taking place in the body of the Ascidian. young During the extraor- dinary change of axis which to describe the probos- lobe (snout, organ of fixa- we are about ciform przeoral tion) remains. station- ary, and the rest of the body actually rotates through an angle of go degrees, using the or- gan of fixation as a pivot about which it In Fig. 105 C rotation turns. the takes place very gradu- which ally is only half performed; while in Fig. 107 it is complete. The method of growth by which this rotation takes place ANATOMY AND DEVELOPMENT. 229 is of a very singular character, and it is difficult to define it in precise terms. In this way then the endostyle (and branchial sac generally) comes to be placed at right angles to its primary position. Since in Amphioxus the endostyle altered its primary axis by a process of independent growth while the long axis of the pharynx was constant throughout the develop- ment, we find that here again, as in so many previous instances, the details by which similar end-results are arrived at are widely dissimilar. This complete change of axis by which the przoral lobe (organ of fixation) becomes placed at the posterior extrem- ity of the body can only be regarded as a cenogenetic feature.* It is therefore chiefly to the primary relations which the various structures bear to one another, before the change of axis, that we must turn for purposes of comparison. If we do this, we find that the following sequence of organs obtains as well in the larva of Amphioxus as in the newly fixed larva of Ciona; namely: 1, praeoral lobe; 2, endo- style; 3, mouth; 4, gill-clefts. Formation of Additional Branchial Stigmata. After the change of axis of the body, the long axes of the stigmata lie transversely. In their further growth they go on elongating in the same (transverse) direction, and after they have attained a certain size their ventral ends — that is to say, the ends nearest the endostyle— bend round towards each other, and from each of the two first- *Tt goes without saying that the primary long axis of the Ascidian larva is homologous with the long axis of Amphioxus. 230 THE ASCIDIANS. formed stigmata a minute portion becomes gradually con- stricted or nipped off. Thus between and cut off from the two original stigmata, there come to lie two intermediate stigmata of much smaller size. (Cf. Fig. 107.) In this way, then, in Ciona, we arrive at the stage with four branchial stigmata on each side of the pharynx. For convenience we shall refer to these by the Roman nu- merals, I., II., III., and IV. It is a remarkable fact that II. and III. do not arise by new perforations, but are cut off from I. and IV. respectively. On account of the close relations which the two first- formed stigmata, I. and IV., bear to one another during the production of the intermediate stigmata, their ventral extremities coming into contact and apparently some- times fusing together so that II. and III. might almost be described as a joint production of I. and IV. rather than as entirely independent offshoots, one is forced to the conclusion that the two first-formed stigmata themselves, though they actually appear simultaneously as separate perforations, in reality represent the two halves of a single primitive gill-slit divided into two by a tongue- bar. If, moreover, we examine the exact origin of these two stigmata (I. and IV.) by means of transverse and horizontal sections, we may become convinced that such is indeed the case; namely, that they represent the two halves of a primitive gill-slit which, on account of the precocious formation of the tongue-bar between them, become perforated separately. For the formation of any two or more consecutive gill- slits, we usually expect to find separate endodermic pockets or pouches of greater or less depth growing out towards the ectoderm. (Cf. Figs. 72 and 92.) We ought to find something analogous to this in Ciona ANATOMY AND DEVELOPMENT. 231 if the two first-formed stigmata had the value of indepen- dent gill-slits. Instead, however, of anything approaching to two endo- dermic outgrowths, we find at the base of the atrial invo- lution a single endodermic ingrowth making its appearance (Fig. 108). The angles made by this ingrowth with the neighbour- ing wall of the branchial sac remain in contact with the floor of the atrium, then fuse with it, and finally become A B ee cc at : : i aoe De Go a CL fp an 'ogZ VM | (LILO yr f O Up i ZEA ey) Zh CLL TT 7s Y : rif tb Gg oO) gs 7b Fig. 108. — Diagrams illustrating the mode of origin of the two first-formed branchial stigmata in Ciona. (After WILLEY.) at, Atrial involution. ec. Ectoderm. ev. Endoderm. gis. Stigmata. 744. Tongue-bar. perforated (Fig. 108). This is the way in which the stig- mata, I. and IV., arise, and it is difficult, if not impossible, to interpret the above-mentioned endodermic ingrowth otherwise than as a precocious tonguc-bar. Even in Amphioxus it was seen how the tongue-bars of the secondary slits arose relatively much earlier than those of the primary slits. If they arose still a trifle earlier, we should have the two halves of each slit becom- ing separately perforated, just as it happens in Ciona. In a species of Balanoglossus an analogous precocious 232 THE ASCIDIANS. formation of tongue-bars, before the perforation of the slits, has been described by Professor T. H. Morcan. From what has been said above, we conclude that the first four pairs of primary branchial stigmata of Ciona (and this probably applies equally to many species of Phallusia) represent and are derivatives of one pair of primitive, ancestral gill-slits. After a comparatively long interval, during which the intermediate stigmata, II. and III., increase in length transversely, two more stigmata, V. and VI., arise at inter- vals, one after the other, by sepa- rate perforations behind those already formed (Fig. 109). On account of the independent origin of V. and VI., it might be supposed that they would have the morphological value of dis- tinct gill-slits, and that we had before us three pairs of ancestral Fig. 109. — Primary branchial gill-slits represented by six pairs eee em of primary branchial stigmata. For this interpretation to hold good, we should expect to find that in other forms in which six primary branchial stigmata were produced, their origin was either the same or reducible to the same type as that of the branchial stigmata of Ciona. This, however, is not the case, since I have found that in Moleula manhattensts,* a simple Ascidian which occurs in great numbers at New Bedford, Mass., the six primary stigmata, corresponding precisely to those in * My observations on the development of J/olgula manhattensis were made at the Marine Biological Laboratory, at Woods Holl, Mass., in the summer of 1893. ANATOMY AND DEVELOPMENT. 233 Ciona, have a somewhat different mode of origin. The two first-formed stigmata (=I. and IV. in Ciona) appear simultaneously as in Ciona. Then after growing to a cer- tain size, they curve round at their ventral ends, not in opposite directions so as to meet each other as they do in Ciona, but in the same direction (Fig. 110). The recurved ends then become constricted off from the parent stig- mata. Later on, a fifth gill-opening arises behind the first four stigmata by independent perforation, and after Fig. 110. — Diagram illustrating the mode of origin of the six primary bran- chial stigmata of Afoleula manhattensis. The numbers are placed at the ventral ends of the slits. The figure is a combination of several hitherto unpublished drawings of different stages in the development. /, ///, and Varose by separate perforation. attaining a certain size, it, in its turn, curves round at its ventral end, and eventually the sixth stigmatic opening is constricted off from the fifth. Since the first six primary stigmata have such different origins in two different species, it is obvious that in attempting to make a comparison with Amphioxus we can only use the two first-formed stigmata, because they agree in the above-mentioned species, and in many others in 234 THE ASCIDIANS. arising simultaneously, and in representing, in all proba- bility, the two halves of a primitive gill-slit, cut in two by a tongue-bar. The stigmata which are added to these must, therefore, be regarded as secondary modifications, hardly comparable to the successive formation of new gill-slits in Amphioxus. In the Ascidians, therefore, we can only detect the representatives of one pair of primitive gill-slits, and there is every reason for supposing them to be homologous with the first pair of gill-slits in Amphioxus as defined above. The six primary stigmata of each side give rise, by re- peated subdivision, to the innumerable stigmata of the adult, both in Ciona and Molgula. The following de- scription, however, applies more particularly to Ciona. In the first place, the primary stigmata grow to a sur- prising transverse length, and then commence to divide into two equal portions by small tongue-like projections, which grow across the aperture indifferently from the anterior or posterior walls of the respective stigmata, and, fusing with the opposite wall, divide the transversely elongated slit into two completely separated halves. Then each of the latter divides again in the same manner, and so the process of subdivision of existing stigmata goes on. In this way six transverse rows of stigmata arise. These may be distinguished as secondary stigmata, since they arise by division from the primary. Gradually, by a peculiar process of growth, the long axes of the secondary stigmata change their direction, and instead of lying transversely they become directed antero- posteriorly. This is their definitive position, and the stigmata now go on rapidly dividing again, and the num- ber of transverse rows of stigmata is in this way doubled, trebled, quadrupled, etc., and we thus arrive at the adult ANATOMY AND DEVELOPMENT. 235 condition. Out of the multitude of stigmata which are present in the adult Ciona only four arise by independent perforation; namely, the primary stigmata I. and IV. (which we regard as the two halves of a primitively single slit) and V. and VI. First Appearance of Musculature. By the time the change of axis of the entire body of the young Ciona has been effected the musculature characteristic of the adult begins to put in an appear- ance. In Fig. 107 circular sphincter muscles are present round the buccal and atrial apertures. The latter are still paired, but are carried by differential growth dorsalwards at a later stage, and finally coalesce together in the dorsal middle line to produce the single atrial aperture of the adult. One strand of the longitudinal muscles of the later muscular mantle is likewise to be seen in Fig. 107. It tends to branch dichotomously. Posteriorly it is inserted on the inner surface of the organ of fixation near the point where it joins on to the body. Later new muscle-bands arise similar to the first, and become distributed over the body-wall in a spreading fan-like fashion, but posteriorly they are all inserted in the same region of the organ of fixation. Alimentary Canal and Pyloric Gland. The course of the alimentary canal can be gathered so plainly from the accompanying figures (Figs. 105 and 107) that it hardly needs a verbal description. From the posterior dorsal corner of the branchial sac the cesophagus leads into the wide stomach, and from the latter, again, the intestine, which often possesses a strangulated appear- 236 THE ASCIDIANS. ance, doubles up obliquely forwards to the left atrial chamber, into which it opens by the anus (Fig. 107). In the angle made by the outgoing intestine with the stomach, a blind diverticulum arises. It is at first a sim- ple ccecum, but soon begins to branch (Fig. 105 C), and finally forms an arborescent growth embracing the in- testine (Fig. 107). This is the so-called pyloric gland, and it is probably homologous with the hepatic cecum of Amphioxus. Appendicularia. It is generally agreed among those who have a voice in the matter, that most of the pelagic Ascidians (Salpa, Doliolum, Pyrosoma) are highly modified forms, spe- cially adapted to a pelagic life, one of the results of which is that their repro- duction is marked by a complicated alternation of generations. It would, therefore, not assist us in our comparison with Amphioxus to describe these types. There is, however, one family of pelagic Ascidians, the Appendicularia, with re- spect to which there are two widely different opinions. Fig. 111.— Appendicularia (Fritil- The Appendiculariz are laria) furcata, from the ventral surface. : ¢ 3 (After LANKESTER.) pelagic, free-swimming As- a. Anus. g/. Unicellular glands. vs. Gill-slits. 4. Dorsal hood-like fold of integument. . Mouth. 4. Tail. tion is so far similar to the cidians, whose adult condi- ANATOMY AND DEVELOPMENT. 237 larval condition of the fixed Ascidians, that they retain the tail as their organ of locomotion throughout life (Fig. 111). The tail is inserted in the middle of the ventral surface of the body proper, and is obviously a mere appendage of the latter. The mouth is terminal or sub-terminal. There is a sin- gle pair of branchial stigmata, which open into a pair of tubular atrial cavities, whose separate external apertures are seen in front, on the ventral surface behind the mouth. The alimentary canal is U-shaped, and the anus opens on the ventral surface to the right of the middle line, some- times behind and some- times (according to the species) in front of the stigmata (Figs. 112). The endostyle is always quite anterior III, in position, and some- times, as in Fig. 112, removed by a consider- ed weg ig. ae Fig. 112. — Diagram of the organisation of able interval from the stigmata. In the posterior ex- tremity of the body are placed the gonads, male and female, in close proximity to one another, the testis in front and the ovary behind. The heart, as a species of Appendicularia, from the right side. (After HERDMAN.) a. Anus; the index line was accidentally drawn about 4% of an inch in front of the anus. 4.5. Branchial sac. ch. Notochord. e. Endostyle. g. Ganglion, from which the nerve-cord proceeds backwards to the tail, passing to the right of the alimentary canal. .g.s, Gill-slit. 4, Heart. cnt. Intestine. 2. Mouth. 2.c. Nerve-cord, with ganglionic enlargements in the tail. o¢. Otocyst; beneath which the hypophysis opens into the branchial sac. ov. Ovary. .d. Peripharyngeal band. s¢,Stomach. /e. Testis. described by LANKESTER, is a unique example of a func- tional organ reduced to the lowest possible level of histo- logical structure. It consists simply of two cells placed 238 THE ASCIDIANS. opposite one another and connected together by contractile protoplasmic threads, which keep up a pulsating motion. The tail is, as might be expected, more elaborately or- ganised than that of the Ascidian larva. The dorsal nerve- cord is solid, and proceeds backwards from the ganglion, passing to the 7zg/¢ of the alimentary canal until it reaches the tail, along which it is continued, lying to the /eft of the notochord; it possesses ganglionic enlargements at intervals in the tail, from which nerves pass out. The caudal musculature also shows somewhat doubtful traces of being segmented in correspondence with the ganglionic swellings of the nerve-cord. In connexion with the cerebral ganglion there is a sense-organ in the form of an otocyst, with an enclosed otolith, and below this a ciliated pit opens into the ante- rior region of the branchial sac, corresponding to the hypophysis, or sub-neural organ, of the fixed Ascidians. According to one view, Appendicularia is the living rep- resentative of the free-swimming ancestor of the Ascidians. According to the other view, it is less primitive than the fixed Ascidians, and was derived from the latter by the gradual increase, from generation to generation, of the du- ration of the pelagic existence of the larvz, until they ceased to metamorphose, and so retained the larval struct- ure throughout life, becoming at the same time sexually mature.® These two views are, of course, antagonistic, and the former of them is held by a number of well-known author- ities. As we are ignorant of the development of Appen- dicularia, it is impossible to decide definitely between them. With the facts which are at our disposal, however, the second view — namely, that the Appendiculariz represent Ascidian larvee which have become secondarily adapted to ANATOMY AND DEVELOPMENT. 239 a pelagic life, and have acquired the faculty of attaining sexual maturity — would be more in harmony with what we know of the relation of Amphioxus to the Ascidians. And it would seem that this affinity can be better demon- strated through the comparison of Amphioxus, both adult and larva, with a fixed Ascidian like Ciona than with Appendicularia.? On the latter view, therefore, the so-called metamerism of the tail of Appendicularia, on which so much stress has been laid, would be simply a secondary elaboration of the tail for the purpose of serving as a permanent locomotor organ. The dorsal nerve-cord of Appendicularia was regarded by For as a simple peripheral nerve. We have described above how a portion of the primitive nerve-tube in Ciona and other Ascidians becomes reduced to a solid nerve. It would be of the greatest interest to discover the mode of origin of this nerve-cord in Appendicularia. Abbreviated Ontogeny of Clavelina. In order to demonstrate clearly the relatively primitive character of the development of Ciona it is sufficient to enumerate a few facts drawn from the development of Clavelina as described by Dr. OswaLp SEELIGER. As mentioned above, Clavelina is a near relative of Ciona, and in the adult condition resembles it very closely in many respects. The development of Clavelina was formerly regarded as being of a primitive character, but is in reality, more especially in the later stages, abbreviated and hastened to a remarkable extent. Like Ciona it possesses in the adult numerous trans- verse rows of stigmata. Each opening, however, arises by 240 THE ASCIDIANS. an independent perforation, so that all those preliminary ontogenetic processes which precede the establishment of the transverse rows of stigmata in Ciona are dropped out of the development of Clavelina.* In Clavelina, again, the change of axis of the body proper occurs in the unhatched larva; so does the fusion of the two atrial apertures to form the dorsal cloacal siphon. The longitudinal muscles of the body proper -commence to appear in the free-swimming larva, while the caudal muscles are enjoying their highest functional activity. The vacuolisation of the notochord does not proceed so far as in Ciona, since the cells are never actu- ally removed from the centre of the notochord, but remain as thin discs stretching across the latter, so that the vacuolar spaces do not become continuous. The behaviour of the organ of fixation in the larva of Clavelina is such that it could hardly be recognised as a preeoral lobe except in the light of Ciona., NOTES. I. (p. 183.) The test or cellulose mantle of the Ascidians con- tains great numbers of cells of various kinds. These were formerly supposed to be derived from the subjacent ectoderm of the body- wall. Kowatevsky has recently shown, however, that the cells of the outer (cellulose) mantle of the Ascidians are derived from wandering mesenchyme-cells which wander from the body-cavity through the ectoderm (either defween the ectodermic cells or actually passing #hvowgh the individual cells) into the mantle. * A mode of formation of the branchial stigmata, intermediate between that of Clavelina and Ciona or Molgula, has been described by GARSTANG for Botryllus. In this genus, the primary branchial stigmata all arise by in- dependent perforations, and then later become divided up into the transverse rows of stigmata. (W. GARSTANG. On the development of the stigmata in Ascidians. Proc. Roy. Soc., Vol. LI. 1892.) NOTES. 241 2. (p. 211.) In Clavefina the atrial involutions do not merely arise as minute circular invaginations of the ectoderm, but at first they appear as short, though quite distinct, longitudinal grooves. Compare also the remarkable longitudinal atrial tubes of Pyrosoma. 3. (p. 238.) There is another possible way of interpreting the structure and systematic position of Appendicularia which may perhaps be nearer the truth than either of the views mentioned in the text. It is not absolutely necessary to suppose that the ancestors of Appendicularia were fixed Ascidians; but both Appendicularia and the fixed Ascidians may have descended from a common free-swimming stock, and have undergone certain modifications in common, such as loss of true vascular system and cceelom. Then, while the Ascidians proper became adapted to a sessile existence, Appendicularia may be supposed to have gone to the opposite extreme, and have become adapted to an absolutely pelagic existence. In becoming adapted to such a purely pelagic or oceanic environment as that of Appendicularia, it is eminently conceivable that an animal would have to undergo as radical a modification of structure as it would in becoming adapted to a sessile existence. (Compare Sa/pa, Doliolum, etc.) V. THE PROTOCHORDATA IN THEIR RELATION TO THE PROBLEM OF VERTEBRATE DE- SCENT. “ Den Schliissel richtigen Verstandnisses gibt nicht das Hinetnpressen neuer Thatsachen tn eine alte Schablone, sondern das Aufsuchen des genetiscthen Zusammenhangs der Erscheinungen.” —WEISMANN. BALANOGLOSSUS. External Features. Or the free-living protochordates, the lowest type of organisation is undoubtedly presented by the Exteropneusta (Hemichorda), the group to which Balanoglossus belongs. Balanoglossus is a remarkable worm-like creature which lives buried in the sand or mud of the sea-shore. By means of numerous unicellular integumentary glands which are distributed over the surface of the body, it secretes a mucous substance to which particles of sand adhere, and so makes for itself tubes of sand in which it lives at about the level of the low tide-mark. It possesses such a characteristic external form and odour (like iodoform) as to render it peculiarly easy of recognition. In front there is a long and extremely sensitive proboscis which is capable of great contraction and extension, and is, in the living animal, of a brilliant yellow or orange colour. Behind the proboscis follows a well-marked collar-region, 242 BALANOGLOSSUS. 243 consisting externally of a collar-like expansion of the integument, with free anterior and posterior margins over- lapping the base of the proboscis in front and the anterior portion of the gz//-s/zts behind. In the ventral middle line, at the base of the proboscis and concealed by the collar, is situated the mouth (Fig. 113). Following behind the collar is the region of the trunk or body proper, which, in the adult of some species, reaches a relatively enormous length, even extending to Fig. 113. — Larva of Balanoglossus Kowalevskii, with five pairs of gill-slits ; from the right side. (After BATESON.) a. Anus. a.~, Temporary pedicle of attachment. ¢. Collar. ck. Notochord. g-5. Gill-slits. 2. Mouth. gr. Proboscis. two or three feet. The ectodermal covering of the body consists in general of ciliated cells, among which are scat- tered unicellular mucous glands ; the cilia, however, appear to be more prominent on the proboscis than elsewhere. In the region of the trunk, which immediately follows upon the collar region, there are a great number of paired 244. THE PROTOCHORDATA. openings on the dorsal side of the body, placing the anterior portion of the digestive tract in communication with the outer world. Theseare the gz//-s/z¢s, and they are arranged strictly in consecutive or metameric pairs to the number of upwards of fifty in the adult. In their structure, and more especially in the possession of tongue-bars, they bear a remarkable resemblance to the gill-slits of Amphioxus. This is particularly striking in young individuals. As the adult form is approached in the development, the bulk of the gill-slits sinks below the surface, only opening at the latter by small slit-like pores, and thus their true character is obscured in a superficial view. Projecting into the interior of the proboscis is a rod-like structure which arises as an outgrowth from the alimentary canal dorsal to the mouth. The lumen of this endodermic diverticulum becomes narrowed down and, in fact, partially obliterated, while the cells constituting its walls give rise to a spongy vacuolar tissue which strongly resembles the notochordal tissue of Amphioxus and the higher Verte- brates. On account of its dorsal position above the mouth, its endodermic origin, and the vacuolisation of its cells, this structure was identified by BATESON in 1885 as the zofo- chord. Nervous System and Gonads. The nervous system of Balanoglossus presents many features of the utmost interest and suggestiveness. It consists essentially of an ectodermal network of nerve-fibres forming the inner layer of the skin (ectoderm) all over the body. In this primitive nervous sheath, which envelops the whole body, there are certain definite local thickenings. Two of these thickenings occur respectively along the whole length of the dorsal and ventral middle lines in the trunk-region, thus producing the dorsal and ventral median BALANOGLOSSUS. 245 longitudinal nerve-cords. In the region of the collar the dorsal nerve-cord becomes entirely separated from the ectoderm, and this portion of it contains, at least in young individuals, a central canal which, from its origin and relations, was shown by BaTeson, and more recently by MorGan, to be homologous with the central canal of the vertebrate spinal cord. Anteriorly the dorsal nerve-cord becomes continuous with a specially dense tract of the general nerve-plexus at the inner posterior surface of the oh be* dn ae be’ H je ein is a b ee ov bc* Com Fig. 114.— Diagram of the organisation of Balanoglossus, from the left side. (From a drawing kindly lent by Professor T. H. MORGAN.) al, Alimentary canal. 6c1. Coelom of proboscis (anterior or przeoral body- cavity). c2. Ccelom of collar. 4c3. Ccelom of trunk. 4.v. Blood-vessel, proceed- ing from the so-called heart (which lies at base of proboscis above the noto- chord) to the ventral blood-vessel. ck. Notochord. com. Commissure, between dorsal and ventral nerve-cords. dm. Dorsal nerve-cord, separated from the integu- ment in the collar-region. d.d.v. Dorsal blood-vessel. .g/, Proboscis-gland; modified coelomic epithelium surrounding heart and front end of notochord. m. Mouth. #.v. Pulsating vesicle, lying inside the ‘‘ heart.” v.d.v. Ventral blood- vessel. v.z. Ventral nerve-cord. proboscis (Fig. 114). This proboscidian plexus thins out somewhat towards the anterior extremity, but nevertheless forms a complete nerve-sheath for the proboscis and indi- cates the sensitive character of the latter (Fig. 115). The ventral nerve-cord does not extend into the region of the collar, but from the point where the collar joins on to the trunk the ventral cord is connected with the dorsal nerve-cord by a commissure-like thickening of the integu- mentary plexus, which passes in the skin on each side round the hinder end of the collar-region (Fig. 114). 246 THE PROTOCHORDATA. The genital organs, testes or ovaries, accord- ing to the sex of the individual, occur as a paired metameric series of pouch-like bodies or gonadic sacs which ex- tend backwards far be- yond the region of the gill-slits. The gonadic sacs are suspended in the Fig. 115. — Diagrammatic transverse sec- tion through hinder region of proboscis of Balanoglossus. (From a drawing kindly lent by Professor T. H. MORGAN.) D. Dorsal. JV. Ventral. 4c1. Proboscis- cavity, almost filled up by mesenchymatous body-cavity by solid cords attached to the dorsal which _ be- come perforated in the integument, and muscular tissue,* proliferated from the original ccelomic epithelial layer (indicated by the black line below the ectoderm). pv. Pulsating vesicle. 4. Heart. ch, Noto- chord. 2.5, Integumentary nerve-plexus. spawning season to ad- mit of the expulsion of the reproductive elements. AMetamerism. Although there is no muscular metamerism in Balano- glossus, yet we have seen that other organs (gill-slits and gonads) are arranged metamerically. And in point of fact, among those Invertebrates which are not included under the phylum of the Articulata, if there is one pecu- liarity of organisation more sporadic in its occurrence than another, it is metamerism. It may affect the most differ- ent organs of the body either collectively or individually, and nothing is more patent than the fact that the meta- meric repetition of parts has arisen independently over and over again in different groups of animals. * This tissue is not represented in Figs. 114 and 116, although it is present throughout the body-cavity. BALANOGLOSSUS. 247 Far from assuming as a self-evident fact that the extreme metamerism of the Annelids and Arthropods is genetically identical with that of the Vertebrates, we have every reason to suppose that it has been elaborated entirely independently in the two cases, and that the apparent simi- larity is due, as already intimated, to a parallel evolution. Body-cavities ; Proboscis-pore ; Collar-pores. the into which Corresponding to three regions the body of Balanoglossus is divided, — namely, probos- cis, collar, and trunk, —the body-cavity is divided up into three systems of cavities. These are (a) the anterior body-cavity or cavity of the proboscis, (8) a pair of collar- cavities, and (y) a pair of body-cavities which form the unsegmented coelom of the trunk (Figs. 114, 115). These cavities arise essen- tially as pouches from the archenteron (Fig. 117), al- though their actual develop- ment differs considerably in different species (MoRGAN). The placed proboscis-cavity is in communication with the exterior by an open- ing through the posterior QOe2Oe Fig. 116. — Diagram of the organisa- tion of Balanoglossus, from the dorsal side. (From a drawing kindly lent by Professor T. H. MORGAN.) c.p. Collar-pores. Gonads. £5. Gill-slits; the dark lines converging be- hind indicate the superficial portions of the gill-slits; below the surface are seen the free ends of the tongue-bars. //. Proboscis-pore, Other letters as above. 20 oes 248 THE PROTOCHORDATA. wall of the proboscis known as the proboscis-pore. In &. Kowalevskii this pore lies asymmetrically to the left of the dorsal middle line (Fig. 115), while in B. Kzupffert a corresponding opening occurs to the right of the middle Fig. 117. — Diagrammatic horizontal section through an embryo of Balanoglos- sus (type of the direct development), to show the origin of the body-cavities as archenteric pouches. (After BATESON.) ap. Tuft of cilia at the apical pole (indication of an apical plate). cl. Probos- cis-cavity. 4c2. Collar-cavities. c038. Trunk- cavities. cé, Circular band of cilia. line, so that in this species there are two proboscis- pores constituting a sym- metrical pair. The left proboscis-pore of Balanoglossus is obvi- ously to be compared with the przoral pit of Amphi- OXuSs. The collar-cavities also open to the exterior by pores, one on each side underneath the dorsal pos- terior free fold of the collar, and on a level with the opening of the first gill-slit. These are the funnel-shaped coflar-pores. SPENGEL states that water is taken in through the collar-pores into the cavity of the collar in order to swell the latter up, so that it may serve as an accessory organ of locomotion in so far as an alternate inflation and collapse of the collar would assist the animal in its slow burrowings in the sand. BALANOGLOSSUS. 249 Alimentary Canal. The mouth cannot be closed, as there is no sphincter muscle, and accordingly, as the animal progresses through the sand, it swallows a large quantity of the latter in which food-particles (unicellular organisms, etc.) may also be involved. As the sand passes through the intestine, it becomes enveloped in the mucous secretion of the intes- tinal epithelium, and is ejected through the anus in a cord of slime. The alimentary canal is a straight tube between mouth and anus. In its hinder portion it is usually sacculated, ze. provided with paired lateral saccular dilatations comparable to the so-called intestinal ceca of the Ne- mertine worms. (See below.) In the region of the pharynx the lumen of the alimentary canal is incompletely divided by lateral constrictions into two portions, an upper or branchial portion carrying the gill-slits, and a lower or digestive portion (Fig. 118). The latter was compared by GEGENBAUR* to the endo- style of the Ascidians, but it is probable that this com- Fig. 118.— Transverse section through the gill-region of Balanoglossus. (After SPENGEL.) al, Digestive portion of gut. dr. Branchial portion of gut. dc3, Third body-cavity (trunk ccelom) ; this is also nearly obliterated in the adult by the pro- liferation of mesenchyme or “ paren- chyme” from its walls. dz.c. Dorsal nerve-cord. 4.6.v. Dorsal blood-vessel. go. Gonad. g.s. Gill-slit. 44. Tongue- bar. v.d.v. Ventral blood-vessel. v.72.c. Ventral nerve-cord. parison, although a very natural and useful one at the time at which it was made, will not hold good, since there is * CARL GEGENBAUR, Elements of Comparative Anatomy. Translated by F. Jeffrey Bell. London, 1878. 250 THE PROTOCHORDATA. nothing in the structure or development of this part of the alimentary tract in Balanoglossus which will bear compari- son with the endostyle.* As indicated in the larve of Amphioxus and the Ascidians, it would seem that the endostyle first became evolved or differentiated at the anterior end of the pharynx, zz front of the gill-slits, in correlation with the dorsal position of the mouth. Development, the Tornaria Larva. The development of Balanoglossus Kowalevskit as made known to us by the admirable work of BaTEson is what is known asa strictly direct development, that is to say, the embryonic, larval, and adult stages follow one another by gradual transitions concomitantly with the simple progres- sive growth of the individual and without any striking metamorphosis. In other species of Balanoglossus the larval form is remarkably different from the adult, and becomes transformed into the latter by a very distinct metamorphosis. The extraordinary larval form here re- ferred to was discovered in 1848 by JOHANNES MULLER, who named it Zornarza, and regarded it, as did his succes- sors Kroun, ALEXANDER AGassiz, and Fritz MULLER, as the larva of an Echinoderm (Starfish). It was not until 1869 that its true character as the larva * A ciliated tract in the floor of the cesophagus of a Tornaria from the Pacific has recently been compared to the endostyle by W. E. Ritter. (On a New Balanoglossus Larva from the Coast of California and its Possession ofan Endostyle. Zool. Anz. XVII. 1894. pp. 24-30.) The comparison is at present somewhat doubtful. More recently GARSTANG has suggested that the endostyle is derived from the adoral ciliated band of the Echinoderm larva. (See Fig. 119.) The suggestion is an interesting one, but Garstang’s idea of the relations of the preoral lobe is very different to the one here set forth. (WALTER GARSTANG, Preliminary Note on a New Theory of the Phylogeny of the Chordata, Zool. Anz. XVII. pp. 122-125.) BALANOGLOSSUS. 251 of a species of Balanoglossus was demonstrated by Extras METSCHNIKOFF. Shortly afterwards, Metschnikoff’s dis- covery was confirmed and amplified by ALEXANDER AGASSIZ. The superficial likeness between Tornaria and such Echi- noderm larve as Bipinnaria or Auricularia is astonishing, and a renewed study of the detailed organisation of Tornaria, recently made by MorGan, appears to have established the fact, originally insisted upon by Metschni- koff, that this resemblance can only be accounted for on the ground of genetic affinity. In Figs. 119 and 120 two types of larvae, Tornaria and Auricularia, are shown side by side; and although unfortunately they are not figured from exactly the same aspect, yet it is obvious at a glance that, in spite of certain differences which will be enumerated below, they both belong to the same category of larval forms. A highly characteristic feature of these larve is the remarkable ectodermal ciliated band which constitutes a perfectly symmetrical but somewhat complicated undulat- ing seam round the body. The larvz are strictly pelagic, and swim about in the open sea by means of their cilia; but the latter, instead of being distributed evenly over the whole surface of the body, are concentrated in the region of the ciliated bands which are composed of thickened ectoderm. In Tornaria there are two ciliated bands, viz.: 1) the above-mentioned undulating seam which is usually known as the eircumoral or longitudinal ciliated band, and 2)a fostoral circular ciliated band. Only the former is present in Auricularia, and the absence of the circular band in this form constitutes one of the chief differences between the two larve. 252 THE PROTOCHORDATA. From a morphological point of view a more striking resemblance between the two larve than that furnished by the longitudinal ciliated bands exists in connexion with the anterior body-cavity or exteroce/, In the Echinoderm Figs. 119 and 120.— Auricularia, larva of Synapta (after SEMON); and Tornaria, larva of Balanoglossus. (After MORGAN.) a. Anus. ag. Apical plate. écl. Anterior body-cavity, communicating with exterior by the water-pore. 4c?, 6c3, Second and third body-cavities of Tornaria. c.o, Circular ciliated band of Tornaria. c¢.c. Contractile cord between apical plate and anterior body-cavity of Tornaria. gf. Gill-pouches. 4.c. Hydroceel of Auricularia (anterior body-cavity). /.c.6. Longitudinal (circumoral) ciliated band. Ze. Left enteroccel (body-cavity). %. Mouth. ». Lateral (paired) nerve-band of Auricularia. #.e. Right enteroccel. sf. Calcareous spicules. s¢. Stomach. wp. Water-pore. N.B.—In Auricularia, the margin of the mouth is surrounded by a ciliated band discovered by SEMON, and known as the adora/ ciliated band. The poste- rior, V-shaped portion of this band lies inside on the ventral floor of the larval cesophagus. larva this cavity arises as a median pouch of the archen- teron, and there is every reason to suppose that it has a similar origin in Tornaria, although this point has not yet BALANOGLOSSUS. 253 been determined. The primary anterior enteroccel in the Echinoderm larva is not quite the same as the correspond- ing cavity in Tornaria, since it contains also the elements of the general body-cavity. Apart from slight differences, the collar-cavities and general body-cavities arise essen- tially in the same way in Tornaria as they do in the case of the direct developing larva of Balanoglossus (see above).* In the Echinoderm larva, however, the paired body- cavities do not arise as independent archenteric pouches, but they become constricted off from the anterior entero- ceel. Making allowance for these deviations in the origin of the body-cavities, — deviations which are by no means fundamental, since in both cases the body-cavities are ultimately reducible to archenteric pouches, —it is an extremely striking fact that both in Tornaria and Auricu- laria the anterior enteroccel acquires an opening to the exterior on the dorsal surface to the left of the middle line. This opening is called the water-pore, since it forms the outlet (possibly both outlet and inlet) of the water-vascular system of the Echinoderm. In Tornaria it persists after the metamorphosis as the prodoscis-pore, which has been described above. The Larva of Asterias vulgaris; Water-pores and Preoral Lobe. In view of what was said above as to the occurrence of paired proboscis-pores in B. Kupfferi, it is interesting to note that sometimes there are two water-pores, a right and a left, in Echinoderm larvae. This has been observed by * As to the origin of the body-cavities in different species of Balanoglos- sus, MORGAN summarises his observations as follows: ‘They may arise as enteric diverticula, as endodermal proliferations, or even arise from mesenchy- matous beginnings.” (See Morcan. No. 125 bibliog.) 254 THE PROTOCHORDATA. Brooks and G. W. FIELD in the larve of a common star- fish, Astertas vulgaris. In this case the primary enteroccel becomes constricted off from the archenteron in the form of two equal pouches. The right and left enteroccelic sacs then take up a symmetrical position on each side of the larval cesophagus, and each sac next opens to the exterior by a water-pore. The pore in connexion with the right sac (Fig. 121) is, however, of a transitory, rudimentary character, and soon closes up, while the left pore per- sists as the definitive water- pore. As in Tornaria, so here, the cavity of the larval body generally, and of the preeoral region (preoral lobe) in particular, is the primary body-cavity blastoceel, and contains scattered mes- At a later or enchyme-cells. Fig. 121. — Young larva of Asterias vulgaris, from the dorsal side. (After G. W. FIELD.) pl. Preoral lobe. 4.c.6. Circumoral (longitudinal) ciliated band. oes, Gésoph- agus. ve. and Ze, Right and left en- teroccelic sacs, each opening by a “ water- pore” to the exterior. s¢. Stomach. iz. Aperture, leading from stomach into in- testine. stage in the larva of As- terias the right and left enteroccelic sacs, having in- creased greatly in length, meet one another in the region of the przoral lobe and fuse together, thus put- ting their two cavities into communication across the median line. The median portion of the enteroccel thus produced extends up into the przoral lobe, and so the primary blastocoelic cavity of the latter is replaced by a secondary ingrowth of the enteroceel (Fig. 122), Similarly with the metamorphosis of Tornaria, the anterior enteroccel, which is at first of very inconsid- BALANOGLOSSUS. 255 erable extent (Fig. 120), increases greatly in size, and assumes its definite position and proportions as the cavity of the przoral lobe (zc. proboscis), thus replacing the original blastoceelic space, while the water-pore remains as the proboscis-pore. As described in the previ- ous chapter, the cavity of the preoral lobe (fixing stolon) of the Ascidian tad- pole is of the nature of a blastoceel or primary body- cavity, containing loose mes- enchyme-cells, and it is Fig. 122. — Older larva (Bipinnaria) of slsterias valgaris, from the ventral tance to note that whether side. (After G. \W. FIELD.) i By a fusion of the two preoral loops the cavity of the preoral of the ciliated band across the apex of the therefore of great impor- preeoral lobe, followed by a separation in the transverse direction, the originally enterocel, the morphological single circumoral band (ef. Figs. rr9 and A i 121) has become divided into two bands, value of the structure itself preoral ciliated band .c.d. and a post- remains the same. oral longitudinal ciliated band /.c.4.. The posterior transverse portion of the proe- oral ciliated band has undergone a fusion Apical Plate of Tornaria, With the front end of the originally dis- tinct adoral band (ef. Fig. 119). ./. Prae- At the anterior end of oral lobe, into which the enteroccael has extended. mm. Mouth. x.e. and Ze. Right the body, or, in other words, and left enteroccelic cavities. s¢, Stomach. a, Anus, lobe is a Odlastoca/ or an at the apex of the praoral lobe, in Tornaria, there is an ectodermic thickening in which nerve-cells and nerve-fibres and a pair of simple eyes have become differentiated. This is the so-called apical plate, and it constitutes the central nervous system of the larva. It can be recognised for some time after the metamorphosis at the tip of the proboscis, but eventually disappears completely. A similar apical plate occurs in 256 THE PROTOCHORDATA. a great number of Invertebrate larvae, and is especially characteristic of the free-swimming larve (Trochophores, or Trochospheres) of Annelids and Molluscs. We shall return to this later. In Tornaria a single contractile cord passes from the apical plate to the anterior enteroccel. There is no apical plate in Auricularia, nor in most of the other Echinoderm larve; but there is reason to sup- pose that it has been secondarily lost, since a transitory ectodermal thickening at the apical pole can frequently be observed in the course of their development ; and, moreover, in what is probably the most primitive Echino- derm larva known (viz. the larva of the Crinoid, Azztedon), there is a well-developed apical plate. Metamorphosis of Tornaria. The metamorphosis of Tornaria, as originally described by Alexander Agassiz, takes place with relative sudden- ness. According to the more recent account of the meta- morphosis given by Morcan, a marked diminution in size occurs; the internal organs are drawn together in such a way that the larval cesophagus, with the gill-pouches (see Fig. 120), is drawn backwards into the body, and the anterior enteroccel, as already described, is carried for- wards into the preoral lobe. The longitudinal (circum- oral) ciliated band, which was the first to develop, is also the first to disappear, while the posterior circular band persists to a somewhat later stage. The Nemertines. It is thus evident that Balanoglossus, especially through its Tornaria larva, shows undoubted marks of affinity to acta es CAtciiai sce- Str te Raise OF mMuc OI the Uniceuwiar M1csu 258 THE PROTOCHORDATA. place from the tip backwards by the in-rolling of its walls. According to the graphic description of HuBRECHT, it is retracted ‘in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the tip.” When at rest within the body the proboscis lies freely within a hollow cylinder, the wall of which is thick and muscular, and constitutes the prodoscis-sheath (Fig. 123). Fig. 123.— Diagrammatic transverse section through the middle of the body of a Nemertine. (After LANG, Zext-b00k of Comp. Anat.) é.m, Basement-membrane. c¢.m. Circular muscles. d@.7. Dorsal or “ medullary" nerve. d@.v. Dorsal blood-vessel. g. Gonads. ivf. Intestine. 2. Longitudinal muscles, /.2, Lateral nerves. /.v, Lateral blood-vessel. ~. Proboscis. 2.5. Pro- boscis-sheath. Sometimes beneath the ectodermal epithelium of the Nemertine proboscis there is a continuous sheath of nerve- fibres, comparable to the nervous plexus in the proboscis of Balanoglossus. Partly, therefore, on account of its structure, and partly on account of its topographical relations when extruded, we are led to suppose that a certain homology exists NEMERTINES. 259 between the retractile proboscis of the Nemertines and the non-retractile proboscis of Balanoglossus (BATESON). In the most primitive Nemertines the nervous system consists essentially of a somewhat complicated pair of cerebral ganglia and a diffuse nerve-plexus, with nerve- cords lying at the base of the ectoderm.* As the cerebral ganglia probably belong to the same category as the cere- bral ganglia of all other typical Invertebrates, and are not represented in Balanoglossus, we can afford to neglect them at present. Confining our attention to the ecto- dermal nerve-plexus, we find occurring in it, along definite lines, local thickenings, after the same principle, but not all on the same lines, as was described above for Balano- glossus. Directly comparable with the dorsal longitudinal nerve-cord of Balanoglossus, there is a similar thickening or concentration of the integumentary nerve-plexus in some of the Nemiertines, in the dorsal middle line (Car- tnina, Cephalothrix). Hubrecht, who discovered this, calls it the medullary nerve. There is, however, no correspond- ing ventral nerve-cord in the Nemertines, but, instead of this, there is a pair of lateral thickenings, constituting the well-known J/ateral nerves of the Nemertines (Fig. 124). It is usually supposed that the lateral nerves of the Nemertines are homologous with the two halves of the ven- tral nerve-cord in the Annelids. In the Annelids the primitive lateral nerves (which are so typical of the Platy- helminths, or flat-worms) have approached one another in the mid-ventral line, and have often undergone intimate fusion together. In some cases, however, they are separated from one another by a wide interval (Sabellaria, etc.). * HUBRECHT compared the lobes of the cerebral ganglia of a Nemertine to the cranial ganglia of the Vertebrates, the lateral nerves to the Rami laterales vagi, and the proboscis-sheath to the notochord. 260 THE PROTOCHORDATA. In the Annelids, in contrast to the Nemertines, the gan- glion-cells are not distributed uniformly along the whole length of the nerve-cord, but are collected together to form definite ganglionic swellings. It is, therefore, very significant that in the Nemertines we have a median dorsal “medullary” nerve, in addition to the elements which constitute the ventral nerve-cord of the Annelids. In many Nemertines the dorsal and lateral nerve-cords do not continue to lie in the ectoderm throughout life, but at Ne, 2 fp: me cm Sniitp i ne - Yn orn Fig. 124.— Diagrammatic view of anterior portion of a Nemertine, from the left side. (After HUBRECHT, from LANG.) a./, Anterior lobe of brain. #./. Posterior lobe of brain. 2. Opening of pro- boscis. mm. Mouth. d.2. Dorsal nerve. /, Lateral nerve. 4.2, Ring-nerves. sink deeper into the body, and so come to be separated from the ectoderm, first by the basement membrane, and then by one or more muscular layers of the body-wall. In the Hoplonemertea (those in which the proboscis is armed with stylets) the medullary nerve is absent. In all cases, however, the longitudinal nerve-cords remain connected with one another by a more or less plexiform arrangement of nerve-fibres ; although sometimes a more definite con- nexion, by means of metameric ring-nerves, has been observed by Husrecnt (Fig. 124). There is no true celom in the Nemertines, and the space between the alimentary canal and body-wall is oc- cupied by a gelatinous mesenchyme, eee nuscul and connective tissue elements. In Balance DEES ity of the celom becomes largely obliterated in the adult, by the proliferation of cells from the epithelium of its walls, thus filling up the cavities with a more or paren vie matous tissue. aden Ph N94) t nro the = provided with paired i lat eral outgrowths or tes z, and a terminal anus. } - Ve earc - } Fr naArh 5 The ~ronadic sacs of the Nemertines offer = 2 semblance to those of Balanoglossus. They occur as a " P = : Sat Si : metameric series oF paired sacs, woicn alternate with the fold above-mentionec and communicate with the exterior by s are at first d, as in } Faeane enhe ntix- } ~ rime hol -ad ry 4 Balanoglossus, subsequently becoming hollowed out and } > ot lot +) ~ t= . ° opening above the lateral cords (Fig. 124) 7 h la ee wntad } ehaar Finally it should be pointed out tha organs, in the form of a well-developed elongated nephridi provided with “end-sacs,”” are present in the Nemertines, nothing of the kind has yet been detected in Balanoglossus. CEPHALODISCUS AND RHABDOPLEURA. It is interesting to note that there are some remarkable in a similar relation do to Amphioxus. aoes not produce and Azaé- 262 THE PROTOCHORDATA. a U-shaped alimentary canal. Both are deep-sea forms, Cephalodiscus having been dredged during the Challenger Expedition, from the Straits of Magellan, at a depth of 245 fathoms ; while Rhabdopleura was first dredged indepen- dently, off the Shetland Islands, at 90 fathoms, by the Rev. VO DAWCiia, ] aE SONIC 7g Fig. 125. — Cephalodiscus dodecalophus, from the ventral side. (After M’'INTOSH.) Actual length of polypide from extremity of branchial plumes to the tip of the pedicle is about 2 mm. é.s. Buccal shield; the shading on its surface indicates pigment-markings, At the tip of the pedicle, buds are produced. Canon Norman, and off the Lofoten Islands, at 200 fath- oms, by Professor G. O. Sars (1866-68). Rhabdopleura is the name given by ALLMAN (1869), who published a short account of it; and it has since been described by Sars, LANKESTER, and G. H. Fow cer. ; soporte cre CEPHALODISCUS. 203 The account which we possess of Cephalodiscus forms one of the Challenger Reports, and was written by Pro- fessor W. C. M'Intoss, who made out the main features of its anatomy. It was further treated, from a morpholog- ical standpoint, by Sipney F. Harmer, who pointed out its remarkably close afhnity to Balanoglossus. The most important morphological features in the anat- omy of Cephalodiscus are shown in Figs. 125-127. The individuals live in colonies, in a “house” or which consists of a ramifying and anastomosing system of tubes, the walls of which are composed of a semi-trans- parent, gelatinous material, whose outer surface Is covered with spinous projections. The walls of the ccencecium are furthermore perforated by numerous apertures, which allow of the ingress and egress of water. The adult members of a colony have no organic con- nexion between themselves, but each one is independent and free to wander about the tunnels of the ccencecium. Although Cephalodiscus has not been studied in the living condition, there is every reason to suppose that it moves about in its tube by means of the large éuccal shield ( Fig. 125) overhanging the mouth, by which it can attach itself to the inner surface of the tube, and then help itself along by the curious pedie/e which occurs ventrally near the hinder end. It thus seems probable that this pedicle can be used as a sucker, but its chief function lies in the production of buds which grow out from it, and eventually become detached. Bateson has described a somewhat similar sucker at the hinder end of the body in voung individuals of Balanoglossus (Fig. 113) Behind and above the buceal shield there is a row of twelve tentacles or branchial plumes. each possessing a central stem or shaft which carries numerous lateral 264 THE PROTOCHORDATA. pinne. An important function of these plumes is to produce currents of water by the action of their cilia, which vibrate in such a direction that the water with food-particles is led into the mouth. The superfluous water is led out from the proximal portion of the aliment- ary canal by a single pair of g7//-s/zts which are not visible in surface view, since they are overhung by a fold of the integument known as the post-oral lamella or operculum, corresponding to the posterior free fold of the collar in Balanoglossus (Fig. 126). In its internal organisa- tion, if due allowance be made for its U-shaped ali- mentary canal, Cephalodis- cus greatly resembles Bala- noglossus (Figs. 126, 127). The buccal shield of the the equivalent of the probos- former is obviously Fig. 126. — Longitudinal frontal (right and left) section through an adult Cephalo- discus. (After HARMER.) éc2, Second portion of body-cavity (collar-ccelom). 6c3. Third portion of body-cavity (trunk ccelom). 47. Pharynx. cp. Collar-pores. gs. Gill-slits. zz. In- cis of the latter, and the it contains corresponds to the probos- cavity which testine. 7.s. Nervous system. of. Oper- culum. oes. Cesophagus. st. Stomach. zt. Base of tentacle. the proboscis-cavity in Cephalo- cis-cavity. Moreover, discus (z.e. the cavity of the buccal shield) communicates with the exterior by ‘two proboscis-pores placed right and left of the dorsal middle line. Following behind the buccal shield is the col/ar-region, from which the branchial plumes arise dorsally, while CEPHALODISCUS. 265 laterally and ventrally it is produced into a free fold to form the above-mentioned operculum. The collar-region contains a section of the ccelom which is precisely homolo- Fig. 127. — Longitudinal sagittal section through an adult Cephalodiscus. (After HARMER.) The section is supposed to be taken sufficiently to one side of the middle line to allow of the representation of one of the ovaries and one of the proboscis-pores. a. Anus. 6.c. Trunk-cceelom. c.c. Collar-ceelom, ch, Notochord. zn. Intes- tine. m. Mouth. #.s. Nervous system. of. Postoral lamella (operculum). ov. Ovary; the oviduct is deeply pigmented. £.c. Praeoral ccoelom (cavity of buccal shield), ff. Pharynx. .f. Proboscis-pore. fed. Base of pedicle. st, Stomach. gous with the collar-cavities of Balanoglossus. As in the latter form, it communicates with the exterior by a pair of collar-pores which open at the level of the gill-slits. 266 THE PROTOCHORDATA. The collar-ccelom is continued posteriorly into the opercu- lum, and anteriorly into the twelve tentacular appendages. Finally, behind the collar comes the region of the body containing the viscera, which are surrounded by the third section of the ccelom. Only the female reproductive organs have been observed up to the present time in Cephalodiscus. They occur as a pair of gonadic sacs, opening to the exterior on each side of the dorsal middle line between the anus and the central nervous system. The latter is very simple, being represented merely by a dorsal thickening of the ectoderm, with nerve-fibres in the region of the collar and posterior portion of proboscis. Finally, a short notochordal diverticulum projects into the base of the buccal shield as in Balanoglossus. Rhabdopleura differs considerably from Cephalodiscus in many respects, but, nevertheless, has some fundamen- tal characteristics in common with it. In Rhabdopleura the individuals of a colony are not independent, but are connected with each other by a common cord or cau/us, which represents the remains of the contractile stalks of the polyps. As the growth of the colony proceeds, the distal portions of the stalks (7.e. the portions farthest away from the animals) become shrunken and hard. The buds arise from the soft portions of the caulus, and never be- come detached as they do in the case of Cephalodiscus. There is only a single pair of tentacular plumes in Rhab- dopleura. Fow er has recently shown that in Rhabdopleura the ceelom, whose existence was first established by Lay- KESTER, exhibits the same subdivisions as have been mentioned above for Cephalodiscus; namely, (1) the cavity of the large buccal shield, (2) the collar-cavity opening PRALORAL LOBE. 267 to the exterior by a pair of dorsally placed collar-pores, and (3) the body-cavity proper surrounding the alimentary canal. According to Fowler, who has recently described them in Rhabdopleura, the nervous system and notochord have essentially similar relations to those which obtain in Cephalodiscus, but there are no proboscis-pores and no gill-slits. THE PRAORAL LOBE OF ECHINODERM LARVA. In the previous pages a good deal of stress has been laid on the existence of a przoral lobe in the various types considered. We have recognised it in the snout of Am- phioxus (preeoral ccelom + przeoral pit), in the proboscis of Balanoglossus, the fixing organ of the Ascidian tadpole, and in the buccal shield of Cephalodiscus and Rhabdo- pleura. From a morphological standpoint the przoral lobe is probably one of the most important, as it is certainly one of the oldest, structures of the body of bilateral animals, and it becomes, therefore, a matter of the first moment to be able to trace the modifications which it has undergone along the different lines of evolution which have culmi- nated in the existing types of animal life. The subject is a very large one, and can only be treated here in its broadest outlines. It is now very generally admitted by zodlogists that the Echinoderms (star-fishes, sea-urchins, etc.) owe the radial symmetry, which is one of the most obvious characteristics of their organisation, to their having been derived from bilaterally symmetrical ancestors, which became adapted to a fixed or sessile existence. If this view is correct, and there is good reason for supposing that it is, it follows that the majority of living Echinoderms have secondarily 268 THE PROTOCHORKRDATA. lost their sessile mode of existence, and have again become free-living g, retaining, however, their radial symmetry. At the present time the fixed habit of life is only retained by the members of one of the subdivisions of the Echino- derm class; namely, the Crzzozdca. Most genera of Crinoids (RAtsocrinus, Pentacrinus, etc.) remain fixed by a long, jointed stalk throughout life ; but the well-known “feather-star,”’ Anedon rosacea, is only fixed during a certain period of its larval development. At the close of the period of fixation the body of the animal, or, as it is called, the ca/yx, breaks away from the stalk by which it was attached to the rocks, and so begins to lead a free existence, being capable of swimming vigorously by the flapping of its arms. Although the existing Crinoids have become extensively modified along their particular line of evolution, yet there is reason to believe that they represent the more im- mediate descendants of the primeval form which ex- changed its primitively free life and bilateral symmetry for a sessile existence and radial symmetry. This view is strengthened by the character of the free-swimming larva of Antedon. This larva does not possess, in any extrava- gant degree, those fantastic structures which are so characteristic of other Echinoderm larve, such as the provisional ciliated processes or arms of the “ Pluteus” (larva of sea-urchins), or the undulating ciliated bands of Auricularia. On the contrary, the larva of Antedon is a simple barrel-shaped organism, with regular ciliated bands pass- ing around it (Fig. 128). Perhaps the structure which, above all, stamps the free- swimming larva of Antedon as having, from a phylogenetic point of view, a more primitive type of organisation than PREORAL LOBE. 269 that of other Echinoderm larve, is the well-developed apical plate at its anterior extremity. We may express this in other words by saying that the larva of Antedon possesses a central nervous system at the apex of its przoral lobe. That the pre- oral lobe in this larva is not sharply marked off from the rest of the body is a detail of no morphological signifi- cance, The apical nervous sys- tem of the Antedon larva was discovered in 1888 by H. Bury, and has been Fig. 128.—Free-swimming larva of more clearly brought out Cea ee at Sse and emphasised in a recent ap. Apical pole. 6.0. Ciliated bands. J. Fixing disc. v. Vestibulum (so-called work by Dr. OSWALD SEELI- jarval mouth, although at this stage cer, At the point which is “PY 7 etedermie groove). marked externally by the anterior tuft of long cilia in Fig. 129 there is a slight groove in the ectoderm below which nerve-fibres and ganglion-cells can be identified. Seeliger further describes a pair of longitudinal nerves running from the nervous area of the apex along the ventro-lateral margins of the body. As already indicated, the apical plate is, as a general rule, conspicuous by its absence in the typical Echinoderm larva. In the free-swimming larva of Antedon, however, it is emphatically present, although destined to become entirely aborted after the fixation of the larva. In most Invertebrate larve in which an apical plate is present (e.g. the Trochophore-larva of Annelids and Mol- luscs) it becomes, during the metamorphosis, involved in other ectodermic thickenings of the przoral lobe, which 270 THE PROTOCHORDA TA. bit Asst A 4 collectively give rise to the cerebral or supracesophageal ganglion. The apical plate may thus be defined as a primitive central nervous system at the apex of the preoral lobe, being the forerunner and formative centre of the cerebral ganglion of the Invertebrates. Although, with the exception of the Crinoids, there is no apical plate in the typical Echinoderm larva, yet, as noted above, in many cases a curious transitory lengthen- ing of the ectodermic cells at the apical pole has been, and can be without great difficulty, observed in larva of star-fishes and sea-urchins. This alone would seem to indicate the former enist- ence of a central nervous system at the apex of the preoral lobe in the bilateral ancestor of the Echinoderms. he way in which the Fig. 129. — Larva of 4sterinc viewed as a transparent object from th ss left side. (Ager LUDWIG.) of the przoral Jobe can be ent.c. Enteric cavitv. de. Left entero- ee i 1713 E ccel, communicating with the right entero- replaced by a dilatation of ce] through ¢./, the preoral lobe. st the enteroccel has been de- tomodaeum. primary blastoceelic cavity No scribed above, both for Tor- naria and for the larva of dAsterias vulgaris (Figs. 121-122), In some cases, as in Astertva giébosa, the przoral lobe is occupied by the enteroceel from the very beginning. In the “Pluteus ” larva of the Echinids (sea-urchins) the preoral lobe is much reduced; but in other Echinoderms, as in the singular larva of Asterina gtédosa, and in the so-called Brachiolaria-larva of the Asterids (star-fishes) in general, it is very prominent, and serves as an effective locomoton (creeping) organ. PRAEORAL LOBE. 271 The very interesting observation has recently been made by MacBripg, that the larva of Asterina gibbosa actually undergoes temporary fixation at the beginning of the metamorphosis, the fixation being effected by the preoral lobe in a manner strikingly similar to that of the larvee of Antedon and of Czona. In the larva of Antedon the adhering disc, by which the larva eventually fixes itself to some foreign surface, is placed near the front end of the przeoral lobe immediately below the apical plate. The central nervous sys- tem of the adult Echinoderm arises in entire indepen- dence of the actual or sup- pressed apical nervous sys- . ee Fig. 130. — Larva of Asterina gibbosa, tem of the larva, and not at viewed as an opaque object from the left all from the ectoderm of the Se ate preeoral lobe. We have thus seen how within the limits of a single group (viz. the Echinoderms) the przoral lobe can become completely emancipated from the central nervous system ; and we have further recognised the fact that whether the cavity of the przoral lobe is a derivative of the primary or secondary body-cavity, whether it contains loose mesen- chyme or is lined by an endothelium, the morphological value of the przeoral lobe itself remains the same. THE PRAORAL LOBE OF THE PROTOCHORDATES. It is probable that the misunderstandings and disagree- ments which are of such frequent occurrence among mor- phologists with regard to the comparison of the types of central nervous system presented respectively by the 272 THE PROTOCHORDATA. Vertebrates and the Invertebrates, are largely due to the failure to detect some general principle of evolution to which that archaic structure, the przoral lobe, has been subjected. Nevertheless, there are many indications which point irresistibly to the conclusion, which I have recently brought forward, that the prime factor which must be recognised in the evolution of the przeoral lobe, from the relations which it presents in the Invertebrates to those which it holds in the Protochordates and Vertebrates, is its emancipation from the central nervous system. In the great groups of the Annelids, Molluscs, and Arthropods, the przoral lobe (prostomium, procephalic lobe) is essentially the seat of the brain or cerebral gan- glion. The latter, through its representative, the apzcal plate, is the main and often the sole element of the central nervous system in the Trochophore-larva of Annelids and Molluscs.* * In speaking of the apical plate as the forerunner or formative centre of the cerebral ganglion, it must not be assumed that these are not distinct structures. The apical plate is essentially median and unpaired, while the cerebral ganglion is paired. They can both, however, be included under the general term, apical nervous system, since they arise from the ectoderm of the preoral lobe. On the other hand, the cerebral ganglion may arise inde- pendently of an apical plate; as, for instance, in Lumdbricus, where there is no apical plate, or in the Memertines, where the apical plate is discarded together with other larval structures (Pilidium). Again, as in Lumbricus and many other cases, the cerebral ganglion, after having separated from the ectoderm of the przeoral lobe, may recede backwards for a considerable dis- tance, so as not to lie in the przeoral lobe in the adult. It is possible that the position of the cerebral ganglia of Nemertines may be accounted for by some such phylogenetic recession from the przeoral lobe. If necessary, it might be said that the praoral lobe can acquire emancipa- tion from the central nervous system by a simple recession of the cerebral ganglion. In the case of the Protochordates, however, on the view here advo- cated, the proeoral lobe has acquired emancipation from the central nervous system, not by the mere recession, but by the complete disappearance of the Invertebrate cerebral ganglion. PREORAL LOBE, 273 é At a later stage of development the longitudinal nerve- cord (confining the description to the Annelids for the sake of simplicity) arises tvdependentiy of the cerebral ganglion, from a pair of longitudinal thickenings of the ectoderm near the mid-ventral line, becoming secondarily connected with the cerebral ganglion by the circumaesoph- ageal nerve-collar or commissure As already indicated, it seems probable, as was sug- gested by Batrour and GrGENBAUR, that the ventral nerve-cord of the Annelids is to be regarded as having arisen phylogenetically by the mutual approximation of two such lateral cords as occur in the Nemertines, and like the latter may be supposed to have originated by a concentration on the ventral side of the body of that primitively continuous sub-epidermic nerve-plexus which is such a characteristic feature of the Nemertines. From a consideration of the adult nervous system in the Echinoderms, Nemertines, Enteropneusta (Balanoglossus), Annelids, and Molluses, it is evident that such a con- centration of nervous tissue has from first to last occurred along very different lines. Speaking in broad terms, it may be said that the only portion of the Invertebrate nervous system which, in its prime essence, is invariable and universal (due allowance being made for exceptional cases) is the cerebral ganglion or its forerunner, the apical plate, the seat of which Hes in the preoral lobe.? Under these circumstances it will suffice to confine our attention to the prasoral lobe, in the belief that if an understanding can be arrived at with regard to that impor- tant structure, one of the chief difficulties in the way of a just conception of the relations existing between Verte- brates and Invertebrates will have been overcome. 274 THE PROTOCHORDATA. Returning now to Balanoglossus, we have to remark that in the Tornaria larva the central nervous system is represented entirely by the apical plate of the przoral lobe, the situation of the apical plate corresponding to the anterior tip of the proboscis of the adult. Unlike the Annelids, however, the apical plate of Tornaria does not become replaced after the manner of the Invertebrates by the development of a cerebral ganglion arising like it from the ectoderm of the przeoral lobe and with it as a formative centre. On the contrary, it completely disappears after the metamorphosis, having become replaced physiologically by the development of the medullary tube in true Verte- brate fashion from the dorsal ectoderm of the collar-region behind the przeoral lobe.* In the Ascidian larva, however, and in Amphioxus, the characteristic Invertebrate apical nervous system no longer appears in any stage of development, its physiological func- tion having been once for all assumed by the medullary tube (cerebral vesicle + spinal cord) which lies par excel- lence behind the przeoral lobe (Fig. 131). Antertor and Posterior Neurenteric Canals, and the Position of the Mouth in the Protochordates. After the postoral medullary tube had led indirectly to the complete obliteration of the preoral apical nervous system, and had attained to such a degree of development as we find, for instance, in the Ascidian tadpole, the central canal of the cerebro-spinal nervous system appears to have acquired remarkable relations with the alimentary canal. At both ends of the body connecting ducts be- * For a detailed account of the formation of the medullary tube in the col- lar-region of Balanoglossus see MorGAN (Bibliography, Nos. 124 and 125). PR.EORAL LOBE. 275 / came established between the nervous and digestive systems, known respectively as the evferior and posterior neurenteric canals. The posterior neurenteric canal is only of transitory occurrence in all existing Vertebrates, and leads from the ch Fig. 131. — Diagrammatic representations of the anterior region of the body in (4) an Ascidian larva, (8) larva of Amphioxus, and (C) Balanoglossus. (After WILLEY,) The figure of Balanoglossus was compiled from Bateson’s figures; the pro- boscis-pore is indicated rather too far forwards, p4, Preeoral lobe (fixing organ, snout, probascis). 2, Endostyle. A. Prvoral pit or proboscis-pore. #. Mouth. #2. Neuropore. wc. Medullary tube, o4, Noto- chord, ¢« Eye. of Otoeyst. gv. and 4, Proboscis-gland and proboscis-heart of s Balanoglossus 276 K ooo Aol ood bs A a oe ‘aia a4, holo D Q 0? Fig. 132. — Sagitta hexaptera from the ventral surface ; nearly three times natural size. (After O. HERTWIG.) a. Anus. 4écl, Head-cavities. dc. Trunk-ceelom. 6¢c3, Caudal caslom. ./. Caudal septum. com, Commissure, from the cerebral ganglion to the single ventral ganglion. /1, 72, 78. Fins. m. Mouth. od, Oviduct. ov, Ovary. 5p. Prehen- sile bristles. s.v. Seminal vesicle. ¢, Tes- tis. v.g. Ventral ganglion. THE PROTOCHORDATA. neural tube into the extreme posterior end of the aliment- ary canal; in fact, into that portion of it which, in the embryos of the higher forms, is known as the post-anal gut. The anterior neuren- teric canal, in its most primi- tive condition, opens into the base of the buccal tube (Fig. 131). On this account we find in the Ascidian tadpole that the mouth is no longer ven- tral, as it isin Balanoglossus, but is placed dorsally, im- mediately in front of the anterior extremity of the medullary tube. This timate relation between the in- mouth and the central ner- vous system gives a reason for the contrast between the dorsal position of the mouth in the Ascidian tadpole and its ventral position in Bala- noglossus. In Amphioxus we have seen that the mouth has been forced aside from its more primitive dorsal position by the forward extension of the notochord to the tip of the PREORAL LOBE. 277 preoral lobe. The origin of the main cavity of the pre- oral lobe in Amphioxus from the right of a symmetrical pair of head-cavities (anterior intestinal diverticula of Hatschek) has been described in a previous chapter. In Balanoglossus there is no such complete division of the preoral body-cavity, but it is throughout a single space, its right and left halves being confluent. If we now com- pare the condition of things in the embryo of Amphioxus, where we have a symmetrical pair of head-cavities, with that of some other form which, in the adult condition, possesses a distinct pair of such cavities, it may assist us in imagining how the mouth could have assumed such opposite relations as have been mentioned above. But first it may be pointed out that in Appendicularia, where, as it would appear, in correlation with the second- ary acquirement of a purely pelagic habit of life (although this point of view is not shared by such authorities as Herdman, Seeliger, and Brooks), the przoral lobe has been reduced to a minimum, or to zero, the mouth has thereby come to lie in a terminal, or sub-terminal, position, with a slight tendency towards the dorsal side.* In the curious pelagic worm, Sagitfa, we meet with another instance of an animal in which the przoral lobe, in the ordinary sense of the term, is reduced to a mini- mum, and the mouth has therefore a sub-terminal position, with a ventral inclination (Fig. 132). But although there is no distinct praoral lobe in Sagitta, there is, neverthe- less, a fatr of head-cavities, which are directly comparable, if not perfectly homologous, with the above-mentioned * Whatever the truth may be as to the precise systematic position and phylogenetic value of Appendicularia, one thing, to my mind, remains abso- lutely certain, namely, that it has descended from a form which possessed a preoral lobe, and that it has secondarily lost that structure. 278 THE PROTOCHORDATA. head-cavities of Amphioxus, although they have a some- what different origin. It should not be forgotten that Sagitta occupies a very isolated position in the zodlogical system, being placed in a group by itself, the Chetoguatha, and that therefore the peculiarities of its organisation cannot be taken as repre- senting any definite intermediate stage in the phylogeny of other forms, yet, from a general standpoint, the con- ditions which it presents in its life-history are highly instructive. The head-cavities of Sagitta arise by constriction from the anterior extremities of the single pair of archenteric pouches which give rise to the ccelom of the adult. They remain distinct and separate on either side of the head throughout life. If, now, we imagine them to grow for- ward and fuse together in front of the mouth, in a simi- lar manner to that described above for the enteroccelic pouches of Asterias, we should have a preoral body-cavity of a similar character to that of Balanoglossus. Now, the ultimate position of the mouth under these new conditions would depend upon circumstances affect- ing the whole organisation of the animal. In an animal whose grade of organisation was on an approximate level with that of Sagitta the mouth would undoubtedly remain on the ventral side of the body. But in an animal whose organisation had reached the stage of evolution represented by that unknown ancestor of Amphioxus (most nearly represented at the present time by the Ascidian tadpole), whose notochord did not extend beyond the anterior limit of the neural tube, the mouth would pass to the dorsal side of the body to come into connexion with the neural canal. PREORAL LOBE. 279 THE PRHORAL LOBE IN THE CRANIATE VERTEBRATES. After what has been said above, in this and the preced- ing chapters, the question as to how the przoral lobe is represented in the craniate Vertebrates need not detain us long. Since, as shown above, the nervous element of the pre- oral lobe (apical plate and cerebral ganglion) is entirely lacking in the Vertebrates, we can only expect to find the mesodermal element represented in the head-cavities of the higher forms. In consequence of the great development of the brain, even in the lowest craniate Vertebrates, as compared with Amphioxus, and in consequence too of the cranial flexure, the head-cavities have been made to assume a more sub- ordinate position, and no longer take part in the formation of a prominent lobe in front of the body. This is a perfect illustration of “le principe du balancement des organes”’ of Geoffroy Saint-Hilaire, the przoral lobe decreasing as the brain increases. A comparison between Figs. 70, 72, 117, and 135 will show at once that the przoral head- cavities of Amphioxus and Balanoglossus are the homo- logues of the premandtbular head-cavities of the craniate Vertebrates. These cavities lie at first below the mid-brain, and later their walls give rise to most of the eye-muscles. In Figs. gi and 135 the median portion of the pramandibular cavities can be seen still in the form of an anterior pocket of the endoderm, and it may be noticed how far it is removed from the anterior extremity of the body to which it extends in Amphioxus, etc. In the craniate Verte- brates the brain extends forwards, and the head-cavities 280 THE PROTOCHORDATA. remain behind. This is, as we should expect, the exact reverse to what obtains in Amphioxus. In connexion with the evolution of the przoral lobe, we thus have an excellent example of repeated change of function. We may conclude, therefore, that the przaoral lobe, which, in the /zvertebrates, is above all the bearer of the cerebral ganglion, and in the Protochordates is released from this function and becomes in part a locomotor (Balanoglossus, Cephalodiscus) fixing (Ascidian) and bur- rowing (Amphioxus) organ, is represented in the craniate Vertebrates by the premandibular head-cavities, whose walls give rise to most of the eye-muscles. THE MOUTH OF THE CRANIATE VERTEBRATES. In consequence of the increase in the size of the brain, its forward extension and its cranial flexure, together with the relative reduction of the head-cavities, it is obvious that the mouth has been carried round from its primitively dorsal position to its final position on the ventral side of the head in the craniate Vertebrates. (Cf. Fig. 91.) This would have been all that need be said about the mouth were it not for the fact that the view, originally started by Dourn, that the Vertebrate mouth was a new formation resulting from the fusion of two gill-slits, has received such wide support and still in a measure holds its own. Since the Annelid mouth perforates the central nervous system in passing through the circumcesophageal nerve- collar, it was necessary to frame a theory which would get over the difficulty that nothing of the kind occurs in the Vertebrates. Accordingly Dohrn supposed that the old Annelid mouth had become aborted, and was replaced MOUTH. 281 by a new mouth derived from a fusion across the mid- ventral line of a pair of gill-clefts. DouRN was a trifle uncertain as to the rudiment of the old mouth, but BEARD was more certain on this point, and thought he had estab- lished the fact that the hy- pophysis cerebri represented the remains of the old An- nelid mouth. Dohrn certainly succeeded in bringing forward some apparently good evidence in support of his theory of the gill-slit origin of the mouth. This evidence was derived from the study of the de- velopment of the mouth in Teleostean or bony fishes. Fig. 133. — Two frontal views of an embryo of Batrachus tau, to show the In many Teleosteans the mouth has at first an appar- ently double origin, in that two separate ectodermal in- growths occur which fuse with the endoderm, instead of the median stomodceal involution which is so char- acteristic of other Verte- brates. This double origin double nature of the stomodceum. (From hitherto unpublished drawings kindly lent by Miss C. M. CLAPP.) The embryo is lying upon the yolk, and the septum which divides the stomo- dceum passes from the upper lip to the surface of the blastoderm which covers the yolk. The lower figure is a drawing of the same embryo as the upper, a few hours later. Above the stomodceum are seen the small nasal pits (rudiments of the external nares), and at the sides of the head are the rudiments of the eyes. of the mouth is particularly well shown in the embryos of the remarkable toad-fish, Batrachus tau, as observed by Miss CorneLiaA CLAPP at the Marine Biological Laboratory of Woods Holl, Mass., in 1889 (Fig. 133). In this case the mouth-cavity is seen to be divided into two halves by a median septum. Subsequently the septum becomes absorbed, and the 282 THE PROTOCHORDATA. two halves of the mouth coalesce. In view of the pre- vious existence of the gill-slit theory of the mouth, some such theory being a necessary accessory to the Annelid- theory, it is not surprising that this undoubted double origin of the mouth in Teleosteans should be regarded as a striking confirmation of Dohrn’s hypothesis. And yet, occurring as it does only in the Teleosteans, whose devel- opment is admittedly in many respects highly modified, the interpretation which Dohrn and his followers have placed upon this observation must always have been open to doubt. The simplest explanation of the double origin of the Teleostean mouth is that, owing to certain condi- tions (possibly mechanical) of development, the two angles of the mouth develop before the median portion. This is the conclusion which H. B. PotLarp has also reached in his recent studies on the development of the head in the Teleostean fish, Godius captto. According to the standpoint I have adopted in the fore- going pages, there is no @ frtorz reason for doubting that the Vertebrate mouth is completely homologous with the Protochordate mouth; and that the latter in its turn is the direct descendant of the typical Invertebrate mouth. Again, the anatomy and development of the Protochor- dates and of the Cyclostomi (Ammoccetes) show no indica- tion whatever of a discontinuity in the evolution of the most highly elaborated mouth of the gnathostomous or jawed Vertebrates. We conclude, therefore, that the ventral mouth of the craniate Vertebrates is the homologue of the primordial dorsal mouth as we find it in the Protochordates, and that its direction of evolution has been, as was so ably main- tained by Ba.Frour, from the cyclostomous to the gnatho- stomous condition. HYPOPHYSIS. 283 SIGNIFICANCE OF THE HYPOPHYSIS CEREBRI. The pituitary body, or hypophysis, belongs to the series of ductless “glands” (pineal body, thyroid gland, thy- mus, etc.) which are such a characteristic feature of the vertebrate organisation. It arises as an ectodermal invo- lution from the roof of the stomodceum, directed towards the base of the primary fore-brain, from which the infun- dibulum grows out. The pituitary involution becomes in most forms nipped off from the stomodceum, and then lies as a closed sac in contiguity with the infundibulum. Later on it produces a system of branches, the lumina of which tend to dis- appear; and in some forms (e.g. Mammalia) it undergoes actual fusion with the infundibulum. The very constant relation of the hypophysis to the infundibulum in the craniate Vertebrates (see Fig. 134) naturally led to the supposition that there must originally have been a functional connexion between the two struct- ures of a similar nature to that which exists between the olfactory pit and neuropore in Amphioxus. Recent re- searches, however, have rendered it probable that such a supposition is erroneous. Von KupFrFeER has discovered the homologue of the lobus olfactorius of Amphioxus in the craniate Vertebrates, and has shown that it occurs at a point far removed from the infundibular region. Until recently it was also very generally thought that the infundibulum represented the anterior end of the brain, which had become bent downwards and backwards by the cranial flexure. Kupffer, however, has brought for- ward weighty reasons for doubting this view. According to him, the infundibulum is essentially a downgrowth or 284 THE PROTOCHORDATA. evagination from the floor of the brain, occurring behind the anterior terminal extremity of the brain. It follows that the morphological anterior extremity of the craniate brain coincides with the median Jodus olfac- torius tmpar, which also represents the point of last con- nexion of the medullary tube with the superjacent ecto- derm. The lobus olfactorius impar lies in the anterior vertical wall, which forms the boundary of the primary fore-brain in front, known as the Jamzna terminalis. RaBL- Ruckuarp has also observed the median olfactory lobe in Fig. 134. — Sagittal section through the head of an embryo of Acanthias. (After RABL-RUCKHARD.) a.c. Position of anterior commissure. a/, Alimentary canal. cev. Cerebellum. ch. Notochord; the black shading below the notochord indicates the aorta. fo. Fore-brain. 4.6. Hind-brain. Ay. Hypophysis, already shut off from the stomodceum and lying as a closed sac at the base of izf the infundibulum. Zo. Lobus olfactorius. . Mouth. m.d, Mid-brain. o.c. Optic chiasma. 2.4. Pineal body (epiphysis). the Selachian embryo (Fig. 134), and it has since been found by BuRCKHARDT in other forms. It can thus hardly be doubted that the median rudi- mentary olfactory lobe of the embryos of the higher Vertebrates is homologous with the lobus olfactorius of Amphioxus (Fig. 51), and, like the latter, represents the remains of the neuropore. In Amphioxus, however, the HYPOPHYSIS. 285 olfactory lobe abuts against the olfactory pit, and, in fact, in young individuals opens into it by the neuropore (Fig. 45). On the view which I have urged above, that the olfactory pit of Amphioxus is homologous with the hypophysis cerebri of the craniate Vertebrates, it must be assumed that in the latter forms, the neuropore hav- ing ceased to be in any way a functional organ, the hy- pophysis, which has likewise become (morphologically) a vestigial structure, has been mechanically separated from the neuropore, with which it was primitively in functional connexion. It must be supposed that this separation of the hypophysis from the neuropore has been effected by the more rapid downward growth of the ectoderm (from which the hypophysis arises) than of the wall of the brain, so that the hypophysis has been carried farther round to the lower side of the head than the neuropore (Fig. 135). The reason for this unequal growth of the external body- wall and of the cerebral wall may, perhaps, be sought for in the great and independent increase in the cubical con- tents of the brain.® We thus arrive at the conclusion that the present relation of the hypophysis to the infundibulum in the craniates, however intimate it may be in some cases, is, nevertheless, incidental and secondary. That this conclusion is not so strained as might appear at first sight is clearly shown by the fact that the in- fundibulum is not the only structure with which the hypophysis enters into close relations. In the exceptional cases of Myxine and Bdellostoma, for instance, the distal end of the hypophysis has nothing to do with the infundibulum, but actually opens into the pharynx. In these hag-fishes, as also in the lamprey 286 THE PROTOCHORDATA. (where there is no internal opening of the hypophysis into the pharynx), the external opening of the hypophysis does not close up, as in the higher forms, but persists throughout life, becoming carried round to the top of the head during the embryonic development by differ- ential growth of neighbouring parts, as has been actually observed in Petromyzon. Fig. 135.— Median sagittal section through the head of young Ammoceetes. (After KUPFFER.) The arrow indicates the extent to which the hypophysis has been (hypothetically) removed from the neighbourhood of the neuropore (lobus olfactorius impar). ch, Notochord. ec. Ectoderm. ex, Endoderm. ef. Epiphysis. Ay. Hypo- physial involution. Zo, Lobus olfactorius impar. 7. Nasal involution. gm. Me- dian portion of preemandibular cavity. s¢, Stomodceum, F.A/.H. Primary fore-, mid-, and hind-brain. In other cases, as, for example, in the embryo of the rabbit, it has been observed that the hypophysis actually undergoes a temporary fusion with the front end of the notochord; and in all cases the distal end of the hypophysis grows inwards as much towards the notochord as towards the infundibulum, so that for the embryonic stages of the craniate Vertebrates it might be said that the relations of HYPOPHYSIS. 287 the hypophysis to the front end of the notochord are as con- stant as its relations to the infundibulum. So close is the apparent relation of the hypophysis to the notochord that at least one zoologist, HuBREcHT, has suggested that there was originally a functional connexion between the two structures. Again, in the embryo of Acztpenser, the sturgeon, as shown by Kuprrer, the distal end of the hypophysis undergoes temporary fusion with the subjacent wall of the alimentary cavity. In spite of the extremely modified character of the embryo of Acipenser (the embryo being flattened out like a disc over the yolk), Kupffer regards this fusion of the hypophysis with the endoderm as being of great morphological significance. On the contrary, for the reasons mentioned above, I would regard all these fusions of the hypophysis in the craniate Vertebrates, whether with the infundibulum, notochord, or endoderm, as being of an entirely incidental character, often due, perhaps, to a tendency of such con- tiguous embryonic tissues to fuse together. I therefore suggest that: The hypophysts arose in con- nexion with a functional neuropore,; when the neuropore ceased to be functional, there was no longer any bond of union between tts inner portion, which opened into the cerebral cavity, and its outer portion, which opened into the buccal cavity; and these two portions became separated by differential growth of the cerebral and body-walts (cf. Fig. 135). The Ascidian Hypophysts. The development of the hypophysis in a typical As- cidian, its constriction from the wall of the cerebral vesicle in the form of a tube, and its opening into the 288 THE PROTOCHORDATA. buccal cavity, or branchial sac, have been described above. The most serious objection which has been raised against the comparison of the hypophysis of the Ascidians with that of the craniate Vertebrates is, that in the former the hypophysis opens, not at an ectodermal surface into the stomodceum, but at an endodermal surface (behind the stomodceum) into the branchial sac. This is undoubtedly the case in some Ascidians, e.g. Drstaplia, and probably also in Clavelina, etc. In Czona, however, as I can state after renewed study of the question, it apparently opens at first into the buccal cavity precisely in the line of junction between the stomodceum and the branchial sac, so that its upper margin is continuous with the stomodceal epithelium, while its lower margin is continuous with the epithelium of the branchial sac. It is probable that too much stress has been laid on the question whether the hypophysis of the Ascidians opens at an endodermic or at an ectodermic surface, and that thus the attention has been diverted from the essential fact that the hypophysis opens into the buccal tube at the entrance to the branchial sac. In the case of the Ascid- ians, therefore, I should also regard the fusion of the hypophysis, whether with the ectoderm of the stomodceum or with the endoderm of the branchial sac, as being in itself non-essential, while the actual opening of the hy- pophysis (itself derived by constriction from the nerve- tube) into the buccal cavity, apart from the question of an ectodermal or endodermal surface, is the essential point. CONCLUSION. 289 CONCLUSION. From the facts that have been recorded and the consid- erations that have been urged in these pages, it would follow that one of the chief factors in the evolution of the Vertebrates has been the concentration of the central nervous system along the dorsal side of the body (in contrast to the position of the longitudinal nerve-cord of Annelids, etc., along the ventral or /ocomotor surface), and its conversion into a hollow tube. If it be admitted that the hypophysis became evolved in connexion with a func- tional neuropore, it is obviously a structure which has arisen within the limits of the Vertebrate phylum, and can, therefore, have no representative in the typical Invertebrate organisation. It has been suggested by ADAM SEDGWICK and vAN WHE that the original function of the central canal of the spinal cord was to promote the respira- tion (oxygenation) of the tissue of the central nervous system, water entering by the neuropore, and passing out through the posterior neurenteric canal. It is not so easy to form a conception as to the prime origin of the other two cardinal characteristics of a Vertebrate (Chordate); namely, gill-slits and notochord. As to the origin of gill-slits, it has been suggested inde- pendently by Harmer and Brooks, that they arose at first not so much to perform the direct function of respiration, as to carry away the bulk of the water which constantly entered the mouth with the food, so as to avoid the neces- sity and discomfort of the never-ceasing flow of water through the entire length of the alimentary canal. In Cephalodiscus, for example, the luxuriant branchial plumes must be sufficient for the respiration of the minute animal, 290 THE PROTOCHORDATA. while the usefulness of the pair of gill-slits, in allowing the surplus water to pass out of the pharynx, is evident. The notochord is more difficult to explain, and the fact of its occurrence in the proboscis of Balanoglossus and in the tail of the Ascidian tadpole is very puzzling. The mode of its occurrence in Balanoglossus is undoubtedly divergent, and not in the direct line of Vertebrate descent. It is possible that the notochord has not arisen through a process of elaborate change of function from a pre-existing structure, but simply as a solidification of the endoderm which was continued into the caudal or post-anal extension of the body to form the axial support for a locomotor tail ; while the subsequent extension of the notochord into the pre-anal region of the body is not difficult to understand. The general capacity of the endoderm for producing skeletal tissue is already present in some of the Medusze and Hydroid polyps whose tentacles are stiffened by a solid endodermal axis. From a purely morphological point of view it now seems as though the praoral lobe and in a lesser degree, perhaps, the hypophysis, would materially assist in furnish- ing the key to a correct appreciation of the relationship between the craniate Vertebrates, the Protochordates, and the Invertebrates. As we have indicated above, in the formulation of the Annelid-theory* no allowance has been made for the prin- ciple of parallelism in evolution; but it is impossible to doubt that this is a very potent factor which should always be borne in mind in estimating the genetic affinity between widely different groups of animals. The closer the super- ficial resemblance between an Annelid and a Vertebrate (in the possession of somites, segmental organs, etc.) is shown to be, the more perfect appears the parallelism CONCLUSION. 291 in their evolution and the more remote their genetic affinity. For the present we may conclude that the proximate ancestor of the Vertebrates was a free-swimming animal intermediate in organisation between an Ascidian tadpole and Amphioxus, possessing the dorsal mouth, hypophysis, and restricted notochord of the former; and the myo- tomes, ccelomic epithelium, and straight alimentary canal of the latter. The ultimate or primordial ancestor of the Vertebrates would, on the contrary, be a worm-like animal whose organisation was approximately on a level with that of the bilateral ancestors of the Echinoderms. NOTES. I. (p. 246.) For the discussion of the phenomena of meta- merism and the enumeration of examples of independent metameric repetition of parts, consult the following: Lanc, ARNOLD. Der Bau von Gunda Segmentata und die Verwandtschaft der Plathel- minthen mit Celenteraten und Hirudineen. Mitth. Zool. Stat. Neapel, Bd. III]. 1882. p.187 e¢seg. SEDGWICK, ADAM. On the Origin of Metameric Segmentation, and Some Other Mor- phological Questions. Quarterly Jour. Micro. Sc. XXIV. 1884. pp. 43-82. Bareson, Witttam. Zhe Ancestry of the Chordata. Quarterly Jour. Micro. Sc. XXVI. 1886. pp. 535-571. CALD- WELL, H. Slastopore, Mesoderm, and Metameric Segmentation. Quarterly Jour. Micro.Sc. XXV. 1885. pp.15-28. HUBRECHT, A.A.W. Report on the Nemertea collected by H. M.S. Challenger, 1873-76. Chall. Rept. Zodl. XIX. 1886. (Also, HUBRECHT. The Relation of the Nemertea to the Vertebrata. (Quarterly Jour. Micro. Sc. XXVII. 1887. pp. 605-644.) Van BENEDEN, Epouarp. Recherches sur le Développement des Arachnactts. Contribution a la Morphologie des Cérianthides. Archives de Biologie, XI. 1891. pp. 115-146. Also consult the recent great work of Bateson, Maverials for the Study of Variation. London, 1894. 292 THE PROTOCHORDATA. 2. (p. 273.) On the subject of the preoral lobe and the api- cal nervous system of Invertebrates, see the following: BALFour, F. M. Comparative Embryology. 1881. Vol. II. Chap. 12. Observations on the Ancestral Form of the Chordata. BEarRD, J. Zhe Old Mouth and the New, A Study in Vertebrate Mor- phology. Anat. Anz. III. 1888. pp. 15-24. Wison, E. B. The Embryology of the Earthworm. Jour. Morph. III. 1889. pp. 387-462. Harscuex, B. Lehrbuch der Zoologie. 3d Liefer- ung. Jena, 1891. Wittey, A. On the Evolution of the Preoral Lobe. Anat. Anz. IX. 1894. pp. 329-332. 3. (p. 285.) From what has been said in the text, it is obvious that the hypophysis of the craniate Vertebrates, in becoming separated from the neuropore, has retained (at least in the embryo) its primitive relations with the buccal cavity, and, like the latter, has been made to assume its present position in consequence of the forward growth of the brain and the ensuing cranial flexure. In Amphioxus, the hypophysis (z.e. olfactory pit) arises as an ectodermic involution immediately over the neuropore, but still independent of the latter. In other words, the neuropore exists in Amphioxus for a considerable length of time before the hypoph- ysis forms ; and this is in accordance with what we should expect from the analogy of the craniate Vertebrates. In the Ascidians, however, the conditions are somewhat different, and there is at first no such obvious differentiation between neuropore and hypoph- ysis. For the simple Ascidians (¢.g. Ciona) it must at present remain doubtful whether the increase in size of the hypophysis takes place entirely by interstitial growth, or whether there is any ingrowth from the wall of the buccal tube at the lips of the aper- ture (dorsal tubercle) of the hypophysis. In any case there are not wanting indications in the Ascidians of a distinction, and even separation, between the distal portion of the hypophysis, which at first opens into the cerebral vesicle, and the proximal portion, which opens into the buccal cavity. In the adult, the proximal portion of the hypophysis has the form of a simple duct, opening by the so-called dorsal tubercle into the buccal cavity, while the subneural gland arises as a proliferation from the ventral wall of the distal portion. In Phallusia mammillata, as was discovered by Juin (Archives de Biologie, V1. 1881. pp. 211-232), num- NOTES. 293 bers of secondary tubules grow out from the principal duct of the hypophysis, and acquire ciliated funnel-like openings into the peribranchial chamber ; subsequently HerpMan (Proc. Roy. Soc. Lidinburgh, XII. 1882-84. p. 145) found that in this form the dorsal tubercle, or opening of the hypophysis into the buccal cavity, is sometimes absent. In Czona intestinalis I have found in young individuals an obliteration of the lumen of the hypophysis between the proximal and the distal portions. In other cases, as in Appen- dicularia, the glandular portion of the hypophysis may be reduced or absent. On the subject of the Ascidian hypophysis, the following papers should also be consulted: SHELDON, Litian. Vote on the Ciliated Pit of Ascidians and its Relation to the Nerve-ganglion and So- called FHlypophysial Gland. (Quarterly Jour. Micro. Sc. XXVIII. 1888. pp.131-148. Hyort, Jouan. Ueber den Entwicklungs- cyclus der Zusammengesetsten Ascidien. Mitth. Zool. Stat. Neapel, X. 1893. pp. 584-617. Mercatr, Maynarp M. Zhe Eyes and Subneural Gland of Salpa. Baltimore, 1893. (Published as Part IV. of Professor Brooks’s Monograph of the Genus Salpa.) 4. (p. 290.) The most complete presentation of the Annelids- theory is contained in the classical A/onographie der Capitel- iden des Golfes von Neapel, by Dr. Huco Etsic. It is needless to add that this monograph will command the gratitude and admiration of zodlogists to the end of time. = to REFERENCES. INTRODUCTION. Carus, J. VicToR. Geschichte der Zoologie. Miinchen, 1872. Dourn, ANTON. Der Ursprung der Wirbelthiere und das Prin- cip des Functionswechsels. Leipzig, 1875. HAECKEL, ERNST. Anthropogenie oder Entwickelungsgeschichte des Menschen. Leipzig, 1874; 4th Edit., 1891. LANKESTER, E. Ray. Article “Vertebrata.” Encycl. Brit., gth Edit. Republished in * Zodlogical Articles,” London, 1891. PERRIER, EDMOND. La Philosophie Zoologique avant Darwin, 2d Edit. Paris, 1886. SEMPER, CARL. Dee Verwandtschaftsbeztehungen der geglieder- ten Thiere. Parts I. to III. Wiirzburg, 1875-76. I. anp II. ANATOMY OF AMPHIOXUS.* AnDREws, E. A. The Bahama Amphioxus (preliminary ac- count). Johns Hopkins University Circulars. Vol. XII. p. 104. June. 1893. ANDREWS, E. A. dn Undescribed Acraniate: Asymmetron lucayanum. Studies from the Biol. Lab. Johns Hopkins Uni- versity, Vol. V. No. 4. 1893. pp. 213-247. Plates XIII.- XIV. Contains bibliography of systematic and faunistic works on Amphioxus. ANTIPA, GR. Ueber die Besiehungen der Thymus su den soge- nannten Kiemenspaltenorganen bet Selachiern. Anat. Anz. VII. 1892. pp. 690-692. One figure in text. * This bibliography does not by any means include all that has been written on the anatomy of Amphioxus. Some of the older and shorter works, as well as some of those relating to special points of histological detail, have been omitted, as they are fully dealt with in many of the memoirs here cited. 205 296 Io II 14 15 16 17 18 20 REFERENCES. BALFour, F.M. A Preliminary Account of the Development of the Elasmobranch Fishes. Quarterly Jour. Micro. Sc. XIV. N.S. 1874. pp. 323-364. Plates 13-15. Paper in which Balfour first published his discovery of the seg- mental origin of excretory tubules. This was made out also in the same year by Semper and Schultz. (Vide infra, Schwltz.) BALFourR, F. M. Ox the Origin and History of the Urino- genital Organs of Vertebrates. Jour. of Anat. and Physiol. X. 1875. pp. 17-48. Eight figures in text. Amplification of his pre- vious work, with bibliography up to date. BALFour, F. M. The Development of Elasmobranch Fishes. Development of the Trunk. Jour. of Anat. and Physiol. XI. 1876. pp. 128-172. Plates 5 and 6. First account of origin of paired limbs from continuous epiblastic thickenings. Batrour, F. M. A Monograph on the Development of Elasmo- branch Fishes. London, 1878. BEDDARD, FRANK Evers. On the Occurrence of Numerous Nephridia in the Same Segment in Certain Earthworms, and on the Relationship between the Excretory System in the Annelida and in the Platyhelminths. Quarterly Jour. Micro. Sc. XXVIII. N.S. 1888. pp. 397-411. Plates 30-31. Contains discovery of neph- ridial network in Pericheta. BENHAM, W. BLAXLAND. The Structure of the Pharyngeal Bars of Amphioxus. Quarterly Jour. Micro. Sc. XXXV.N.S. 1893. pp. 97-118. Plates 6-7. BOURNE, ALFRED GIBBS. Contributions to the Anatomy of the Hirudinea. Quarterly Jour. Micro. Sc. XXIV. N.S. 1884. PPp- 419-506. Plates 24-34. Contains discovery of nephridial network in Pontobdella. BOVERI, THEODOR. Ueber die Niere des Amphioxus. Miin- chener Medicin. Wochenschrift. No. 26. 1890. Sep. Abd. pp. I-13. Two figures in text. (Preliminary note.) Boveri, THEODOR. Die Mierencandlchen des Amphioxus. Ein Beitrag sur Phylogenie des Urogenitalsystems der Wirbelthiere. Zoolog. Jahrbiicher. Abth. fiir Morphol. V. 1892. pp. 429-510. Taf. 31-34 and five figures in text. Costa, O. GABRIELE. Cent zoologict ossia descrizione som- maria delle specte nuove di antmali discoperti in diverse contrade del regno nell’ anno 1834. Napoli, 1834. See also Fauna del regno di Napoli. 1839-50. CuEnoT, L. Etudes sur le sang et les glandes lymphatigues dans la série animale. Archives de zool. expérimentale, XIX. 1891. Amphioxus. pp. 55-56. 21 23 24 25 26 27 28 29 REFERENCES. 2907 Notes absence of blood-corpuscles in Amphioxus. Those described by previous authors must therefore require another ex- planation. DOHRN, ANTON. Studien zur Urgeschichte des VW irbelthier- korpers. LV. Section 5. Entstehung und Bedeutung der Thymus der Selachter. Mitth. Zool. Stat. Neapel. V. 1884. pp. 141-151. Taf. 8. Figs. 1 and 2. Eisic, Huco. Dze Segmentalorgane der Capitelliden. Mitth. Zool. Stat. Neapel. 1. 1879. pp. 93-118. Taf. IV. Discovery of numerous nephridia in single segments and an- astomoses between successive nephridia. EMERY, CARLO. Le specie del genere Fierasfer nel Golfo at Napoli. 2d Monograph in the “ Fauna und Flora des Golfes von Neapel.” Leipzig, 1880. EMERY, CaRLo. Zur Morphologie der Kopfniere der Teleostier. Biologisches Centralblatt, I. 1881. pp. 527-529. See also Zoologischer Anzeiger, VIII. 1885. pp. 742-744. Fusari, Romeo. Beitrag sum Studium des peripherischen Nervensystems von Amphioxus lanceolatus. Internationale Mo- natsschrift fiir Anatomie und Physiologie, VI. 1889. pp. 120-140. Taf. VII.-VIII. Goopsir, JOHN. Ox the Anatomy of Amphioxus lanceolatus. Transactions of the Royal Society of Edinburgh, Vol. XV. Part I. 1841. pp. 241-263. GRENACHER, H. Bettrage zur nahern Kenntniss der Muscu- latur der Cyclostomen und Leptocardier. (Leptocardia proposed by Haeckel as a classificatory name on account of the simple tubular “heart” of Amphioxus.) Zeitschr. fiir Wiss. Zoologie, XVII. 1867. pp. 577-597. Taf. XXXVI. First isolation of muscle-plates of Amphioxus. GUNTHER, ALBERT. Synopsis of Genus Branchiostoma. In Report on Zodl. Collections of H. M.S. Alert. 1881-82. pp. 31- 33. London, 1884. HaTSCHEK, BERTHOLD. Die Metamerie des Amphioxus und des Ammocetes. Verh. Anat. Gesellschaft, 6th Versammlung. Wien, 1892. pp. 137-161. Eleven figures in text. 29 bis. HATSCHEK, BERTHOLD. Zur Metamerie der Warbelthiere. 30 Anat. Anz. VII. Dec. 1892. pp. 89-91. Huxvey, T. H. Preliminary Note upon the Brain and Skull of Amphioxus lanceolatus. Proceedings of the Royal Society, XXIII. 1874. pp. 127-132. Points out that in Myxine and Ammocceetes a velum is present separating the buccal (stomodceal) from the branchial cavity- 298 31 33 34 35 36 37 38 REFERENCES. The resemblance of the buccal cavity and tentacles (cirri) of Ammoceetes to the corresponding parts in Amphioxus is so close that there can hardly be any doubt the two are homologous. The anterior end of the nerve-tube of Amphioxus corresponds to the lamina terminalis of the craniate Vertebrates. Huxtey, T. H. Ox the Classification of the Animal Kingdom. Journal of the Linnzan Society (London), XII. 1876. pp. 199- 226. (Read 3d Dec., 1874.) Section on “epical,” p. 216 ef seg. Atrial cavity of Amphi- oxus and Ascidians is an epiccel like the opercular cavity of the Amphibian tadpole. KOLLIKER, ALBERT. Ueber das Geruchsorgan von Amphioxus. Miiller’s Archiv fiir Anat. Physiol., etc. 1843. pp. 32-35. Taf. Il. Fig. 5. Discovery of olfactory pit and first description of the spermatozoa of Amphioxus. KOpPEN, Max. Beitrage sur vergleichenden Anatomie des Centralnervensystems der Wiorbelthiere. Zur Anatomie des Eidechsengehirns. Morphologische Arbeiten (Schwalbe), I. 1892. pp. 496-515. Taf. 22-24. Contains discovery of giant-fibres in caudal portion of spinal cord of Lacerta viridis. Kou, K. Einige Bemerkungen iiber Sinnesorgane des Amphi- oxus lanceolatus. Zool. Anz. 1890. pp. 182-185. States that sometimes there is a shallow olfactory groove on the right side as well as that in the left. Such grooves are often due to artificial crumpling, and the observation requires confirmation. KRUKENBERG, C. FR. W. Zur Kenntnis des chemischen Baues von Amphioxus lanceolatus und der Cephalopoden. Zool. Anz. 1881. pp. 64-66. See also HOPPE-SEYLER’S reply. pp. 185-187. Compare also CUENOT (supra). KUPFFER, CARL VON. Studien sur vergleichende Entwick- lunesgeschichte des Kopfes der Krantoten.L. Die Entwicklung des Kopfes von Acipenser sturio an Medianschnitten untersucht. 95 pp. 8°. 9 Tafeln. Miinchen und Leipzig, 1893. Contains also a chapter on brain of Amphioxus, with figures. LANGERHANS, PAUL. Zur Anatomie des Amphioxus lanceolatus. Archiv fiir mikroskopische Anatomie, XII. 1876. pp. 290-348. Taf. XII.-XV. Standard work on the histology of Amphioxus. LANKESTER, E. Ray. On Some New Points in the Structure of Amphioxus and their Bearing on the Morphology of Vertebrata. Quarterly Jour. Micro. Sc. XV. N.S. 1875. pp. 257-267. REVERENCE S. 299 39 LANKESTER, E. Ray. Contributions to the Knowledge of Amphi- oxus lanceolatus, Yarrell. \b., Vol. XXIX. 1889. pp. 365-408. Five plates. 4o Lworr, Basttius. Uber den Zusammenhang von Markrohr und Chorda betm Amphioxus und ahnliche Verhaltnisse bet Anneliden. Zeitschrift fur wiss. Zoologie. Bd. 65. 1893. pp. 299-308. Taf. XVII. Describes those supporting fibres of the spinal cord of Amphi- oxus which descend in successive paired groups to the notochordal sheath and penetrate the latter in order to insert themselves on the inner surface of the sheath. The openings in the notochordal sheath of Amphioxus, through which the ventral supporting fibres pass, were first observed by WILHELM MULLER in 1871. (W. MUuvter, Ueber den Bau der Chorda dorsalis. Jenaische Zeit- schrift, VI. 1871. pp. 327-354.) See also PLATT (infra) and Lworr (88). Latter contains complete bibliography of literature relating to structure of notochord. 41 Maver, Paut. Ober dic Intwicklung des Herzens und der grossen Gefassstimme bet den Selachiern. Mitth. Zool. Stat. Neapel. VII. 1887. pp. 338-370. ‘Taf. 11-12. 42 Mever, Epuarp. Studien iiber den Korperbau der Anneliden. Mitth. Zool. Stat. Neapel. VII. 1887. pp. 592-741. Taf. 22-27. 42 bis. Mortiau, CAMILLE. Recherches sur la Structure de la Corde dorsale de CAmphioxus. Bull. Acad. Belg. Tome 39. No. 3. 1875. 22 pp. One plate. 43 Mitver, Wituetm. Ueber adie Stammesentwicklung des Sehorgans der Wirbelthiere. 76 pp. Five plates. 4°. Leipzig, 1874. 44 MULLER, WituneLM. Ceber das Urogenttalsystem des Amphi- oxus und der Cyclostomen. Jenaische Zeitschr. fiir Naturwissen- schaft, Bd. Il. (neue Folge). 1875. Sep. Abdruck. pp. 1-38. Two plates. This is the important work in which the pronephros and mesonephros were for the first time clearly distinguished from one another. The author was, however, in error regarding Johannes Miiller’s renal papillae of Amphioxus. 45 MULurr, Jouannes. Uber den Bau und die Lebenserscheinun- gen des Branchiostoma lubricum Costa, Amphioxus lanceolatus, Varrell. Berlin, 1844. 4°. 40 pp. Five plates. Read at the kénigl Akademie, 1841. 46 NAnseN, Fripryor. Zhe Structure and Combination of the His- tological Ihlements of the Central Nervous System. Bergens Museums Aarsberetning for 1886. Bergen, 1887. 300 REFERENCES. 47 OwsJANNIKOW, Puitip. Ueber das Centralnervensystem des Amphioxus lanceolatus. Bulletin de l’Acad. imp. des Sciences de St. Pétersbourg, Tome XII. 1868. pp. 287-302, with one plate. Also in Mélanges Biologiques, T. VI. pp. 427-450. Introduced a method of maceration by which he was able to shake out the central nervous system and thus isolate it from the body. In this way he was able to correct the erroneous descrip- tions of de Quatrefages and others (who stated that there were ganglionic enlargements in the spinal cord), and to discover the alternate arrangement of the spinal nerves. 48 PLatTT, JULIA B. Frbres connecting the Central Nervous System and Chorda in Amphioxus. Anat. Anz. VII. 1892. pp. 282-' 284. Three figures in text. 49 POLLARD, E. C. A Mew Sporozoin in Amphioxus. (Quarterly Jour. Micro. Sc. XXXIV. N. S. 1893. pp. 311-316. Plate XXIX. Unicellular parasites in intestinal epithelium. 49 b¢s. PoucuET, GEorGES. On the Laminar Tissue of Amphioxus. Quarterly Jour. Micro. Sc. XX.N.S. pp. 421-430. Plate XXIX. 50 DE QUATREFAGES, ARMAND. JAZémoire sur le systéme nerveux et sur Vhistologie du Branchtiostome ou Amphioxus. Annales des sciences nat. Zoologie. 3d series. IV. 1845. pp. 197-248. Plates 10-13. First observation of passage of ova through atriopore; and discovery of the peripheral ganglion-cells in connexion with the cranial nerves. 51 RATHKE, HEINRICH. Bemerkungen iiber den Bau des Amphi- oxus lanceolatus, eines Fisches aus der Ordnung der Cyclostomen. Konigsberg, 1841. 4°. pp. 1-38. One plate. 52 ReEtzius, GusTAV. Zur Kenntniss des centralen Nervensystems von Amphioxus lanceolatus. Biologische Untersuchungen. Neue Folge II. pp. 29-46. Taf. XI.-XIV. Stockholm, 18go. 52 des. RETzIUS, GusTAvV. Das hintere Ende des Riickenmarks und sein Verhalten zur Chorda dorsalis bet Amphioxus lanceolatus. Verh. Biol. Vereins. (Biologiska Foreningens Forhandlingar.) Stockholm. Bd.IV. pp. 10-15. 9 figs. 1891. 53 RoubE, Emit. Aistologische Untersuchungen tiber das Nerven- system von Amphioxus lanceolatus. In Anton Schneider's Zoo- logische Beitrage. Bd. II., Heft 2. Breslau, 1888. pp. 169-211. Plates XV.-XVI. Standard work on the central nervous system of Amphioxus. 54 Ronon, JOSEF Victor. Untersuchungen iiber Amphioxus lanceolatus. Lin Beitrag zur vergleichenden Anatomie der Wir- 55 56 57 58 59 60 61 REFERENCES. 301 belthiere. In Denkschriften der Math.-Naturwiss. Classe der kais. Akad. der Wissenschaften. Bd. XLV. Wien, 1882. 64 pp. 4°. Six plates. Relates chiefly to nervous system. Describes also the smooth muscle-fibres in wall of pharynx, etc. Finds that the majority of sensory nerve-fibres to the skin end freely between the cells of the ectoderm in bush-like ramifications. For the rest, see NANSEN ROHDE, RETZzIUS, and FuSARI. Rovpu, W. Untersuchungen iiber den Bau des Amphioxus lanceolatus. Morphologisches Jahrbuch, I]. 1876. pp. 87-164. Taf. V.-VII.; also figures in text. RUCKERT, JOHANNES. Lutwickelung der Excretionsorgane. Ergebnisse der Anatomie und Entwicklungsgeschichte (Merkel und Bonnet), I. 1891. pp. 606-695. Includes an extensive bibli- ography. SCHNEIDER, ANTON. Settrage zur vergleichenden Anatomie und Entwicklungsgeschichte der Wrorbelthiere. lL. Amphioxus lanceolatus. pp. 3-31. Taf. XIV.-XVI. 4°. Berlin, 1879. SCHULTZ, ALEXANDER. Zur Entwickelungsgeschichte des Sela- chieretes. Archiv. fiir Mikr. Anat. XI. 1875. pp. 569-580. Taf. 34. Preliminary notes of both Semper and Schultz, regarding the segmental origin of the excretory tubules, were published in the Centralblatt fiir Medicinische Wissenschaft, 1874. SEMON, RICHARD. Studien tiber den Bauplan des Urogenital- systems der Wirbelthtere; dargelegt an der Entwickelung dieses Oreansystems bet Ichthyophis glutinosus. Jenaische Zeitschrift, XXVI. 1891. pp. 89-203. Taf. I1.-XIV. SPENGEL, J. W. Settrag zur Kenntniss der Kiemen des Amphi- oxus. Zool. Jahrbiicher. Abth. fiir Morphol. IV. 1890. pp. 257- 296. Taf. 17-18. SPENGEL, J. W. Benham’s Krittk metner Angaben iiber die Kiemen des Amphioxus. Anat. Anz. VIII. 1893. pp. 762-765. STIEDA, LupwiG. Studien iiber den Amphioxus lanceolatus. Mém. de l’Acad. Impériale des Sciences de St. Pétersbourg, 7th series, Vol. XIX. No.7. 7o pp. Four plates. 1873. Contains some good observations on the central nervous system. First to show that the split-like structure above central canal did not correspond to the posterior fissure of the vertebrate spinal cord, but was a portion of the original central canal itself, the lumen of which had been partially obliterated by approximation of its walls. First identification of ventral (motor) roots of spinal nerves in Amphioxus. 302 63 64 65 66 67 68 69 REFERENCES. THACHER, JAMES K. MAedian and Paired Fins ; a Contribution to the History of Vertebrate Limbs. Transactions Connecticut Academy, III. No. 7. 1877. pp. 281-310. Plates 49-60. WEISS, F. ERNEST. Excretory Tubules in Amphioxus lanceolatus. Quarterly Jour. of Micro. Sc. XXXI. N.S. 1890. pp. 489-497. Plates 34-35. VAN WIJHE, J.W. Ueber Amphioxus. Anat. Anz. VIII. 1893. pp. 152-172. VAN WIJHE, J.W. Due Kopfregion der Cranioten beim Amphi- oxus, nebst Bemerkungen iiber die Wirbeltheorte des Schédels. Anat. Anz. IV. 1889. pp. 558-566. VAN WIJHE, J. W. Ueber die Mesodermsegmente des Rumpfes und aie Entwicklung des Excretionssystems bet Selachiern. Archiv. f. Mikr. Anat. XXXII]. 1889. pp. 461-516. Taf. 30-32. WILLEY, ARTHUR. Lefort on a Collection of Amphioxus, made by Professor A. C. Haddon, in Torres Straits, 1888-89. Quarterly Jour. Micro. Sc. XXXV.N.S. January, 1894. pp. 361-371. One figure in text. Branchiostoma cultellum. Peters. III. DEVELOPMENT OF AMPHIOXUS. AYERS, HowarpD. Sdellostoma Dombeyi, Lac. A Study Srom the Hopkins Marine Laboratory. Biological Lectures, Marine Biological Laboratory, Woods Holl. 1893. No. VII. Boston, 1894. 69 cs. BERT, PAUL. On the Anatomy and Physiology of Amphioxus. Annals and Mag. of Nat. Hist., 3d Series. Vol. XX. 1867. pp. 302-304. (Translated from Comptes Rendus. Aug. 26th, 1867. pp. 364-367.) Breeding season of Amphioxus at Arcachon is from March to May. Was the first to observe the ejection of the sperm through the atriopore. Calls attention to remarkable lack of regenerative power in Amphioxus. Individuals cut in two will live for several days, but will not regenerate. “If the extremity of the body of an Amphioxus be cut off, the wound does not cicatrize; on the contrary, the tissues become gradually disintegrated. I have seen animals, with only the tail mutilated, become gradually eaten away up to the middle of the branchial region, and live thus without any intestines, without abdominal walls, and without branchiz for several days.” These observations of Paul Bert are “I bo “J wn 76 “I ws “I nN VEFERENCES, 39 Oo capable of easy confirmation, and should be borne in mind in view of the extraordinary regenerative power which Wilson dis- covered in the segmentation Bovert, THEODOR. Ger titte der Geschlechts- ariisen und ate Entstehung der Cae es beim Antphi- exus. Anat. Anz. VII. 1892. pp. 170-81. Twelve figures. DOHRN, ANTON. Studten sur Urgeschichte des Wrrbelthier- korpers. IMI. Die Entstehung und Bedeutung der Hy pophysts bet Petromyson Planert. Mitth. Zool. Stat. Neapel. IV. 882. DourN, ANTON. Studien, VII. Dre Thyreotdtea bet Petrosty- son, Amphtoxus und Tunicaten. Ib. VI. 1885. Dohrn lays unnecessary stress upon the fact that often in transverse section, especially in the anterior region of the pharynx, the endostyle of Amphioxus projects up into the cavity of the pharynx in the form of a convex lens-shaped ridge. This is merely due to the muscular contraction of the pharynx, which almost invariably takes place when Amphioxus is placed in a Killing reagent. It is, therefore, not an anatomical feature of any significance. DOHRN. ANTON. Studien, XI]. TZhyreotdea und Ay pobran- chialrinne, Spritslochsack und Pseudobranchialrinne bet Fischen, Ammocetes und Tuntkaten. 1b. VII. 1887. Donrx, ANTON. Studien, NIT. Oder Nerven und Gefiisse bet Ammocetes und Petromyson Planert. \b. VT. 888. FRORIEP, AuGuST. Extwickelungygeschichte des Kopfes Ergebnisse der Anat. und Ee (Merkel und Bonnet), I. Sgr. pp. 561-605. Eleven figures. Includes an extensive bibliography. HATSCHEK, BERTHOLD. Staaten iiber Entwicklung des Aint phr- oxus. Arbeiten a. d. Zool. Institute. Wein, 1881. 88 pp. Nine plates. HATSCHEK, BERTHOLD. Jttthetlungen tiber Amphrioxus. Zoologischer Anzeiger, VII. 1884. pp. 517-520. Olfactory pit, sense-organ of proral pit, anterior proral * nephridium.” HATSCHEK, BERTHOLD. Cer den Schichtenbau von Amphi- oxus. Anat. Anz. II. 1888. pp. 662-667. Five figures. ages of the embryo. ¥¢ Origin of sclerotome, ete. KASTSCHENKO, N. Zur Entwwicklungsgeschi embryos. Anat. Anz. III. 888. pp. 445-467 One of the first to bring forward definite embryological facts to prove that the anterior (prve-auditory) head-cavities of VAN WIJHE (Ueber die Mesodermsegmente. ete., des Selachierkoptes. Amster- 304 80 81 83 84 85 86 REFERENCES. dam, 1882) are not homodynamous with the true somites. He was followed in this respect by RABL (Theorie des Mesoderms. Morphologisches Jahrbuch, XV. 1889). KorscHELtT, E., und HEIDER, Kk. Lehrbuch der vergleichen- den Entwicklungsgeschichte der wirbellosen Thiere. 3a Heft. Jena, 1893. KOWALEVSKY, ALEXANDER. Lutwichlungsgeschichte des Am- phioxus lanceolatus. Mém. de Acad. Imp. des Sciences de St. Pétersbourg. VII. Series. T. XI. No. 4. 1867. Three plates. KOWALEVSKY, ALEXANDER. JVeitere Studien iiber die Ent- wicklungsgeschichte des Amphioxus lanceolatus, nebst einem Beitrage sur Homologie des Nervensystems der Wiirmer und Wirbelthiere. Arch. f. Mikr. Anat. XIII. 1877. pp. 181-204. Two plates. Among the definite discoveries communicated by Kowalevsky in these two memoirs may be mentioned the following: General features of segmentation and gastrulation, origin of mesoderm from archenteric pouches, unique method of formation of nerve-tube (see text), origin of notochord, neurenteric canal, asymmetrical origin of gill-slits and mouth, and zz fart the metamorphosis. KUPFFER, CARL VON. Die Entwicklung von Petromyzon Planert. Arch. f. Mikr. Anat. XXXV. 1890. pp. 469-558. Six plates. Origin of head-cavities, hypophysis, etc. KUPFFER, CARL VON. Dre Entwicklung der Kopfnerven der Vertebraten. Verhandl. Anat. Gesellschaft in Miinchen. 18or. pp. 22-55. Eleven figures. (Erganzungsheft zum Anat. Anz. VI. 1891.) Ammoceetes (see Fig. 92 in text). KUPFFER, CARL VON. Studien sur vergleichende Entwick- lungsgeschichte des Kopfes der Kranioten I. Die Entwicklung des Kopfes von Acipenser sturio an Medianschnitten untersucht. pp. 95. Nine plates. Seven figures in text. Jfinchen and Leipsig, 1893. Important contribution to the delimitation of the wall of the brain. On page 84 is a reconstruction of head-cavities of Am- moceetes (see Fig. 72). Figs. 21 and 22 in the plates repre- sent cerebral vesicle of Amphioxus. (Cf. Fig. 51.) LANKESTER, E. Ray, and WILLEY, A. Zhe Development of the Atrial Chamber of Amphioxus. Quarterly Jour. Micro. Sc. XXXI. 1890. pp. 445-466. Four plates. 87 88 89 go gl g2 93 REFERENCES. 305 LEUCKART, RUDOLPH, und PAGENSTECHER, ALEX. Unter- suchungen iiber niedere Seethiere. Amphioxus lanceolatus. Miiller’s Archiv f. Anat. u. Physiol. 1858. pp. 558-569. Taf. XVIII. Description of larvae of Amphioxus taken off Heligoland. Drew attention to larval asymmetry, and to the existence of the brain-ventricle (cerebral vesicle). In absence of knowledge of early development their interpretation of many of the structures (especially praoral pit, mouth, and _ gill-slits) was incorrect. Latter applies also to Schultze’s observations. Lworr, Basttius. Uber Bau und Entwicklung der Chorda von Amphioxus. Mittheilungen a. d. Zool. Station. Neapel. 1X. 1891. pp. 483-502. One plate. Consult this memoir for previous literature on histology of notochord. Lworr, BasiLtus. Ueber einige wichtige Punkte in der Ent- wicklung des Amphioxus. Biologisches Centralblatt, XI]. 1892. pp- 729-744. Eight figures. Notes absence of mesodermal “ pole-cells.”. From frequency of mitoses in dorsal ectoderm of gastrula, concludes that the material destined to form dorsal wall of archenteron, from which notochord and myoccelomic pouches arise, grows in from the ectoderm round dorsal lip of blastopore. Hence notochord and mesoderm are essentially derived from ectoderm! MARSHALL, A. MILNES. Vertebrate Embryology. London, 1893. Muuer, Jouannes. Uber die Fugendzustinde einiger See- thiere. Monatsbericht der k6nigl. preuss. Akad. der Wissen- schaften zu Berlin. 1851. pp. 468-474. First accurate description of larva of Amphioxus, p. 474. In 1847 Johannes Miiller obtained a young Amphioxus of 2} mm. at Helsingfors. He says that the appearance of the gill-slits was peculiar, in that there were two rows of slits in the pharyngeal wall, placed one above the other. In the upper row were /ve round slits, while the lower slits were vertically elongated and were fourteen in number. He adds that it was doubtful whether it represented the young “ Branchiostoma lubricum ” or belonged to a new species. Mutcer, WitnELM. Ueber die Hypobranchialrinne der Tunt- katen und deren Vorhandensein bet Amphioxus und den Cyklo- stomen. Jenaische Zeitschrift f. Naturwiss. VII. 1873. pp. 327-332. Piatt, Junta B. /urther Contribution to the Morphology of the Vertebrate Head. Anat. Anz. VI. 1891. pp. 251-265. 306 94 95 96 97 99 100 Iol 103 REFERENCES. Rasy, Carr. Uber die Differensierung des Mesoderms. Anat. Anz. III]. 1888. pp. 667-673. Eight figures. Discovery of the sclerotome-diverticulum in embryo of Pristiurus. Ricr, Henry J. Observations upon the Habits, Structure, and Development of Amphioxus lanceolatus. American Nat. XIV. 1880. pp. 171-210. Plates 14 and 15. Author was the first to find Amphioxus in Chesapeake Bay. With regard to development, he gives some fairly good figures of larvee, and observed some of the more obvious features of the metamorphosis, as already described by Kowalevsky. RUCKERT, JOHANNES. Ueber der Entstehung der Lxcretions- organe bet Selachiern. Arch. fiir Anat. u. Physiol. (Anatomische Abtheilung). 1888. pp. 205-278. Three plates. Contains also the discovery of segmental origin of gonads. SCHNEIDER, ANTON. Seitrdge sur vergleichenden Anatomie und Entwicklunesgeschichte der Wrrbelthiere, 11. Anatomie und fntwickl. von Petromyszon und Ammocates. 4°. Ten plates. Berlin, 1879. Figure of the ciliated grooves in pharynx of Ammoceetes, at page 84. SCHULTZE, MAx. Beobachtung junger Lexvemplare von Ampht- oxus. Zeit. f. Wiss. Zool. II]. 1851-2. pp. 416-419. Two larve from Heligoland. Good description of structure of notochord. vAN Wijk, J. W. Ueber Amphiorus. Anat. Anz. VIII. 1893. pp. 152-172. Wiiey, A. Ox the Development of the Atrial Chamber of Amphioxus. (Preliminary communication.) Proceedings of the Royal Society, XLVIII. 1890. pp. 80-89. Witiey, A. Zhe Later Larval Development of Amphioxus. Quarterly Jour. Micro. Sc. XXXII. 1891. pp. 183-234. Three plates. WILson, EpMuND B. On Afidtiple and Partial Devolopment in Amphioxus. Anat. Anz. VII. 1892. pp. 732-740. Eleven figures. In this and the following more detailed paper, the author describes and interprets a remarkable series of experiments on the artificial production of twins and dwarfs. Besides this, there are many important observations on the normal cleavage of the egg. WILson, EpMUND B. Amphiovus and the Mosaic Theory of Development. Journal of Morphology, VII. 1893. pp. 579- 638. Ten plates. 104 REFERENCES. 307 ZIEGLER, H. Ernst. Der Ursprung der mesenchymatischen Gewebe bet den Selachiern. Archiv f. Mikr. Anat. XXXII. 1888. Pp- 378-400. One plate. Independent discovery of sclerotome-diverticulum. (See Rabl.) IV. ASCIDIANS. For bibliography relating to the Ascidians, see Professor W. A. HERD- MAN’S Reports on the Tunicata collected during the ‘‘ Challenger” expedition — Parts J.-III. 1882-88; and also KoORSCHELT und HEIDER, “Lerhbuch der vergleichenden Entwicklungsgeschichte der wirbellosen Thiere.” Heft III. Jena, 1893. 105 106 107 108 109 IIo V. PROTOCHORDATES, ETC. AYERS, HowarRD. Concerning Vertebrate Cephalogenesis. Jour. Morph. IV. 1890-91. pp. 221-245. BATESON, WILLIAM. JAZemoirs on the Development of Balano- glossus. Quarterly Jour. Micro. Sc. Vols. XXIV.-XXVI. 1884-86. Brooks, W. K. The Systematic Affinity of Salpa in its Relation to the Conditions of Primitive Pelagic Life ; the Phylogeny of the Tuntcata ; and the Ancestry of the Chordata. Part II. of Monograph of the Genus Salpa. Johns Hopkins University. Baltimore, 1893. BURCKHARDT, RuDOLF. Dze Homologieen des Zwischenhirn- daches und thre Bedeutung fiir die Morphologie des Hirns bet niederen Vertebraten. Anat. Anz. 1X. 1894. pp. 152-155 and 320-324. Relates to neuropore of craniate Vertebrates. Author calls the lobus olfactorius impar of Kupffer, the recessus neuroporicus. CLAPP, CORNELIA M. Some Points in the Development of the Toad-jish (Batrachus Tau). Jour. Morph. V. 1891. pp. 494- pol. Observations on the double origin of mouth, made in 1889, not published in this paper. DavipoFF, M. von. Ueber den “Canalis neurentericus antertor bei den Asctdien.” Anat. Anz. VIII. 1893. pp. 301-303. 308 REFERENCES. III Dourn, ANTON. Studien zur Urgeschichte des Woarbelthier- korpers, I. Der Mund der Knochenfische. Mitth. Zool. Stat. Neapel. III. 1881-2. pp. 253-263. 112 FIELD, GEORGE W. The Larva of Asterias vulgaris. Quarterly Jour. Micro. Sc. XXXIV. 1892. pp. 105-128. 113 Fow.er, G. HERBERT. The Morphology of khabdopleura Normani Allman. Festschrift fiir Rudolf Leuckart. pp. 293-297. Leipzig, 1892. 114 HarM_ER, S. F. See M’INTOSH. 115 HERDMAN, W. A. Article ‘‘ Tunicata.” Ency. Brit. 9th ed., republished in “ Zodlogical Articles ” by Lankester, etc. 116 Huprecut, A. A. W. Article “ Nemertines... Ency. Brit. oth ed., republished in “ Zodlogical Articles” by Lankester, etc. 116 é¢s. HUBRECHT, A. A. W. On the Ancestral Form of the Chordata. Quarterly Jour. Micro. Sc. XXIII. 1883. pp. 349-368. For later works on this subject see Notes to Chap. V. 117 KUPFFER, C. VON. Lutwickelungsgeschichte des Kopfes. In Merkeland Bonnet’s Ergebnisse der Anatomie und Entwickelungs- geschichte, II]. 1893. pp. 501-564. 118 LANG, ARNOLD. Zum Verstandnis der Organisation von Cephalodiscus dodecalophus M’Int. Jenaische Zeitschrift f. Naturwiss. XXV. 1891. 11g LanG, ARNOLD. Ueber den Finfiuss der festsitzenden Lebens- weise auf die Thiere. Jena, 1888. 120 LANKESTER, E. Ray. Degeneration: a Chapter in Darwinism. Nature Series. London, 1880. Republished in “ The Advance- ment of Science; Occasional Essays and Addresses.” London, 1890. : 121 LANKESTER, E. Ray. A Contribution to the Knowledge of Rhabdopleura. Quarterly Jour. Micro. Sc. XXIV. 1884. pp. 622-647. 122 MacBripe, E. W. Zhe Organogeny of Asterina Gibbosa. Proceedings Royal Society. Vol. 54. 1893. pp. 431-436. 123 M’INTOSH, WILLIAM C. Report on Cephalodiscus dodecalo- phus, M’Intosh. ‘* Challenger” Reports. Zodlogy,XX. 1887. With Appendix by S. F. HARMER. 124 MorGan, T.H. Zhe Growth and Metamorphosis of Tornaria. Jour. Morph. V. 1891. pp. 407-458. 125 MorGan, T. H. The Development of Balanoglossus. Jour. Morph. IX. 1894. pp. 1-86. 126 PLATT, JuLIA B. Hurther Contribution to the Morphology of the Vertebrate Head. Anat. Anz. VI. 1891. pp. 251-265. Describes the double origin of mouth in Batrachus. REFERENCES. 309 127 POLLARD, H. B. Odservations on the Development of the Head i Gobius capito. Quarterly Jour. Micro. Sc. XXXV. 1894. PP. 335-352- 127 dis. POLLARD, H. B. The “ Cirrhostomial” Origin of the Head in Vertebrates. Anat. Anz. 1X. 1894. pp. 349-359. 128 RABL-RUCKHARD, H. Der Lobus Olfactorius Impar der Selachier. Anat. Anz. VIII. 1893. pp. 728-731. 129 SEDGWICK, ADAM. The Original Function of the Canal of the Central Nervous System of Vertebrata. Studies from Morph. Lab. Cambridge, II. 1884. pp. 160-164. 130 SEDGWICK, ADAM. JVotes on Elasmobranch Development. Quarterly Jour. Micro. Sc. XXXIII. 1891-92. pp. 559-586. Contains important observations on the first appearance of the mouth, and its relation to the pituitary body. 131 SEELIGER, OSWALD.* Studien zur Entwicklungsgeschichte der Crinoiden. (Antedon rosacea.) TZoologische Jahrbiicher. Abth. f. Anat. VI. 1892. pp. 161-444. 132 VAN WIJHE, J. W. Ueber den vorderen Neuroporus und die phylogenetische Function des Canalis Neurentericus der Wirbel- thiere. Zool. Anz. VII. 1884. pp. 683-687. 133 WILLEY, A. Studies on the Protochordata, 1-I/[. Quarterly Jour. Micro. Sc. XXXIV.-XXXV._ 1893. Contain further bibliographical references. INDEX. Acipenser sturio, 102, 129, 287. Acrania, 17, 46. AGASSIZ, A., 250, 251, 256. ALLMAN, 262. Ammocetes, 163-170, 173, 178, 182, 186, 282. ANDREWS, 39, 41. Annelid theory, 5, 79, 82, 97, 176, 282, 290, 293. Annelids, excretory system of, 78-82, 99. giant fibres of, 97, 103. nervous system of, 95-97. segmentation of, 4. vascular system of, 55. Antedon rosacea, 256, 268-269, 271. Anus, 14, 25, 118, 131, 187. Aorta, dorsal, 49, 50, 53. Aperture, buccal, 182. cloacal, 182, 183, 210. Appendicularia, 180, 236-239, 241, 277. Archenteron, IIo. Artery, branchial, 47, 50, 98, 139. genital, 98. Ascidians, pelagic, 181, 236. sessile, 181. Asterias vulgaris, 254, 270. Asterina gibbosa, 270, 271. Asymmetron lucayanum, 40, 41. Asymmetry, 155-162, 177. Atriopore, 14, 77, 105. Atrium (see also Cavity, peribranchial), 14, 22, 186, 195. development of, 75-78, 210-212. post-atrioporal extension of, 25. Audition, 44. AUDOUIN, 197. Auricularia, 251-253, 256, 268. Axis (see Relations, axial). AYERS, 18, 173. Balancers, 42. Balanoglossus, 29, 43, 98, 128, 221, 222, 231, 242-253, 259, 261, 264, 265, 274, 276. Balanoglossus, nervous system of, 244- 246. Kowalevskit, 248, 250. ‘upfrert, 248, 253. BALFOUR, 5, 38, 79, 175, 190, 203, 273, 283, 292. Band, adoral ciliated, 250. circumoral ciliated, 251, 256. longitudinal ciliated, 251. post-oral (circular) ciliated, 251, 256. Bands, mesodermic, 120, 217, 218. peripharyngeal, 34, 140, 145, 168-169, 179, 185, 195, 226. Bars, branchial (see Gill-bars). BATESON, 98, 221, 244, 245, 250, 259, 263, 291. Batrachus tau, 281. Bdellostoma, 173, 285. BEARD, 208, 281, 292. BEDDARD, 81. VAN BENEDEN, 187, I9I, 197, 200, 224, 291. BENHAM, 33, 42. BERT, 174. Bipinnaria, 251. Blastoccel, 108, 254, 255. Blastomeres, 107. Blastopore, 110, 112, 197. Blastula, 108, 197. Blood-sinuses, 191, 192. Blood-vessels, contractile, 47, 98. origin of, 122. Bodies, polar, 106. Body, pineal, 207. pitituary (see Hypophysis). Body-cavity (see also Coelom), 217, 220- 222, 247. préeoral, 128, 218. Bojanus, organ of, 194. Botryllus, 181, 240. | BOULENGER, 14. BOURNE, A. G., 81. BOVERI, 42, 48, 60, 98, 99, 100, 151, 177. Brachiolaria, 270. 311 312 Brain, 92, Ior. Branchiomery, 65, 132. Branchiostoma cultellum, 40. lubricum, 8. Breeding-season, 105. Brood-pouch, 215. BROOKS, 254, 277, 289. Bulbils, vascular, 48. BURCKHARDT, 284. Bury, H., 269. CALDWELL, 291. Canal, alimentary, 24, 111, 187, 196, 214, 235, 249, 264. neurenteric, 114, 118, 199, 202, 275. Capillaries, 49, 98. Capitellida, 81. Cartilages, buccal, 18, 147. labial, 18. Caulus, 266. Cavity, opercular, 22. peribranchial (see also Atrium), 22, 183, 186, 195, 209. peritoneal, 22. Cells, epithelio-muscular, Ig1. Cellulose, 182. Cenogenesis, 177. Cephalisation, 75, 89. Cephalochorda, 13. Cephalodiscus, 261-267, 280, 289. Chetognatha, 278. Ciona intestinalis, 203, 210, 215, 222, 224, 226, 229, 230-235, 240, 271, 288, 292, 293. Cirri, buccal, 12, 20, 145. Cladoselachida@, 44. CLAPP, CORNELIA, 281. Clavelina, 181, 185, 187, 200, 215, 225, 241, 288. Cleavage, 107, 197. polymorphic, 108. Ceeca, intestinal, 249, 261. Caciliani, 67. Ccoecum, hepatic, 24, 236. Coelom, 22, 26, 31, 33, III, I2I, 122, 220- 222, 247-248, 265, 266. perigonadial, 153, 177. ‘Coencecium, 263. Collar-pores, 98, 248, 265. Collar-region, 242, 264. Collector, 45, 165. Commissure, circumcesophageal, 96, 273, 280, Compression, bilateral, 15, 43, 115. INDEX. Contraction, peristaltic, 98, 192. Cordon ganglionnaire viscéral, 224. COSTA, 7, Io. Craniota, 17. Crinoidea, 268. Cross-bars, 28. CUNNINGHAM, J. T., 80. Cutis, 38, 41, 122. CUVIER, 3. Cyclostomata, 8, 10, 45, 208. Cyclostome, 46. Cynthia papillosa, 200. DAVIDOFF, 200. DEAN, B., 44. Degeneration, 5. Development, abbreviated, 214, 215, 239. adolescent period of, 149, 150. direct, 250. duration of larval, 149, 169, 203, 215- embryonic, 114, 201. larval, 117, 130. latent, 145, 160. precocious, 161, 212. Differentiation, sexual, 154. Dissepiments (see Septa). Distaplia magnilarva, 206, 225, 288. Distribution, 11, 40-41. Diverticula, anterior intestinal (see also Head-cavities), 115. DOHRN, 5, 30, 167, 173, 176, 178, 179, 280, 281, 282. Duct, mesonephric, 66. pronephric, 69, 78, 99. Dura mater, 87. Echinoderms, 250-256, 267-271, 291. Ectoderm, 24, 78. ciliated, 112, 113, 130, 175, 243, 257. definitive, 111. primitive, 110. EISIG, 45, 81, 94, 103, 293. Embryo, ciliated, 113, 214. ventral curvature of Ascidian, 201. EMERY, 67. Endoderm, definitive, 111. primitive, 110. Endostyle, 9, 24, 31, 39, 130, 138, 149, 150, 167, 177, 185, 195, 227, 229, 250. Enteroccel, 252, 254, 255. Lxnteropneusta, 242, Epiceele, 41. Epithelium, atrial, 33,59, 100, 209. coelomic, 33, 122, 220-222, INDEX. 313 Equilibration, 44, 205. Equilibrium, Io, 43. ERLANGER, 220. Evolution, parallel, 80, 247, 290. Eye of Ascidian tadpole, 102, 206. Eye, median, 18, 102, 130. myelonic, 207. pineal, 207-209. Eyes, paired, Io2. Fascia, 36, 123. FELIX, 99. Fertilisation, 106, 188. Fibres, giant, 92-94, 103. Miillerian, 94. of Mauthner, 94. supporting, 89. FIELD, G. W., 254. Fierasfer, 67. Fin, definitive caudal, 131. provisional caudal, 115. Fin-rays, 15. Fins, 15, 44. lateral, 38, 42. Fixation, organ of, 222, 229, 271, 280. FLEMMING, 99. Flexure, cranial, 92, 279. FOL, 239. Folds, medullary, 199. metapleural, 15, 38, 42, 43, 76, 132, 176. Follicle, tos. Food, 9, 39, 185, 249. FOWLER, G. H., 262, 266, 267. FRORIEP, 175. Function, change of, 176, 280. Funnels, atrio-ccelomic, 58, 98. brown (same as preceding). ceelomic (see also Nephrostomes), 62. FUSARI, 87, 163. Fusari, plexus of, 87, 178. FURBRINGER, 99. Ganglia, peripheral, 85, 88. spinal, 84, 103. Ganglion, Ascidian, 188, 224, 225. cerebral, 96, 270, 272-274. Ganglion-cells, 89, 91. bipolar, 95. giant, 92. multipolar, 92. GARSTANG, 240, 250. Gastrula, 110, 197. significance of, III. Gastrulation, Io9. GEGENBAUR, 249, 273. Germ-layers, primitive, 110, 114. Gill-bars, 28, 32-34. blood-vessels of, 48-49. Gill-pouches, 165, 166. Gill-slit, first, 117, 118, 132, 141, 166, 170— 172. Gill-slits (see also Stigmata), 17, 27, 100, 130-132, 135-138, 139, 148-149, 160, 173-174, 195, 229, 234, 243, 244, 264, 289. asymmetry of, 157-158. atrophy of, 140, 143, 149. Gland, club-shaped, 116, 117, 134, 138, I4I, 170-172, 176. Pyloric, 236. subneural, 188-191, 225. thyroid, 169-170. thymus, 29, 30. Glands, fixing, 204. Glomerulus, 64, 65, 69, I00, Gnathostome, 46. Gobius capito, 282. GoopsIR, 8. DE GRAAF, 208. Groove of Hatschek, 21, 51, 135. Groove, epibranchial, 226. hyperbranchial, 34, 39, 195. hyperpharyngeal (same as preceding). hypobranchial (see also Endostyle), 9, 167. medullary, 112, 198. pericoronal (see Bands, peripharyn- geal). peripharyngeal (see Bands, peripha- tyngeal). Gut, post-anal, 203, HAECKEL, 5, 46, III, 177. HANCOCK, Igo. HARMER, 263, 289. VAN HASSELT, 193. HATSCHEK, 4I, QI, 102, 103, 104, I12, IIS, 118,174, 175, 292. Hatschek’s nephridium, 172. Head-cavities of Ammoceetes, 129. of Amphioxus, 126-128. preemandibular, 128, 175, 279-280. of Sagitta, 277. Heart, 46, 51-53, 191, 192. recurrent action of, 193. HEIDER (see KORSCHELT and HEIDER). Heptanchus, 173. 314 HERDMAN, 183, 277, 293. Hermaphrodite, 187, 196. Hexanchus, 173. HJorT, 225, 293. HOCHSTETTER, 54. Hood, nerve-plexus of oral, 84, 178. oral, 12, 147, 150, 178. HUBRECHT, 258, 259, 260, 287, 291. HUXLEY, 20, 22, 41, III. Hypophysis, 160, 165, 178, 190, I1g1, 195, 225, 283-288, 290, 292. Ichthyophis glutinosus, 67. Infundibulum, 102, 283, 285. dnsects, compared with Vertebrates, 2-4. Involutions, atrial, 209, 241. JULIN, 187, 190, 197, 200, 224, 225, 226, 292. KASTSCHENKO, 175. Kidney, 65. KLINCKOWSTROM, 207. KGlliker's olfactory pit, 19. KOPPEN, 103. KORSCHELT and HEIDER, 178. KOWALEVSKY, 4, 104, 114, 174, 196, 216, 240. KROHN, 197, 250. KUPFFER, I0I, 102, 128, 129, 175, 283, 287. Lamella, post-oral, 264. Lamina, dorsal, 183, 185, 195, 226. terminalis, 284. Lamprey (see Petromyzon). LANG, 291. LANGERHANS, 21, 56, 98, IOI, 154. Lanice conchilega, 80. LANKESTER, 38, 41, 58, 62, 98, III, 237, 262, 266. LEUCKART, Ioo. LEYDIG, 4. Ligamentum denticulatum, 25, 63, 164. Limax lanceolatus, 7. Line, lateral, 21, 42-45. Liver, 24. Lobe, przeoral, 218, 222, 228, 229, 254, 267-280, 290, 292. procephalic, 272. Lobus olfactorius impar, 102, 283, 284. Locomotion, caudal, 103, 203. ciliary, 121. muscular, 121. Loimia medusa, 80. INDEX. Lumbricus, 79, 272. LWOFF, 175. Lymph-spaces, 15, 51. MACBRIDE, 271. Mantle, cellulose, 183. muscular, 183. MARSHALL, MILNES, 177. Maturation, 106. Mauthner, fibres of, 94. MAYER, PAUL, 99, I0o. Medulla oblongata, gr. Membrane, interccelic, 152. vitelline, 105. Merlucius, 67. Mesenchyme, 201, 217, 220-222, 261. Mesoderm, III, II4, 120, 122, 199-201, 221. Mesonephros, 66. Metamerism, 64, 132, 196, 246-247, 291. Metamorphosis, 136, 150, 215, 223, 250, 256. Metanephros, 66. METCALF, 293. METSCHNIKOFF, 251. MEYER, EDUARD, 80. MILNE-EDWARDS, 197. MINOT, I55. M'INTOSH, 263. Molgula, 194. Molgula manhattensis, 210, 232, 240. Moroav\, T. H., 232, 245, 247, 253, 256, 274. Mouth, 19, 117, 131, 143-144, 146, 150, 176, 178, 229, 276, 280-282. asymmetry of, 157-160. MULLER, FRITZ, 250. MULLER, J., 8, 18, 50, 56, 59, 250. MULLER, W., Io2, 167. Muscles, 34-37, 86, 122, I95, 203, 222, 235. Muscle-fibres, origin of, 121. Musculature (see Muscles). Myoccel, 121. Myotomes, 13, I50. Myxine, gill-slits of, 171. hypophysis of, 285. pronephric duct of, roo. NANSEN, 103. NASSONOFF, Igo. Nemertines, 249, 256-261, 272, 273. lateral nerves of, 259. medullary nerve of, 259, 260. Nephridium, 62, 79, 99, 261. INDEX. Nephrostomes, 65, 69, 72. Nerve-cord, ventral, 96, 259, 273, 289. Nerves, cranial, 85. motor, 86, 100. R. branchialis vagi, 163, 164. Rr. cutanei ventrales, 44. R. recurrens trigemini et facialis, 45. R. cutaneus quinti (same as preced- ing). R. lateralis trigemini (same as pre- ceding). R. dorsalis, 85, 103. R. lateralis vagi, 45, 259. R. ventralis, 85, 103. R. visceralis, 86. sensory, 86, spinal, 83. Nerve-tube (see Tube, medullary). Nervous system, origin of central, 111, IIg, 198. Neuropore, I9, 90, I15, 160, 199, 202, 223, 225, 283, 285, 287, 292. NORMAN, CANON, 262. Notidanide, 173. Notochord, 8, 13, III, II15, 124-126, 158, 161-162, 199, 216, 222, 244, 266, 286, 287, 290. Ontogeny, 177. Operculum, 264. Organs, renal, 55, 194. reproductive (see also Pouches, gonadic), 122, 151-155, 187-188, 246, 266. Otocyst, 205. Otolith, 10, 205, 224. Oviduct, 187. Ovum, 105. OWSJANNIKOW, Ioo. PAGENSTECHER, 100. Palingenesis, 177. PALLAS, 7. Paludina vivipara, 220. Papillze, adhesive, 204. renal, 56-57, 59. Pericardium, 191, 218. Pericheta, 81. Petromyzon, 93, 163, 169, 286. Phallusia, 203, 232, 292. Pharynx, 27, 183. Phylogeny, 177. Pigment, 18, 26, 33, 102, 130, 131, 134, 204. 315 Pigment-cells, 135. Pilidium, 272. Pit, olfactory, 19, 90, 145, 160, 165, 195, 283, 285, 292. preeoral, 51, 128, 135, 144, 148, 267. Plate, apical, 255-256, 269, 270, 272-274, 292. medullary, 113, 115, 118, 198. Plates, skeletal (endostylar), 32. PLATT, JULIA, 175. . Pleuronectid@, 3, 40, 162, 178. Plexus, branchial, 163, 164, 165. Pluteus, 268, 270. | Pole-cells, mesoblastic, 175. POLLARD, H. B., 282. Pontobdella, 81. Porus branchialis, 23. Pouches, archenteric, 114, 115, 120, 247, 248. gonadic, 13, 25, 40, 153-154. myoccelomic, 122. POUCHET, 82. Pristiurus, 99. Proboscis, 221, 242, 247, 257, 264. Proboscis-cavity, 247. Proboscis-pore, 128, 248, 253, 264. Proboscis-sheath, 258. Products, genital, 174. Pronephros, 66-75, 78. blood-vessels of, 63, 69, 74, 100. development of, 69, 78. Prostomium, 272. Protopterus, 14. Pyrosoma, 181, 236, 241. QUATREFAGES, 88, 174. RABL, 175. RABL-RUCKHARD, 284. Raderorgan, 21, 148. RATHKE, 8. Recessus opticus, 102. Rectus abdominis, 35. Relations, axial, 226-229. RETZIUS, 82, 100, 103. Rhabdopleura, 261, 262, 266, 267. Ridge, epibranchial, 226. Ridges, subatrial, 76. RITTER, 250. Rods, skeletal, 28. ROHDE, 100, IoI, 103. ROHON, 82, 86, 163, 165. ROLPH, 23, 41, 56, 86, 98. RUCKERT, 60, 99, 100, 154. 316 Sac, branchial (see also Pharynx), 183, 195, 227. Sagitta, 13, 277-278. SAINT-HILAIRE, I, 279. principles of, 2, 279. SALENSKY, 206. Salpa, 180, 182, 193, 236, 241. Sarcolemma, 36. SARS, G. O., 262. SAVIGNY, Igo. Schizoceel, 175. SCHMIDT, KARL, 182. SCHNEIDER, ANTON, 35, 38, 98, 100, 178. Sclerotome, 123, 175, 221. SEDGWICK, ADAM, I12, 289, 291. SEELIGER, 239-240, 269, 277. Segmentation (see Cleavage). Segmentation-cavity, 108. SEMON, 67. SEMPER, 5, 79, 99, 176. Sense-cells, 20, 21. Sense-organ of przoral pit (see Groove of Hatschek). Septa, 13, 37, 122. Sheath, notochordal, 38, 123. SHELDON, LILIAN, 293. Shield, buccal, 263. Skeleton, axial, 13. Snout, 115, 218. Somites, mesodermic, 115, 121. Spawning, Ios. Species of Amphioxus, 41. SPEE, GRAF, 99. SPENCER, BALDWIN, 207, 208, 209. SPENGEL, 38, 41, 248. Spermatozoa, 105. Spinal cord, 83, 222. central canal of, 89, 289. Spiracle, 173. Spiraculum, 23. Splanchnoceel, 122. Stage, critical, 149, 174. STANNIUS, 45. STIEDA, Ioo. Stigmata, 183, 195, 196, 227. formation of, 229-235. Stomodceum, 165, 209. Sympathetic system, 35, 86. Synapticula (see Cross-bars). Table, showing order of development of Ascidian and Amphioxus, 213. INDEX. | Tadpole, Batrachian, 14. | Tail of Ascidian tadpole, 201-204, 212, 222. Teleosteans, 45, 281. Tentacles, velar, 20, 195. Test, 182, 240. THACHER, 38. Thymus, 29. Tissue, connective, 37, 41, 122. mesenchymatous, 221. Tongue-bars, 28, 140, 142, 148, 231. Tornaria, 250-253, 255-256, 270, 274. Trochophore, 256, 272. Tube, medullary, 114, 120, 198, 274. neuro-hypophysial, 225. Tubercle, dorsal, 189, 225. Tuberculum posterius, 102. Tubules, excretory, 59-65, 72, 100, I22. mesonephric, 70, 177. pronephric, 67, 70, 78, 100. uriniferous, 65. Tunic (see Test). Ureter, 66. Urmund, IIo. Ussow, Igo. Vacuolisation of notochord, 125, 216, 240, 244. Vas deferens, 187, Vein, cardinal, 54. caudal, 54. hepatic, 49, 98. portal, 53, 98. sub-intestinal, 49, 53-55. Velum, 20, 50, 150, 178. Vesicle, cerebral, 90, 100, 204, 223, 224, 226. Water-pore, 253, 254. WEISS, F. E., 57, 59. VAN WIJHE, 39, 50, 51, 88,99, 128, 163, 164, 165, 178, 280. WILDER, BuRT G.,, 14. WILSON, E. B., 108, 174, 175, 292. WooDWaARD, A. S., 44. YARRELL, 8. | ZIEGLER, H. E,, 175. | Zoarces, 67. Columbia University Biological Series. EDITED BY HENRY FAIRFIELD OSBORN, Da Costa Professor of Biology in Columbia College. This series is founded upon a course of popular University lectures given during the winter of 1892-3, in connection with the opening of the new department of Biology in Columbia College. The lectures are in a measure consecutive in charac- ter, illustrating phases in the discovery and application of the theory of Evolution. Thus the first course outlined the de- velopment of the Descent theory; the second, the application of this theory to the problem of the ancestry of the Vertebrates, largely based upon embryological data; the third, the applica- tion of the Descent theory to the interpretation of the structure and phylogeny of the Fishes or lowest Vertebrates, chiefly based upon comparative anatomy ; the fourth, upon the problems of individual development and Inheritance, chiefly based upon the structure and functions of the cell. Since their original delivery the lectures have been carefully rewritten and illustrated so as to adapt them to the use of Col- lege and University students and of general readers. The vol- umes as at present arranged for include: I. From the Greeks to Darwin. By Henry Farrrisip OSBORN. II. Amphioxus and the Ancestry of the Vertebrates. By ArtHur WILLEY. III. Fishes, Living and Fossil. By Basnrorp Dean. IV. The Cell in Development and Inheritance. By Epuunp B. WILson. Two other volumes are in preparation. MACMILLAN & CO., 66 FIFTH AVENUE, NEW YORK. I. FROM THE GREEKS TO DARWIN. THE DEVELOPMENT OF THE EVOLUTION IDEA. BY HENRY FAIRFIELD OSBORN, Sc.D. PRINCETON, Da Costa Professor of Biology in Columbia College. Ready in September. This opening volume, “ From the Greeks to Darwin,” is an outline of the development from the earliest times of the idea of the origin of life by evolution. It brings together in a continu- ous treatment the progress of this idea from the Greek philoso- pher Thales (640 3B.c.) to Darwin and Wallace. It is based partly upon critical studies of the original authorities, partly upon the studies of Zeller, Perrier, Quatrefages, Martin, and other writers less known to English readers. This history differs from the outlines which have been pre- viously published, in attempting to establish a complete conti- nuity of thought in the growth of the various elements in the Evolution idea, and especially in the more critical and exact study of the pre-Darwinian writers, such as Buffon, Goethe, Erasmus Darwin, Treviranus, Lamarck, and St. Hilaire, about whose actual share in the establishment of the Evolution theory vague ideas are still current. TABLE OF CONTENTS. I. THE ANTICIPATION AND INTERPRETATION OF NATURE. II. Amone THE GREEKS. III. THE THEOLOGIANS AND NATURAL PHILOSOPHERS. IV. THe EVoLurionists oF THE EIGHTEENTH CENTURY. V. From Lamarck To St. HILAIRE. VI. Tue First HALF-cENTURY AND DARWIN. In the opening chapter the elements and environment of the Evolution idea are discussed, and in the second chapter the re- markable parallelism between the growth of this idea in Greece and in modern times is pointed out. In the succeeding chap- ters the various periods of European thought on the subject are covered, concluding with the first half of the present century, especially with the development of the Evolution idea in the mind of Darwin. Il. AMPHIOXUS AND THE ANCESTRY OF THE VERTEBRATES. BY ARTHUR WILLEY, B.Sc. LONo., Tutor in Biology, Columbia College ; Balfour Student of the University of Cambridge. Ready in September. The purpose of this volume is to consider the problem of the ancestry of the Vertebrates from the standpoint of the anat- omy and development of Amphioxus and other members of the group Protochordata. The work opens with an Introduction, in which is given a brief historical sketch of the speculations of the celebrated anatomists and embryologists, from Etienne Geoffroy St. Hilaire down to our own day, upon this problem. The remainder of the first and the whole of the second chapter is devoted to a detailed account of the anatomy of Amphioxus as compared with that of higher Vertebrates. The third chapter deals with the embryonic and larval development of Amphioxus, while the fourth deals more briefly with the anatomy, embryology, and relationships of the Ascidians; then the other allied forms, Balanoglossus, Cephalodiscus, are described. The work concludes with a series of discussions touch- ing the problem proposed in the Introduction, in which it is attempted to define certain general principles of Evolution by which the descent of the Vertebrates from Invertebrate ancestors may be supposed to have taken place. The work contains an extensive bibliography, full notes, and 135 illustrations. TABLE OF CONTENTS. INTRODUCTION. CHaprerR I. ANATOMY OF AMPHIOXUS. Il. Ditto. II]. DevELOPMENT OF AMPHIOXUS. IV. Tue ASCIDIANS. V. THE PROTOCHORDATA IN THEIR RELATION TO THE PROBLEM OF VERTEBRATE DESCENT. III. FISHES, LIVING AND FOSSIL. AN INTRODUCTORY STUDY. BASHFORD DEAN, PH.D. COLUMBIA, Instructor in Biology, Columbia College. This work has been prepared to meet the needs of the gen- eral student for a concise knowledge of the Fishes. It contains a review of the four larger groups of the strictly fishlike forms, Sharks, Chimaeroids, Teleostomes, and the Dipnoauns, and adds to this a chapter on the Lampreys. It presents in figures the prominent members, living and fossil, of each group; illustrates characteristic structures; adds notes upon the important phases of development, and formulates the views of investigators as to relationships and descent. The recent contributions to the knowledge of extinct Fishes are taken into special account in the treatment of the entire subject, and restorations have been attempted, as of Dinichthys, Ctenodus, and Cladoselache. The writer has also indicated diagrammatically, as far as generally accepted, the genetic relationships of fossil and living forms. The aim of the book has been mainly to furnish the student with a well-marked ground-plan of Ichthyology, to enable him to better understand special works, such as those of Smith Wood- ward and Giinther. The work is fully illustrated, mainly from the writer’s original pen-drawings. TABLE OF CONTENTS. CHAPTER “ 2 I. Fisaes. Their Essential Characters. Sharks, Chimaeroids, Teleo- stomes, and Lung-tishes. Their Appearance in Time and their Distribution. II. Tur Lampreys. Their Position with Reference to Fishes. Bdel- lostoma, Myxine, Petromyzon, Palaeospondylus. III. Tue SHark Group. Anatomical Characters. Its Extinct Members, Acanthodian, Cladoselachid, Xenacanthid, Cestracionts. 1V. Cuimazroips. Structures of Callorhyuchus and Chimaera. Squalo- raja and Myriacanthus. Life-habits and Probable Relationships. V. Teveostomes. The Forms of Recent ‘‘ Ganoids.” Habits and Dis- tribution. The Relations of Prominent Extinct Forms. Crosso- pterygians. Typical ‘‘ Bony Fishes.” VI. Tue Evo.ution oF THE Groups oF Fisnes. Aquatic Metamerism. Numerical Lines. Evolution of Gill-cleft Characters, Paired and Unpaired Fins, Aquatic Sense-organs. VIL. Tue DEVELOPMENT oF FisHEs. Prominent Features in Embryonic and Larval Development of Members of each Group. Summaries. aes tse ton So et > eae Breast ears haar Hat Strata aah} Peete Pays? mi Vey b uy Bite RaMissuth AM SER eh d Het Stes A Aap taht mate Ragan re foe ye Sea PRR Lye} sea om ae Rye irc mtaCaee Man eeas te tated ry NSC Pete vont eae