ATL
I
hp
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a
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Le; oS)
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a
a
Fic. 707.—Amphioxus lanceolatus. Embryo. A, from the side; B, in horizontal
section. ak, ectoderm; cz, neurenteric canal; dh, archenteron; 7k, endoderm ; mk, meso-
dermal folds ; 7,’neural tube ; wd, archenteron ; ws, first celomic pouch ; wsh, ccelomic cavity ;
V, anterior ; H, posterior end. (From Korschelt and Heider, after Hatschek.)
end of the archenteron: its final extension to the end of the
snout 1s a subsequent process.
New ccelomic pouches are formed in regular order from before
backwards, the embryo at the same time elongating and becoming
laterally compressed and pointed fore and aft. At the anterior end
the mouth (Fig. 708, 1) appears on the left side of the body as a
small aperture, which soon increases greatly in size. On the ventral
surface another small aperture, the first gill-slit (ks) makes its
appearance, and soon shifts over to the mght side: it forms a
direct communication between the pharynx and the exterior,
like the stigmata of Appendicularia (p. 21): there is at present no
trace of the atrium.
The anterior end of the archenteron has meanwhile grown out
o4 ZOOLOGY SECT.
into a pair of pouches, which become shut off as closed sacs: of
these the right gives rise to the ccelome of the head (/), the left
to a depression called the pre-oral pit (w), trom which the groove
of Hatschek is afterwards formed. On the floor of the Brecher
in the neighbourhood of the mouth, a depression appears, which
gives rise to a structure called the club-shaped gland (kh).
Posteriorly the neurenteric canal closes and the anus appears.
We left the mesoderm in the form of separate paired ccelomic
sacs, arranged metamerically in the dorsal region of the embryo.
The sacs increase in size, and extend both upwards and downwards,
each presenting a somatic layer (Fig. 706, D, mk?) in contact with
the external ectoderm, and a splanchnic layer (mk?) in contact
i
K \
Jit
Fic. 708.—Amphioxus lanceolatus. A, young larva; B, anterior end more highly
magnified. c, provisional tail-fin ; ch, notochord ; cn, neurenteric canal; «/, enteric canal ;
h, celome of head ; :, club-shaped gland ; k’, its external aperture ;
mi’. nerve-tube ; np. neuropore ; sv. sub-intestinal vein ; w, pree.
and Heider, after Hatschek.)
, first gill slit ; #7. mouth 3
ral pit. (From Korschelt
with the nervous system and notochord dorsally, and with the
enteric canal ventrally. At about the level of the ventral surtace
of the notochord, a horizontal partition is formed in each coelomic
sac (Fig. 706, D), separating it into a dorsal and ventral portion.
The dorsal section is distinguished as the protovertebra (is), and
its cavity as the myocwle or muscle-cavity: the ventral section
is called the lateral plate, and its cavity forms a segment of the
ccelome.
The ventral plates now unite with one another in pairs below
the enteric canal, their cavities becoming continuous: at the
same time the cavities of successive eel plates are placed
in communication with one another by the absorption of their
XU PHYLUM CHORDATA dD
adjacent (anterior and posterior) walls.
In this way the cavities of the entire
series of ventral plates, right and left,
unite to form the single unsegmented
ccelome of the adult, thei av walls giving
rise to the ecelomie epithelium.
At the same time the cells of the
splanchnic layer of the protovertebree
become converted into muscular fibres,
which nearly fill the myoceele,and give
rise to the myomeres: the myocommas
arise from the adjacent anterior and
posterior walls of the protovertebre.
An outpushing of the splanchnic layer,
at about the level of the ventral sur-
face of the notochord, grows upwards
between the myomere externally and
the notochord and nerve-tube intern-
ally: from the cells lining this pouch
the connective-tissue sheath of the
notochord and nervous system arises,
and perhaps also the fin-rays. From
the parietal layer of the protovertebree
arises the derm or connective tissue
layer of the skin. o
The larva increases in size,and be- “S 5
comes very long and narrow, with a
pointed anterior end and a provisional
caudal tin posteriorly (Fig. 709, ¢). As
growth proceeds, new segments are
added behind those already formed, the
notochord grows forwards to the an-
terior end of the snout, and the eye-
spot (au.) and olfactory pit appear, the
latter as an ectodermal pit which com-
municates with the neurocele by the
still open neuropore (mp.). The mouth
(m.) attains a relatively immense size,
still remaining on the left sicle.
Additional gill-shts appear behind
the one already mentioned: they all
make their appearance near the middle
ventral line, and gradually shift over
to the rmght side: at first they corre-
spond aaa the myomeres, so that the
segmentation of the phary nx 1s part
of the general metamerism of the
body. Altogether fourteen clefts are
l-slits.
tu, eye-spot 5
Korschelt
56 ZOOLOGY SECT.
produced in a single longitudinal series. Above, @.e. dorsal to
them, a second longitudinal series makes its appearance, contaimmng
eight clefts, so that at this stage there are two parallel rows of
gill-slits on the right side of the body, and none on the left. But
as growth goes on, the first or ventral series gradually travels over
to the left side, producing a symmetrical arrangement, and at the
same time the first slit and the last five of the first or definitively
left series close up and disappear, so that the numbers are
equalised on the two sides. At first each gill-sht is simple, but
before long a fold grows down from its dorsal edge, and, proceeding
ventrally, divides the single aperture into two: this fold is the
secondary or tongue lamella, the original bars of tissue between
the undivided shts becoming the primary lamellee.
While the development of the gill-slits 1s proceeding, the atrium
is in course of formation. Paired longitudinal ridges, the meta-
[cnn
41 _g ae,
\ wr
we
ae
Pic. 710.—Amphioxus lanceolatus. Ventralaspect of three larvee showing the develop-
ment of the atrium. ap. atriopore ; k, gill-slits ; /7. left metapleural fold ; in. mouth; rf. vight
metapleural fold ; w, pree-oral pit. (From Korschelt and Heider, after Lankester and Willey.)
pleural folds (Fig. 710, Uf. vf., Fig. 711, sf.) appear on the ventral
side of the body, behind the gill-slits, and gradually extend for-
wards, dorsal to the latter, their arrangement being very unsym-
metrical in correspondence with that of the clefts themselves.
On the inner face of each fold, i.e. the face which looks towards
its fellow of the opposite side, a longitudinal sub-atrial rudge
(Fig. 711, A, s/) appears, and, the two sub-atrial ridges meeting and
coalescing, a canal (B, 7) is formed immediately below the ventral
body-wall. This canal is the commencement of the atrium: it is
at first quite narrow, but gradually extends upwards on each side
(C, p) until it attains its full dimensions. It is open, at first, both
in front and behind: the posterior opening remains as the atrio-
pore: the anterior opening becomes gradually shifted forwards as
the fusion of the sub-atrial ridges proceeds (Fig. 710, Band C), and
is finally completely closed. In this way the gill-slits come to open,
XIII PHYLUM CHORDATA 57
not directly on the exterior, but into a cavity formed by the union
of paired ridges of ae body-wall, and theretore lined by ectoderm.
The mouth gradual] y passes to the ventral surface, and under
goes a relative aaa in size: a fold of integument develops
Found it, and forms the oral hood, which is probably to be looked
upon as a stomodeeum. The endostyle appears on the right of
the pharynx (Fig. 709, 7), and is at first rod-shaped, then V-shaped :
ultimately the limbs of the V unite in the middle ventral line.
The gill-shts imerease in number, and become more and more
vertically elongated. The provisional caudal fin disappears. The
gonads arise from the outer and ventral regions of the proto-
Fic. 711.—Amphioxus lanceolatus., Diagrammatic transverse sections of three larvae
to show the development of the atrium. «ao. aorta; c, dermis; d, intestine; jf. fas
wh, cavity for dorsal fin-ray ; m. myomere ; 7. nerve-tube ; p, atrium; sf. metapleural folds
si. sub-intestinal vein ; sh, sheath of notochord and neuron; s/. sub-atrial ridge ; sp. coelome.
(From Korschelt and Heider, after Lankester and Willey.)
vertebree in the form of pouches, which gradually assume their
permanent form. The development of the nephridia is not
known, but an organ, considered to be a provisional nephridium
(Fig. 709, x), is formed in the mesoderm of the first metamere,
and opens into the pharynx: it disappears in the adult.
Distribution.—Amphioxus has been found in the North
Atlantie and Mediterranean, on the west coast of North America,
the East Indies, the east coast of South America, Australia,
New Zealand, and the Malayan Islands. Asymmetron was first
known from the Bahamas, and a second closely allied species has
been found in the Louisiade Archipelago. As might be expected,
no fossil remains of the group are known.
58 ZOOLOGY SECT.
Distinctive Characters.—The Acrania may be defined as
Vertebrata in which the notochord extends to the anterior end of
the snout, in advance of the central nervous system. There is no
skull, and no trace of limbs. The ectoderm consists of a single
layer of cells which may be ciliated. The pharynx is of immense
size, perforated by very numerous gill-slits, and surrounded by an
atrium. The liver isa hollow pouch of the intestine. There is
no heart, and the blood is colourless. The nephridia remain dis-
tinct and open into the atrium. The brain is very impertectly
differentiated ; there are only two pairs of cerebral nerves : and the
dorsal and ventral spinal nerves do not unite. There are no paired
eyes, but there is a median pigment spot in the wall of the brain; the
auditory organ is absent. The gonads are metamerically arranged
and have no ducts. There is a typical invaginate gastrula, and
the mesoderm arises in the form of metameric ccelomic pouches.
The celome is an enteroceele.
Affinities.— Amphioxus has had a somewhat chequered zoologi-
cal history. Its first discoverer placed it among the Gastropoda,
considering it to be a Slug. When its vertebrate character was
made out, it was for a long time placed definitely among Fishes as
the type of a distinct order of that class, but it became obvious,
from a full consideration of the case, that an animal with neither
skull, brain, heart, auditory organs, nor paired eyes, with colourless
blood, with no kidneys in the ordinary sense of the word, and with
its pharynx surrounded by an atrium, was more widely separated
from the lowest Fish than the lowest Fish from a Bird or Mammal.
There was still, however, no real suspicion of “invertebrate ”
affinities until the development both of Amphioxus and_ the
Urochorda was worked out, and it was shown that in many
fundamental points, notably in the formation of the nervous
system and the notochord, there was the closest resemblance
between the two. The likeness was further emphasised by the
presence in both forms of an endostyle, an epipharyngeal groove,
and peripharyngeal bands, and of an atrium, and by the
obvious homology of the gill-slits of Tunicates with those of
Amphioxus. The Urochorda being obviously a degenerate group,
it was suggested that the peculiarities of the adult Amphioxus
might also be due to a retrogressive metamorphosis. Of this,
however, there is no evidence, and all recent investigations and
especially the discovery of the nephridia, have tended to bring the
Acrania nearer to the Craniate Vertebrata, and to remove them
urther from the lower Chordata.
DIVISION B.—CRANIATA.
The group of Craniate Vertebrata includes all those animals
known as Fishes, Amphibians, Reptiles, Birds, and Mammals, or, in
XI PHYLUM CHORDATA 59
other words, Vertebrata having a skull, a highly complex brain, a
heart of three or four chambers, and red blood-corpuscles.
In spite of the obvious and striking diversity of organisation
obtaining among Craniata, between, for instance, a Lamprey, a
Pigeon, and a Dog, there is a fimdamental unity of plan running
through the whole group, both as to the general arrangement of
the various systems of organs and the structure of the organs them-
selves, far greater than in any of the principal invertebrate groups.
The range of variation in the whole of the six classes included
in the division is, in fact, considerably less than in many single
classes of Invertebrata, for instance, Hydrozoa or Crustacea.
Hence, while the plan hitherto adopted of treating the group class
by class will be followed, it will be found convenient to begin by
devoting a considerable space to a preliminary account of the
Craniata as a whole, since in this way much needless repetition
will be avoided.
The Craniata include the following classes and sub-classes :—
Cuass I—Cycbosromava, f¢-11¥
Including the Lampreys and Hags.
Cuass II.—Piscrs, pp. | 2+
Including the true Fishes, which are again divisible into
Sub-class 1.—Hlasmobranchit, ps >,
Including the Sharks and Rays.
Sub-class 2.—Holocephalt, p.17 3.
Including only the Cat-fish (Chimera) and the Elephant-fish
(Callorhynchus).
Sub-class 3.—Teleostomt, jr.i% 3
Including the bony Fishes, such as Perch, Cod, Trout, &. and the
Sturgeons and their allies.
Sub-Class 4.—Dipnoi) [12 & 4
Including the Amphibious Fishes or Mud-fishes.
Cuass III.—Ampuipia, ),24 5,
Including Frogs, Toads, Newts, and Salamanders.
1 The animals included in Classes I and II are all ‘‘ Fishes” in the broad
sense of the word
60 ZOOLOGY SECT. XIII
Ciass IV.—REPpTILIA,
Including Lizards, Snakes, Crocodiles, Turtles, and Tortoises.
Ciass V.—AVES,
Including Birds.
Ciass VI.—MamMALIA,
Including Hairy Quadrupeds, Seals, Whales, Bats, Monkeys, and
Man.
External Characters.—The body of Craniata (Fig. 712) is
bilaterally symmetrical, elongated in an antero-posterior direction,
and usually more or less cylindrical. It is divisible into three
regions: the head, which contains the brain, the chief sensory
organs, and the mouth and pharynx; the trunk, to which the
ccelome is confined, and which contains the principal digestive and
circulatory as well as the excretory and reproductive organs; and
the tal, or region situated posteriorly to the ccelome and anus, and
containing no essential organs. Between the head and trunk
there is frequently a narrow region or neck, into which the ccelome
does not extend. In aquatic Vertebrates the tail is of great size,
not marked off externally from the trunk, and is the chief organ
of locomotion : in terrestrial forms it becomes greatly reduced in
diameter, and has the appearance of a mere unpaired posterior
appendage.
The mouth (mth.) is a transverse aperture placed at or near the
anterior end of the head. Near it, sometimes dorsal, sometimes
ventral in position, are the paired nostrils or anterior nares (na.)—
or in Cyclostomata the single nostril—leading to the organs of
smell. Farther back, on the sides of the head, are the large paired
eyes (e.), and on the dorsal surface there is sometimes more or less
indication of a vestigial median or pineal sense organ (pn. e.), which
may take the form of an eye. Posterior to the paired eyes are
the wuditory organs (au.), the position of which is indicated in the
higher forms by an auddtory aperture.
On the sides of the head, behind the mouth, are a series of
openings, the gill-slits or external branchial apertures (e. br. a. 1
—7): they are never more than seven in number, and in air-
breathing forms disappear more or less completely in the adult.
In the higher Fishes a fold called the operculum (Fig. 726, op.)
springs from the side of the head immediately in front of the
first gill-slit and extends backwards, covering the branchial
apertures.
On the ventral surface at the junction of the trunk and tail is
the anus (an.). Distinct urinary and genital apertures, or a single
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62 ZOOLOGY SECT.
urino-genital aperture, are sometimes found either in front of or
behind the anus, but more commonly the urinary and genital ducts
open into the termination of the enteric canal, or cloaca, so that
there is only a single egestive opening, known as the cloacal
aperture. On either side of this there may be a small abdominal
pore (ab. p.) leading into the ccelome.
In Fishes and some Amphibians, the trunk and tail are produced
in the middle dorsal line into a vertical fold or median fin, which is
continued round the end of the tail and forwards in the middle
line to the anus. Frequently this continuous fin becomes broken
up into distinct dorsal (d.f. Land 2), ventral (wv. f.), and caudal (c. 7.)
Jins, which may assume very various forms: in the higher classes
all trace of median fins disappears.
Fishes also possess paired fins. Immediately posterior to the
last gill-slit is a more or less horizontal outgrowth, the pectoral fin
(pet. f.), while a similar
but smaller structure,
the pelvic fin (pv. f.),
arises at the side of the
anus. In the embryonic
condition there is some-
times found to be a low
ridge (7.) connecting the
pectoral and pelvic fins
of each side with one
another, and from this
and other considerations
there is reason for think-
ing that the paired fins
are detached and en-
larged portions of a
continuous lateral _ fin,
having similar anatomi-
cal relations to the meta-
pleural folds of Amphi-
oxus.
In all Craniata above
Fishes, 2.c., from Am-
phibia upwards, the
Fic. 713.—Diagrammatie vertical section of the skin of a paired fins are replaced
Fish. 2, unicellular mucous glands; Co, derm; Ey. r a ‘ ye i @
epiderm ; F. fat; G@, blood-vessels; Ko, OBIS Calige by Joi e- and hind-limbs
Ko, granule-cells ; S, vertical, and W, horizontal bun- ( a / AAA Rae
dles of connective tissue. (From Wiedersheim’s (7. hi h. -)s each consist
Vertebrata.) ing of three divisions—
; upper-arm, fore-arm, and
hand in the one case; thigh, shank, and Soot in the ‘other. Both
hand and foot normally terminate in five fingers or digits, and
the pentaductyle limb thus formed is very characteristic of all the
>
NIM PHYLUM CHORDATA 63
higher Vertebrata. The paired fins or limbs, as the ease nay be,
are the only lateral appendages possessed by Vertebrates
Body-wall and Internal Cavities.— Tho body is covered
externally by a shin consisting of two layers, an outer or epithelial
layer, the epidermis (Fig. 713, Zp.), derived from the ectoderm of
the embryo, and an inner or con- Oo
nective-tissue layer, the dermis aN »
(Co), of mesodermal origin. The \ a a5
epidermis is always many-layered, Ws =
the cells of the lower layers,
forming the stratum Malpighti,
being protoplasmic and capable
of active multipheation, while
those of the superticial layers
often become flattened and hi rny,
and constitute the stratum cor-
neum. Glands are often present
in the skin in the form of tubular
or flask-shaped in-pushings of
the epidermis or of isolated gland-
cells (B).
Beneath the skin comes the
muscular layer. This is always
highly developed, and, in the
lower Craniata, has the same
general arrangement as in Am-
phioxus, 7c. consists of zig-zag
muscle-segments or = myomeres
(Fig. 714, mym.), separated from
one another by partitions of con-
nective tissue, or myocommas
(mye.), and formed of longitudin-
ally disposed muscle-fibres. The
myomeres are not placed at right
angles to the long axis of the
body, but are directed from the
3 pet. f. pectoral fin ;
kin in the middle of the body remov
a strip of sl
view of Dogfish (Must
sagittal plane outwards and back- S28
5 An
wards, and are at the same time (Ee
convex in front and concave be- agse
hind, so as to have a cone-in-
cone arrangement (Fig. 715, C).
Hach myomere, moreover, is
divisible into a dorsal (@. m.) and a ventral (v m.) portion. In
the higher groups this segmental arrangement, though present in
the embryo. is lost in the adult, the myomeres becoming converted
into more or less longitudinal bands, having an extremely complex
arrangement.
Fia.
64 ZOOLOGY SECT. XIID
In the trunk, as shown by a section of that region, the muscles
form a definite layer beneath the skin and enclosing the cwlome
(Fig. 715, A and C,cel.). The muscular layer, asin Amphioxus, 1s
not of even diameter throughout, but is greatly thickened dorsally,
so that the ccelome is,as it were, thrown towards the ventral side.
Its dorsal portion, moreover, is excavated by a canal, the newral or
cerebro-spinal cavity (c.s.c.), im which the central nervous system 1s
contained, and the anterior portion of which is always dilated, as
the cranial cavity, for the brain. Thus a transverse section of the
trunk has the form of a double tube. In the head, neck, and tail,
(B, D), the ccelome is absent in the adult, and the muscles occupy
practically the whole of the interval between the skin and the
skeleton, presently to be referred to: in the tail, however, there is
found a hemal canal (h. ¢.) containing connective tissue, and
representing a virtual backward extension of the cceelome. The
fins, or fore- and hind-limbs, are moved by longitudinal muscles
derived from those of the trunk. All the voluntary or body-
muscles of Craniata are of the striped kind.
The cceelome is lined by peritonewm (C, pr.), a membrane con-
sisting of an outer layer of connective tissue, next the muscles,
and an inner layer of ccelomic epithelium bounding the cavity,
and thus forming the innermost layer of the body-wall. In Fishes
the ccelome is divided into two chambers, a large wbdominal cavity
containing the chief viscera, and a small forwardly-placed peri-
cardial cavity (A. pe.) containing the heart, and lined by a de-
tached portion of peritoneum known as the pericardium. In
Mammals there is a vertical muscular partition, the diaphragm,
dividing the ccelome into an anterior chamber or thorax, containing
the heart and lungs, and a posterior chamber or abdomen containing
the remaining viscera.
Skeleton.—The hard parts or supporting structures of Craniata
fall into two categories, the exoskeleton and the endoskeleton. The
exoskeleton consists of bony or horny deposits in the skin, and
may be either epidermal or dermal, but is never, like the armour
of an Arthropod or the shell of a Mollusc, cuticular. The epidermal
exoskeleton is always formed by the cornification or conversion
into horn of epidermal cells, and may take the form of scales—as
in Reptiles—feathers, hairs, claws, nails, horns, and hoofs. The
dermal exoskeleton occurs in the form of either bony or horn-like
deposits in the derm, such as the scales and fin-rays of Fishes, and
the bony armour of the Sturgeon, Crocodile, or Armadillo. Some
recent researches tend to show that the dermal exoskeleton may
be ectodermal and not mesodermal in its ultimate origin.
The endoskeleton, or “ skeleton” in the ordinary ‘sense of the
word, forms one of the most complex portions of the body, and
presents an immense range of variation in the different classes and
orders. As in Amphioxus, the axis of the entire skeletal system
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ce Man. e. commen; ch. choroid (dotted) ; membhi ane (p. c.f) over the
C.P. ciliary processes ; v. ¢. epithelium of comea; ciliary processes and the
€. cj. conjunctiv f. 0. yellow spot; J. iris; J, y Cie 5 etn ate
ON. optic nerve ; OS. ora serrata ; o—, optic posterior face of the 1rl1s.
axis; p.c. R, anterior non- -visual portion of retina ; Tho 2 ee rate
Pe Bs a mented epithelium (black); &. retina ; The optic nerve ‘ (ON.)
sp. 1. Suspensory ligament; Sc/. sclerotic ; V. I, pierces the sclerotic and
vitreous. (From Foster and Shore’s Physiology.) .
choroid and becomes con-
tinuous with the retina, its
fibres spreading over the inner surface of the latter. Microscopic
examination shows that these fibres, which form the imnermost
layer of the retina (Fig. 740, 0. n.), turn outwards and become
connected with a layer of nerve-cells (7. ¢.). External to these
come other layers of nerve-cells and granules, supported by a
framework of delicate fibres, and finally, “forming the outer surface
of the retina proper, a layer of bodies called, “from their shape,
the rods and cones (7.). These are placed perpendicularly to the
surface of the retina, and their outer ends are imbedded in a
single layer of hexagonal pigment cells, loaded with granules of
the black eee Ales racy referred to.
Immediately behind and in close contact with the iris is the
transparent biconvex lens (Fig. 739, Z.), formed of concentric layers
of fibres each derived from a single cell. The lens is enclosed in
a delicate capsule, attached by a suspensory ligament (sp.l.) to the
ciliary processes. The suspensory ligament exerts a pull upon the
elastic lens so as to render it less convex than when left to itself;
when the ciliary muscles contract they draw the suspensory
ligament towards the ins and allow the lens to assume its normal
XIII PHYLUM CHORDATA 105
eurvature. It is im this way that the accommodation of the eye
to near and distant objects is effected.
The space between the cornea in front and the iris and lens
behind is called the anterior chamber of the eye, and is filled by a
watery fud—the agueous humour. The main cavity of the eye,
bounded in front by the lens and the ciliary processes and for the
rest OL its extent by the retina, is ealled the posterior chamber
and is filled by a gelatinous substance, the vi/reous humour ( Zz HL).
The cornea, aqueous, lens, and vitreous together constitute the
dioptric apparatus of the eve, and serve to focus the rays of light
nu’
gr
gr
0.10
iacram of the retina, the supporting structures to the left, the nervous and epithelial
right ; a—d. fibrous supporting structures ; 97. gr. gvanular layers ; 7.c. n.C.
2.0%. 2.0". TD cells; nv. nuclear layer of rods and cones; 0.7. fibres of optic nerve ;
7. rods and cones. (From Wiedersheim’s Vertebrata.)
Fic.
trom external objects on the retina. The iris is the diaphragm by
which the amount of light entering the eye is regulated. The
percipient portion or actual organ of sight 1s the retina, or, more
strictly, the layer of rods and cones. The great peculiarity of the
vertebrate eye, as compared with that of a Cephalopod (Vol. I,
p. 720), to which it bears a close superficial resemblance, is that
the sensory cells form the outer instead of the inner layer of the
retina, so that the rays of light have to penetrate the remaining
layers before affecting them.
106 ZOOLOGY SECT.
The mode of development of the eye is as characteristic as its
structure. At an early stage of development a hollow outgrowth—
the optic vesicle (Fig. 741, A, opt. v)\—is given off from each side of
the diencephalon (dien.). It extends towards the side of the head.
where it meets with an in-pushing of the ectoderm (inv. /.) which
deepens and forms a pouch, and finally, separating from the
ectoderm, a closed sac (B, /.) with a very small cavity and thick
walls. Thissacis the rudiment of the lens: as it enlargesit pushes
against the optic vesicle, and causes it to become invaginated (/),
the single-layered optic vesicle thus becomes converted into a two-
layered optic cup (opt. ¢., opt. c.), its cavity, originally continuous with
the diaccele, becoming obliterated. The invagination of the vesicle
to form the cup does not take place symmetically, but obliquely from
the external (posterior) and ventral aspect of the vesicle, so that
the optic cup 1s incomplete along one side where there is a cleft—
the choroid fissure—aftterwards more or less completely closed by the
wie
Fic. 741.—Early (A) and later (B) stages in the development of the eye of a Craniate.
dicen, Gencephalon ; inv. |. invagination of ectoderm to form lens; /. lens; opt. ¢. outer layer
of optic cup; opt. ec’. inner layer ; opt. st. optic stalk; opt. v. optic vesicle ; ph. pharynx ;
ply. pituitary body. (Altered from Marshall.)
union of its edges. The outer layer of the optic cup becomes the
pigmentary layer of the retina: from its inner layer the rest of that
membrane, including the rods and cones, is formed. The stalk of
the optic cup occupies, in the embryonic eye, the place of the optic
nerve, but the actual fibres of the nerve are formed as backward
growths from the nerve-cells of the retina to the brain.
During the formation of the lens, mesoderm grows in between
the pouch from which it arises and the external ectoderm: from
this the main substance of the cornea and its inner or posterior
epithelium are formed, the adjacent ectoderm becoming the
external epithelium. Mesoderm also makes its way into the optic
cup, through the choroid fissure, and becomes the vitreous. Lastly,
the mesoderm immediately surrounding the optic cup is differenti-
ated to form the choroid, the iris, and the sclerotic.
Thus the paired eye of Vertebrates has a threefold origin: the
sclerotic, choroid, iris, vitreous, and the greater part of the cornea
XIIL PHYLUM CHORDATA LOT
are mesodermal: the lens and external epithelium of the eormea
are derived from the ectoderm of the head: the retina ana optic
nerve are developed from a hollow pouch of the brain, and are
therefore, in their ultimate origin, ectodermal. The sensory cells
of the retina, the rods and cones, although not directly formed from
the external ectoderm, as in Invertebrates, are ultimately traceable
into the superficial layer of ectoderm, since they are developed
from the inner layer ot the optie vesicle, which is a prolongation
of the inner layer of the brain, which is continuous, before the
closure of the medullary groove, with the eetoderm covering’ the
general surtace of the body.
The eye-ball is moved by six muscles (Fig. 742). Four of these
arise from the inner wall of the orbit, and pass, diverging as they
go, to their insertion round the equator of the eye. One of them
1s dorsal in position, and is
called the superior rectus (s. 7.)
a second ventral, the inferior
rectus (in. 7.), a third anterior,
the anterior or internal rectus
(27.7.), and a fourth posterior,
the posterior or external rectus
(e.7). The usual names (in-
ternal and external) of the
two last-named muscles origin-
ate from their position in Man,
where, owing to the eye look-
ing forwards instead of out-
wards, its anterior surface be-
Fic. 742.—Muscles of the eye of a Skate and
comes internal, its posterior their nerves (semi-diagrammatic). ///, oeulo-
e a mens motor nerve ; /V, trochlear; V/, abducent.
surface external. The two re- e. 7, external rectus; in.o. inferior oblique ;
ini rom sela “176 ee in. 7”. inferior rectus; 7.7. internal rectus;
maining muscles usually aris OF Soinlio i oubiewetoNctpeniGnG Niue tear:
from the anterior (in Man superior rectus.
inner) corner of the orbit, and
are inserted respectively into the dorsal and ventral surface of the
eye-ball. They are the superior (s.0.) and inferior oblique (1. 0.)
muscles.
The median or pineal eye (Fig. 743), is formed, in certain cases,
from the distal end of the epiphysial diverticulum already men-
tioned. It has the form of a rounded capsule, the outer or
anterior portion of the wall of which is a lens (/.) formed of
elongated cells, while its posterior portion has the character ot
a retina (M,7). The latter has a layer of nerve fibres on its
outer, and one of rod-like visual elements (7.) on its inner sur-
face: it thus agrees with the usual types of Invertebrate retina,
and not with that of the paired eye.
The organ of hearing, like that of sight, presents quite peculiar
features. It arises in the embryo as a paired invagination of the
108 ZOOLOGY SECT.
ectoderm in the region of the hind-brain, a shallow depression being
formed which deepens and becomes flask-shaped, and finally, as a
rule, loses its connection with the external ectoderm, becoming
a closed sac surrounded by mesoderm. At first simple, it soon
becomes divided by a constriction into dorsal and ventral com-
partments. The dorsal compartment is differentiated into an
irregular chamber, the wériculus (Fig. 744, u.), and, usually, three
tubes. the semicircular canals. Of these two, the anterior (ca.)
Fic. 743.—Section of the pineal eye of Hatteria. 4g, blood-vessel ; , cavity of eye, filled with
fluid ; 2, connective tissue capsule ; 7. lens; If. molecular layer of retina; 7, layer of rods and
cones ; sf, nerve; 2, cells in nerve. (From Wiedersheim’s Vertebrata, after Baldwin Spencer.)
and posterior (up.) canals, are vertical in position and have their
adjacent limbs united so that the two canals have only three
openings between them into the utriculus: the third or eaternal
canal (ae.) 18 horizontal, and opens into the utriculus at either
end. Each canal is dilated at one of its ends into an ampulla
(ae., we., ap.), placed anteriorly in the anterior and external canals,
posteriorly in the posterior canal.
The ventral compartment of the auditory sac is called the
sveculus (s.): 1t gives off posteriorly a blind pouch, the cochlea (1),
XIII PHYLUM
CHORDATA Loo
which attains considerable dimensions in the higher classes, while
)
SG
— ass
Fic. 744.—External view of organ of hearing
of Craniata (semi-diagrammatic). aa,
ampulla of anterior canal; ae, of horizontal
canal; ap, of posterior canal; ass. apex of
superior utricular sinus ; ca, anterior semi-
circular canal ; ae, horizontal ; ap, posterior ;
cus, canal uniting sacculus with utriculus ;
de, endolymph atic duct; /, cochlea; rec.
utricular recess; s, sacculus; se, endo-
lymphatic sac; sp, posterior utricular
sinus; ss. superior utricular sinus; wv.
utriculus. (From Wiedersheim’s Vertebrata.)
from its inner face is given off
a narrow tube, the endolyin-
phatic duct (de.), which either
ends blindly or opens on the
dorsal surface of the head. The
utricle and sacculi are some-
times imperfectly differentiated,
and are then spoken ot te gether
as the vestibule.
Patches of sensory cells (Fig.
745, ae.) elongated cells pro-
duced into hair-like processes
(a. h.)—oceur in the ampulla
and in the utricle and saccule :
they are known as macula
acustice and criste acustica
(ce. 7.), and to them the fibres of
the auditory nerve (z.) are dis-
tributed. A fluid, the endo-
lymph, fills the whole of the
auditory organ, or membranous
labyrinth, and in it are formed
otoliths of varying size and
number. There is every reason
for thinking that the labyrinth,
as in the lower animals, func-
tions as an organ of equilibration as well as of hearing.
As the membranous labyrinth develops in the embryo it. be-
comes surrounded
and enclosed by
the auditory cap-
sule, the cartilage
of which adapts
itself to the form
of the labyrinth,
presenting a large
excavation for ie
utricle and _ sac-
cule and tunnel-
like passages for
the canals. The
auditory organ
does not, however,
i i 0 3. 745.—Longitudinal section through ance pallas ae. miditory
fit tightly mto Be Pa ouea a. h. auditory hairs ; ¢. part of pemieoulat cani ali
l syste of cr. crista acustica; ct. connective tissue; ¢.¢, epithehum, 1.
lus “J stem aerae uw. junction with utriculus. (From Foster and Shore's
cavities, but be- Physiology)
110 ZOOLOGY SECT.
tween it and the cartilage is a space, filled by a fluid called
perilymph, which acts as a buffer to the delicate organ floating
in it.
Urinogenital Organs.—In all Craniata there is so close a
connection between the organs of renal excretion and those of
reproduction that the two systems are conveniently considered
together as the urinogenital organs.
Speaking generally, the excretory organ consists of three parts,
all paired and situated along the dorsal wall of the ccelome; the
Jore-kidney or pronephros (Fig. 715, A, p. nph.), the mid-kidney or
mesonephros (ms. nph.), and the hind-kidney or metanephros (mt. nph.).
Each of these is provided with a duct, the pro- (pn. d.), meso-
(msn. d.), and imeta-nephric (mt. n. d.) ducts, which open into the
cloaca. The gonads (gon.) lie in the coelome suspended to its dorsal
wall by a fold of peritoneum: in some cases their products are
discharged into the ceelome and make their exit by abdominal
pores, but more usually the pronephric duct in the female
assumes the functions of an oviduct and the mesonephric duct in
the male those of a spermiduct. The pronephros is almost always
functionless in the adult, and usually disappears altogether. The
mesonephros is usually the functional kidney in the lower Craniata,
in which, as a rule, no metanephros is-developed, and the mesone-
phric duct, in addition to carrying the seminal fluid of the male,
acts as a ureter. In the higher forms the mesonephros atrophies,
and the metanephros is the functional kidney, the metanephric
duct becoming the ureter.
The Aidney—meso- or meta-nephros—of the adult is a massive
gland of a deep red colour made up of convoluted wrinary tubules
(Fig. 746), separated from one another by connective tissue con-
taining an abundant supply of blood vessels. The tubules are
lined by a single layer of glandular epithelial cells (B, C) and
each ends blindly in a globular dilatation, the Malpighian capsule
(A, gl.), lined with squamous epithelium. In many of the lower
Craniata, a branch goes off from the tubule, near the Malpighian
capsules, and, passing to the ventral surface of the kidney, ends
in a ciliated funnel-like body (Fig. 747, nst.), resembling the
nephrostome of a worm, and, like 1t, opening into the ccelome.
At their opposite ends the tubules join with one another, and
finally discharge into the ureter.
The renal arteries branch extensively in the kidney, and give
off to each Malpighian capsule a minute afferent artery (Fig. 746,
A, v.a.): this pushes the wall of the capsule before it, and breaks
up into a bunch of looped capillaries, called the glomerulus, sus-
pended in the interior of the capsule. The blood is carried off
from the glomerulus by an efferent vessel (v.¢.), which joins the
general capillary system of the kidneys, forming a network over the
Xu PHYLUM CHORDATA 111
urmary tubules: finally, the blood is returned from this network
to the renal vein. The watery constituents of the urine are
separated from the blood in ‘traversing the glomerulus, and
Howing down the tubule, take up and dissolve the remaining
constituents—urea, uric acid, &e—which are seereted by ie
cells of the tubules. ; ;
The development of the kidney reveals a resemblance to the
nephridia of worms which would hardly be suspected from its
adult structure. The pronephros (Fig. 747, A, p. nph.) originates
as two or three coiled tubes formed from mesoderm in hel body-
wall at the anterior end of the eelome: they are arranged meta-
merically and each opens into the ecelome by a ciliated funnel
B C
lie fi
SY peso
\ NG
5.
Fic. 746.—A, part of a urinary tubule with blood-vessels. a, artery ; g/, Malpighian capsule con-
glomerulus ; v. veinlet returning blood from capillary network (to the ght) to vein
vi; va, afferent vessel of glomerulus; re, efferent vessel. B, longitudinal, and C, transverse
sections of urinary tubules. a, secreting part of tubules; 6, conducting part of tubules ;
c. capillaries ; 7. nuclei. (From Foster and Shore’s Physiology.)
(nst.). Obviously such tubes are imesonephridia : their chief pecu-
liarity is that their outer ends do not open directly on the exterior,
but into a longitudinal tube, the archinephric or segmental duet
(sg. d.), which passes backwards and discharges into the cloaca.
It seems probable that this arrangement is to be explained by
supposing that the nephridia originally opened externally into a
longitudinal groove, which, by the apposition of its edges, was
converted into a tube. All three nephridia of the pronephros
open, by their ciliated funnels, into the narrow anterior end of
the ccelome, into which projects a branch of the aorta ending 1n
a single large glomerulus.
The pronephros soon degenerates, its nephridia losing their
connection with the segmental duct (B), but in the meantime
fresh nephridia appear in the segments posterior to the pro-
nephros, and together constitute the mesonephros or Wolfian body
112
ZOOLOGY SECT.
(B, ms. nph.) from which the permanent kidney is formed in most
of the lower Craniata. The mesonephric nephridia open at one
Fic.
A “5; ad
747.—Diagrams illustrating the development of the urinogenital organs of Craniata
A, development of pronephros and segmental duct; B, atrophy of pronephros, development
of mesonephros ; C, differentiation of pro- and meso-nephric ducts ; D, development of meta-
nephros, male type; E, female type. al. bl. allantoric bladder; an. anus; el. cloaca; gov.
yonad; int. intestine; m.c. Malpighian capsule; ms. n.d. mesonephric duct; ms. naplh-.
mesonephros ; it. n.d. metanephric duct; mt. nph. metanephros; nst. nephrostome; 0:
ovary ; p.2.d. pronephric duct; p. nph. pronephros ; sg. d. segmental duct; ¢. testis; ¢. c.
vasa efferentia.
end into the segmental duct (sy d.), at the other, by cihated.
funnels (nst.), into the coelome; a short distance from the funnel.
xm “PHYLUM CHORDATA 113
each gives off a blind pouch which dilates at the end and forms a
Malpighian capsule (a. ¢.), and a branch from the aorta entering it
gives rise to a glomerulus.
In some forms the archinephrie duct now becomes divided by a
longitudinal partition into two tubes: one retains its connection
with the mesonephros and is known as the mesonephrie or Wolfian
duet (C, ms.n.d.): the other has no connection with the nephridia, but
opens into the celome in the region of the vanishing pronephros :
it is the pronephric or Miillerian duct (p.n.d.). In some Craniata
the Miillerian appears quite independently of the Wolffian duct :
the latter is then simply the segmental duct after the union with
it of the mesonephric tubules.
In the higher Vertebrata, from Reptiles to Mammals, a diverti-
culum (D, E, mé. 2. d.) is given off from the posterior end of the
Wolffian duct, which grows forwards and becomes connected with
the hindmost nephridia. In this way is formed a metanephros
(mt. nph.), which becomes the permanent kidney, and a metane-
phrie duct (wt. 2. d.), which becomes the ureter. The Wolffian
body ceases to discharge a renal function, and becomes a purely
vestigial organ.
In many Fishes there is a dilatation of the ureter, the urinary
bladder, which serves as a receptacle for the urine. In the higher
Craniata the ventral wall of the cloaca sends off a pouch, the
allantore bladder (al. bl.), which serves the same purpose although
morphologically an entirely different structure.
The gonads (gon.) are developed as ridges growing from the
dorsal wall of the ccelome, and covered by ccelomic epithelium,
from the cells of which, as in so many of the lower animals, the
ova and sperms are derived. The testis consists of crypts or
tubules, lined with epithelium, and usually discharging their pro-
ducts, through delicate vasa efferentia (D, v. ¢.), into the Wolffian
duct, but in some groups into the caelome. The sperms are
always motile. The ovary is formed of a basis of connective
tissue or stroma, covered by epithelium, certain of the cells of
which become enlarged to form ova. In the majority of cases the
ova are discharged from the surface of the ovary into the open
ends of the Miillerian ducts (E, p. n. d.), which thus function simply
as oviducts, having no connection in the adult with the urinary
system. In-some groups the ova, like the sperms, are shed into
the ccelome and escape by the genital pores, and in many teleo-
stean or bony Fishes, the ovary is a hollow organ, as in Arthro-
poda, discharging its ova into an internal cavity, whence they are
carried off by a duct continuous with the gonad.
A few Craniata are normally hermaphrodite, but the vast
majority are dicecious, hermaphroditism occurring, however, occa-
sionally, as an abnormality.
In close connection with the urinogenital organs are found
VOL, TI J
1l4 ZOOLOGY SEOT.
certain “ductless glands,” the adrenals or supra-renal bodies. They
ave developed partly from ridges of the dorsal wall of the ccelome
i.¢., from mesoderm, partly from the sympathetic ganglia. There
may be numerous adrenals segmentally arranged, or a single pair.
Their function is quite unknown, but their abundant blood-supply
points to their possessing a high physiological importance.
Development.—The ova of Craniata are usually telolecithal, but
the amount of food-yolk varies within wide limits. When it is
small in quantity segmentation is complete but usually unequal,
when abundant, incomplete and discoidal. In the latter case the
embryo proper is formed, as in Cephalopods, from a comparatively
small portion of the oosperm, the rest giving rise to a large
yolk-sac.
There is never a typical invaginate gastrula, as in Amphioxus,
but in some of the lower Craniata a gastrula stage is formed by a
and later (B) embryos of Frog. cel. ccelome ; cel’. pro-
longation of ccelome into protovertebra ; ent. mesenteron ; med. gv. medullary groove 3 isd.
mesoderm ; 7ch. notochord ; pro. protovertebra ; sg. d. segmental duct; som. somatic layer of
mesoderm ; sp. c. spinal cord; sp/. splanchnic layer of mesoderm ; yk. yolk cells. (After
Marshall.)
Fic. 748.—Transverse section of earlier (A
combination of in-pushing and over-growth: the details will be given
in the sections on the various groups. In the higher forms a
gastrula cannot be recognised with absolute certainty.
The mode of development of the mesoderm and of the ccelome
differs strikingly from the process we are familiar with in Amphi-
oxus. Atan early stage the mesoderm is found in the form of
paired longitudinal bands (Fig. 748, A, isd.) lying one on each side
of the middle line, where they are separated from one another by
the medullary tube (md. gv.) and the notochord (nch.), and com-
pletely filling the space between the ectoderm and the endoderm.
In all probability the mesoderm is derived from both of the primi-
tive germ-layers. Each mesoderm band becomes differentiated
into a dorsal portion, the vertebral plate, bounding the nervous
XUI PHYLUM CHORDATA 115
system and notochord, and a ventral portion, the lateral plate,
bounding the mesenteron. The vertebral plate undergoes meta-
meric segmentation, becoming divided into a row of squarish
masses, the protovertebra or mesodernuil segments (B, pr. v.): the
lateral plate splits into two layers, a somatic (som.) adherent
to the ectoderm, a splanchnic (sp/.) to the endoderm. The space
between the two is the ccelome (ew7.), which is thus a schizocele or
cavity hollowed out of the mesoderm and is at no stage in com-
munication with the mesenteron, like the ccelomic pouches of
Amphioxus. A dorsal offshoot of the ccelome (cel’) may pass into
each protovertebra, but such an arrangement is temporary. From
the dorsal portions of the protovertebre the myomeres are formed,
from their ventral portions the vertebra.
The developmeut of the principal organs has been described, in
general terms, in the preceding account of the organs themselves :
it will be convenient to defer further consideration of this subject
until we come to deal with the development of the various types
of Craniata, and with the embryological characteristics of the
classes and sub-classes.
Distinctive characters.—The Craniata may be defined as
Vertebrata in which the notochord is not continued to the end of the
snout, but stops short beneath the fore-brain, some distance from its
anterior end. A skull is always present, and there are usually paired
limbs. The ectoderm is many-layered and is never ciliated in the
adult, and only rarely in the larva. The pharynx is of moderate
dimensions, and is perforated by not more than seven pairs of
gill-shts. There is no atrium. The liver is large, massive, and not
obviously tubular. There is a muscular chambered heart, and the
blood contains red corpuscles. The nephridia (mesonephridia) unite
to form large paired kidneys and open into ducts which discharge
into or near the posterior end of the intestine. The brain is com-
plex, and there are at least ten pairs of cerebral nerves: the spinal
nerves are, except in Cyclostomes, formed by the union of’ dorsal
and ventral roots. Paired eyes of great complexity, derived in
part from the brain, are present, and there is a pair of auditory
organs. There is a single pair of gonads, and the reproductive
products are usually discharged by ducts derived from the nephri-
dial system. There is never a typical invaginate gastrula, and
the mesoderm arises in the form of paired longitudinal bands which
subsequently become segmented. The ccelome is a schizoccele.
CLASS I —CYCLOSTOMATA.
The Cyclostomata, or Lampreys and Hags, are eel-like Fishes,
distinguished from all other Craniata by the possession of a
suctorial mouth devoid of functional jaws, by the single olfactory
organ, and by the absence of lateral appendages or paired oe
i
116 ZOOLOGY SECT.
1. EXAMPLE OF THE CrLass.—THE LAMPREY (Petroniy201).
Three species of Lamprey are common in the Northern Hemi-
sphere: the Sea-lamprey (P. marinus), which attains a length of a
metre; the Lampern, or common fresh-water Lamprey (2. flucia-
tilis), about 60 cm. in length; and the Sand-pride, or lesser
fresh-water Lamprey (P. branchialis), not exceeding 30 cm. in
length. In the Southern Hemisphere the Lampreys belong to two:
genera: Mordacia, found on the coasts of Chili and Tasmania, and
(feotria, in the rivers of Chili, Australia, and New Zealand. Both
genera differ from Petromyzon in minor details only.
External characters.—The head and trunk (Fig. 749) are
nearly cylindrical, the tail-region compressed or flattened frony
B
AAA y
Pic. 749.—Petromyzon marinus. Ventral (A), lateral (B), and dorsal (C) views of the head.
1. 1, first gill-cleft ; buc. 7. buccal funnel; eye, eye; mth. mouth ; na. ap. nasal aperture =
apille ; pv. pineal area; ¢1. 7/2. 8. teeth of buccal funnel; #4. teeth of tongue. (After
Pp. Pay
W. K. Parker.)
side to side. At the anterior end, and directed downwards, is a
large basin-like depression, the Duccal funnel (buc. f.), sarrounded
with papille (p.) and beset internally with yellow, horny ¢ecth
(18). At the bottom of the funnel projects the end of the
tongue (t*), also bearmg teeth, and having immediately above it
the narrow mouth (mth.). On the dorsal surface of the head is the
single median nostril (na. ap.), and immediately behind it a trans-
parent area of skin (pz.) indicates the position of the pineal organ.
The paired eyes have no eyelids, but are covered by a transparent
area of skin. The gill-slits (br. cl. 1) are seven pairs of small aper-
tures on the sides of the head, the first a little behind the eyes.
On the ventral surface, marking the junction between trunk and
XU PHYLUM CHORDATA 117
tail, is the very small anus (Fig. 758, «.), lying in a slight depres-
sion and having mmediately behind it a small papilla pierced at
its extremity by the wrinogenital aperture (2). It has been sug-
gested that a pair of ridges, lying one on each side of the anus,
represent vestiges of pelvic fins; otherwise there is no trace of
paired appendages. Two dorsal fins and a caudal fin are present,
the second dorsal being continuous with the caudal,
Lampreys live on small Crustacea, Worms, and other aquatic
organisms, but also prey upon Fishes, attaching themselves to the
bodies of the latter by the sucker-like mouth, and rasping oft
their Hesh with the armed tongue. They are often found holding
on to stones by the buecal funnel, and under these circumstances
perform regular respiratory movements, the branchial region ex-
panding and contracting like the thorax of a Mammal. The
be brbs C2
oe Te Se
olfc
pde naap \,
brbi
f [
50.—Petromyzon marinus. Skull, with branchial basket and anterior part of verte-
bral column. The cartilaginous parts are dotted. a. d. c. anterior dorsal cartilage ; a. lat. ec.
anterior lateral cartilage ; an. c. annular cartilage; au. c. auditory capsule ; br. b. 1—7, verti-
cal bars of branchial basket ; br. c/. 1—7, external branchial clefts ; cn. c. cornual cartilage ;
er. 7, cranial roof; /. c. I—4, longitudinal bars of branchial basket ; /y. ¢. lingual cartilage ;
i. v. c, median ventral cartilage; na. ap. nasal aperture ; nch. notochord ; Nv. 2, foramen for
optic nerve; o/7. ¢. olfactory capsule; pe. c. pericardial cartilage; p. /. c. posterior dorsal
cartilage ; p. /at. c. posterior lateral cartilage ; sb. oc. a. sub-ocular arch ; st. p, styloid process ;
sty. c. styliform ca age; ¢. teeth. (After W. K. Parker.)
reason of this is that when the animal is adhering by the mouth
the respiratory current cannot take its usual course—entering at
the mouth and leaving by the gill-shts—but is pumped by
muscular action both into and out of the branchial apertures.
The skin is soft and slimy, mottled greenish-brown in P. marinus,
bluish above and silvery on the sides in the fresh-water species.
‘The epiderm contains unicellular glands, the secretion of which
gives its slimy character to the skin. The segmental sense organs
take the form of a double lateral line and of minute pits on the
head. There is no trace of exoskeleton.
Skeleton.—The «zia/ skeleton of the trunk is very simple.
There is a persistent notochord (Fig. 750, neh.) with a tough
sheath composed of an inner fibrous and an outer elastic layer.
Attached to the sides of the notochord are little vertical rods of
cartilage (7. 7.) arranged segmentally and bounding the spinal
118 ZOOLOGY SECT.
canal on each side: they are rudimentary neural arches. For the
rest of its extent the spinal canal is enclosed only by tough,
pigmented connective tissue.
The cranium also exhibits a very primitive type of structure.
Its floor is formed by a basal plate (Fig. 751, 0. pl.), made by the
union of the parachordals and trabecule, and surrounding pos-
teriorly the fore-end of the notochord. Immediately in front of
the termination of the notochord is a large aperture, the basi-
cranial fontanelle (b. cr. f.), due to the non-union of the posterior
ends of the trabecule ; through it passes the pituitary pouch, pre-
sently to be referred to (Fig. 754), on its way from the olfactory sac
to the ventral surface of the notochord. Lateral walls extend
upwards from each side of the basal plate, but the roof of the
cranium is formed by membrane except at one point, where a
narrow transverse bar (cr. 7.) extends across between the side-walls.
and furnishes a rudimentary roof. United with the posterior end
of the basal plate are the auditory capsules (au. c.), and the side-
walls are pierced with apertures for the cerebral nerves (Nv. ?,
Nv. 5, Nv. &.).
So far the ’skull is thoroughly typical, though in an extremely
simple or embryonic condition; the remaining parts of it differ a
good deal from the ordinary structure as described in the preceding
section, and are in many cases very difficult of interpretation.
The olfactory capsule (olf. c.) is an unpaired concavo-convex plate
which supports the posterior wall of the olfactory sac and is pierced
by paired apertures for the olfactory nerves. It is unique in being
united to the cranium by fibrous tissue only.
Extending outwards and downwards from each side of the basal
plate is an inverted arch of cartilage, called the sub-ocular arch
(Figs. 750 and 751, sb. oc. «.), from the fact that it affords a support:
to the eye. From its posterior enda slender stylotd process (st. p.)
passes directly downwards and is connected at its lower end with a
small cornuwal cartilage (en. ¢.). In all probability the sub-ocular \
arch answers to the palato-quadrate or primary upper jaw, the |
styloid and cornual cartilages to the main part of the hyoid arch. \
In close relation with the angle of the sub-ocular arch is an up-
wardly directed plate, the posterior lateral cartilage (p.lat.c.), which
probably answers to the primary lower jaw, or Meckel’s cartilage.
Connected with the anterior end of the basal plate is the large
bilobed posterior dorsal cartilage (p. d.c.); it appears to be formed
from the united anterior ends of the trabecule. Below and pro-
jecting in front of it is the anterior dorsal cartiluge (a. d. ¢.), which
is probably homologous with the upper labial cartilage of some
Fishes and Amphibia (see below). Also belonging to the series of’
labial cartilages are the paired anterior lateral cartilages (a. 1. ¢.)
and the great ring-shaped annular cartilage (an. c.) which supports.
the edge of the buccal funnel.
XIII PHYLUM CHORDATA 119
The tongue is supported by a long unpaired lingual cartilage
(Fig. 750, ly. ¢.), which probably answers to the» basi-hyal or
€70.€
Fic. 751.—Petromyzon marinus. Dorsal (A), ventral (B), and sectional (C), views of skull.
The cartilaginous parts are dotted. a. ¢. ¢, anterior dorsal cartilage ; wn. c. annular cartilage ;
au. ¢. auditory capsule; b. cr. 7. basi nial fontanelle; b. p/. basal plate; cn. c. cornual
cartilage ; cr. 7. cranial roof; na. ap. nasal aperture ; neh. notochord *, 1, olfactory nerve ;
Nr. 2,5, and 8, foramina for the optic, trigeminal, and auditory nerv , fifth nerve ;
olf. c. olfactory capsule; p. d. ¢. posterior dorsal cartilage ; p. (at. c. posterior lateral cartilage >
sb. oc. a. sub-ocular arch ; st. p. styloid process. (After W. K. Parker.)
median ventral element of the hyoid arch of other Cramiata (see
p- 71); it is tipped in front by a small median and a pair of still
smaller lateral cartilages. Below it is a slender T-shaped median
120 ZOOLOGY SECT.
ventral cartilage (m.v.c.), which may possibly be the median ventral
element of the mandibular arch. Lastly, attached to each side of
the annular cartilage and passing backwards and downwards, are a
pair of tapering, rod-like styliform cartilayes (sty. ¢.).
The visceral skeleton also differs in a remarkable manner from
the ordinary craniate type. It consists of a branchial basket.
formed, on each side, of nine irregularly curved vertical bars of
cartilage (Fig. 750, br. b. 1—9), the first placed almost imme-
diately posterior to the styloid cartilage, the second imme-
diately in front of the first gill-cleft, the remaining seven just
behind the seven gill-clefts. These bars are united together by
four longitudinal rods (Je. 1—4), of which one lies alongside the
notochord and is connected in front with the cranium, two others
are placed respectively above and below the gill-clefts, while the
fourth is situated close to the middle ventral line and is partly
fused with its fellow of the opposite side. The posterior vertical
bar is connected with a cup-like cartilage (pe. ¢.), which supports
the posterior and lateral walls of the pericardiitm. The whole
branchial basket lies external to the gill-pouches and branchial
arteries, not, like typical visceral arches, in the walls of the
pharynx.
The median fins are supported by delicate cartilaginous fin-rays
or pterygiophores, which are more numerous than the myomeres,
and lie parallel to one’ another in the substance of the fin, extending
downwards to the fibrous neural tube. , ‘
The muscles of the trunk and tail are arranged in myomeres
which take a zigzag course. In the branchial region they are
divided into dorsal and yentral bands which pass respectively
above and below the gill-slits. A great mass of radiating muscle
is inserted into the buccal funnel, and the tongue has an ex-
tremely complex musculature. .
Digestive Organs.—The ¢eeth are laminated horny cones:
beneath them lie mesodermal papille covered with ectoderm
which bear a superficial resemblance to the germs of true calcified
teeth. The mouth leads into a buccal cavity (Fig. 752, m.) formed
from the stomodeum of the’ embryo, and communicating behind
with two tubes placed one above the other: the dorsal of these is
the gullet (es.), the ventral the respiratory tube (r.t., see below):
guarding the entrance to the latter is a curtain-like fold, the
velum (vl.). The gullet bends over the pericardium and enters
the intestine (int.) by a valvular aperture. The intestine passes
without convolutions to the anus: its anterior end is slightly
dilated and ig the only representative of a stomach: its posterior
end is widened to form the rectum (Fig. 758, r.). The whole
of the intestine is formed from the mesenteron of the embryo,
and the blastopore becomes the anus, there being no proctodeeum.
The lumen of the intestine is semilunar, owing to the presence
PHYLUM CHORDATA {21
XIII
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) or nephridium, with a single
Malpighian capsule (¢),into which
a branch
enters and forms a glomerulus.
Myxine is hermaphrodite and
protandrous—ze.,
young
sperms, and at a later period
to ova.
from the aorta (d)
the
animal
gonad of
produces
The eggs of
both genera are of ereat pro-
Fic. 763. —Auditory organ of Myxine.
ampulls s, endolym-
Cc. Sea canal ; uti.
(After Retzius.)
end.
winp., ainp.’
phatie sac
a utriculo
acculus.
portional
Myxine are enclosed when laid
size,
and those of
in a horny shell bearing numer-
ous hooked processes at each pole:
are entangled together, and probably
also attached to seaweed, The develop-
ment of the Hags is not known.
4.—GENERAL REMARKS.
The Lampreys and Hags are undoubt-
edly the lowest of cramiate Vertebrata
but are im many respects so highly
specialised that it is a matter of great
diticulty to determine their Brant iee
with the remaining classes. The struc-
ture of the verte ye column and of the
cranium are undoubtedly primitive in
the extreme :
what may be called the accessory por-
tions of the skull, such as labial cartilages,
they show a singularly high
specialisation. The emciial packet 1s
quite su generis, the theory that its ver-
tical bars are true branchial arches, dis-
placed outwards during development,
being quite unproved.
functional Jaws is very remarkable, seeing
that in the remaining Craniata these
structures always bound the mouth at a
period when the skull is in the stage of
development in which it remains perma-
nently in Cyclostomes: it is quite pos-
sible that their functionless condition
may be due to degeneration accompany-
ing the evolution of a suctorial mouth.
but in the development of
degre eof
The absence of
by means of these the eggs
1G. 764.—A, portion of kidney of
Bdellostoma. B, segment
of same, highly magnified, «a,
ureter; 4, urinary tubule; ec,
Malpighian capsule ; d, afferent
artery; ¢, efferent artery.
(From Gegenbaur'’s Comparatir:
Anatomy.
XT PHYLUM CHORDATA 133
The brain, in spite of its small size, is In some respects—notably
in the presence of cerebral hemispheres—of a more advanced
type than that of some of the true Fishes. The circumstance
that the pituitary pouch perforates the
skull-floor from above and becomes early
associated with the olfactory sac, 1s
unique among the Vertebrata. The
kidney of Bdellostoma is of the most
primitive type, and the presence of a
large pronephros is a significant archaic
character. The total absence of hmbs
may be a result of degeneration.
The geographical distribution of the class
is interesting from the fact that each order 11
contains some genera which are mainly oO
northern, others which are exclusively a
6 = on
southern. Petromyzon is found on the Oy
—W)
coasts and in the rivers of Europe, North
America, Japan, and West Africa; it 1s
therefore mainly Holaretic. Ichthyomyzon
is found on the western coasts of North
America, Mordacia in Tasmania and Chilt,
Geotria in the rivers of Chili, Australia,
roadie
and New Zealand. Myxine occurs in the a
North Atlantic and on the Pacific Coast BE
of South America; Bdellostoma on the ay
coasts of South Africa, New Zealand, ah
and Chili. EX
Until quite recently no undoubted YA A
fossil remains of Cyclostomes have been ZEN\
known, but there is some reason to believe Z ANN
that a little fossil fish, Palwospondylus ZAM
gunni (Fig. 765), lately discovered in the Zeal
Devonian rocks of Scotland, is referable Zi
to this class. It is about an inch long Za
and shows two regions, the cranium and
the vertebral column; there is no trace
of jaws, branchial basket, or limbs. The 4
vertebral column is composed of calcified — j,., 765,—palzeospondylus
centra with neural arches; he#mal arches gunni (magnified). ¢. ami;
E i : p.a. parachordal and auditory
are present in the caudal region; the Beuion se grcbenule ree
5 + i é : rion ; «. backward processes
structure of this part of the skeleton 1s of skull. (After Traquair.)
thus of a distinctly higher type than in re-
cent Cyclostomes, and lends support to the view that the latter are
degenerate. There is a caudal fin supported by forked rays. The
cranium consists of an anterior, probably trabecular, region (Ca),
and of a posterior region (jp. d.) which seems to answer to the
134 ZOOLOGY SECT.
parachordals and auditory capsules. Just in advance of the anterior
region is a ring-shaped opening surrounded by cirri (c.), and con-
sidered to be the nasal aperture. The posterior region gives off
paired plates (2.) which may perhaps correspond with the dorsal
longitudinal bars of the branchial basket in the Lamprey.
CLASS II.—PISCES.
The Pisces, including the cartilaginous and bony Fishes and
~ the Dipnoi, are Craniata which have the organs both of re-
spiration and of locomotion adapted for an aquatic mode of life.
The chief, and in the majority the only, organs of respiration
are the gills, which are in the form of series of vascular processes
attached to the branchial arches and persisting throughout life.
The organs of locomotion are the paired pectoral and pelvic fins, and
the unpaired dorsal, ventral, and caudal; these are all supported by
fin-rays of dermal origin. A dermal exoskeleton is usually present.
In the endoskeleton the notochord is usually more or less com-
pletely replaced by vertebre ; there is a well-developed skull and a
system of well-formed visceral arches, of which the first forms upper
and lower jaws, the latter movably articulating with the skull, and
both nearly always bearing teeth. There is frequently an air-
bladder, which in certain exceptional cases acquires the function
of a lung or chamber for breathing air. The hypophysis is not
in any way connected with the nasal chambers, and lies within
the cranial cavity. There is a pair of nasal chambers which only
exceptionally communicate internally with the mouth-cavity.
The auditory labyrinth contains the three typical semicircular
canals. The kidney is a persistent mesonephros.
Sub-Class I.—Elasmobranchii.
The sub-class Elasmobranchii comprises the Sharks, Dog-fishes,
and Rays. The skeleton of these fishes, like that of the Cyclo-
stomata, is composed essentially of cartilage, and, though there may
be ossifications in the substance of the cartilage, distinct bones,
such as are found in all higher groups, with the exception of the
Holocephali, are not present. The dermal fin-rays, supported on
the cartilaginous skeleton of the fin, are of horn-like constitution.
There is never (in recent forms) an operculum or gill-cover. There
is a cloaca, the external opening of which serves as a common outlet
for the rectum and the renal and reproductive ducts. Among
some of the. fossil representatives of this group are to be found
the most primitive of all known Fishes.
XIII PHYLUM CHORDATA 135
1—EXAMPLE OF THE SuB-CLass: THE Doc-FIsu (Seyllinm
canicula or Chiloseyllium Juscun).
General external features.—The gencral shape of the body
(Fig. 766) may be roughly described as fusiform: at the anterior,
or head, end it is broader and depressed ; posteriorly it tapers
gradually and is compressed from side to side. The head termi-
nates anteriorly in a short blunt snout. ‘The tail is narrow and
bent upwards towards the extremity. The colour is grey with
brown markings, or dark-brown above, hghter undermeath. The
entire surface is covered closely with very minute hard placoid
scales or dermal tecth somewhat larger on the upper surface than
on the lower. These are pointed, with the points directed some-
what backwards, so that the surface appears rougher when the
hand is passed over it forwards than when it is passed in the
opposite direction. When examined closely each scale is found to
be a minute spine situated on a broader base. The spine consists
Fic. 766.—Dog-Fish (Chiloscyllium modestum). Lateral view. (After Giinther.)
of dentine covered with a layer of enamel; the base is composed of
bone, and the whole scale has thus the same essential structure as
a tooth. Along each side of the head and body runs a faint
depressed longitudinal line or slight narrow groove—the lateral
line.
As in Fishes in general, two sets of fins are to be recognised—the
unpartied or median fins, and the paired or lateral. These are all
flap-like outgrowths, running vertically and longitudinally in the
case of the median fins, nearly horizontally in the case of the lateral :
they are flexible, but stiffish, particularly towards the base, owing
to the presence of a supporting framework of cartilage. Of the
median fins two—the dorsal—are situated, as the name indicates,
on the dorsal surface : they are of triangular shape; the anterior,
which is the larger, is situated at about the middle of the length of
the body, the other a little further back. The caudal fringes the
tail: it consists of a narrower dorsal portion and a broader ventral,
continuous with one another round the extremity of the tail, the
latter civided by a notch into a larger, anterior, and a smaller,
posterior lobe. The tail is heterocercal, i.c., the posterior extremity
of the spinal column is bent upwards and lies in the dorsal portion
of the caudal fin. The ventral or so-called anal fin is situated on
136 ZOOLOGY SECT.
the ventral surface, opposite the interval between the anterior and
posterior dorsals (in Seylliwm); it resembles the latter in size and
shape.
Of the lateral fins there are two pairs, the pectoral and the
pelvic. The pectoral are situated at the sides of the body, just
behind the head. The pelvic, which are the smaller, are placed
on the ventral surface, close together, in front of the middle of
the body. In the males the bases of the pelvic fins are united
together in the middle line, and each has connected with it a
clasper or copulatory organ. The latter is a stiff rod, on the inner
and dorsal aspect of which is a groove leading forwards into a
pouch-like depression in the base of the fin. ; ;
The mouth—a transverse, somewhat crescentic opening—is
situated on the ventral surface of the head, near its anterior end.
In front and behind it is bounded by the upper and lower jaws,
each bearing several rows of teeth with sharp points directed back-
wards. The nostrils are situated one in front of each angle of the
mouth, with which each is connected by a wide groove—the naso-
buccal groove. In Chiloscyllium the outer edge of the groove 1s
prolonged into a narrow subcylindrical appendage—the barbel. A
small rounded aperture, the spiracle—placed just’ behind the eye
—leads into the large mouth cavity, or pharynx. Five pairs of
slits running vertically on each side of the neck—the branchial
slits—also lead internally into the mouth cavity. A large median
opening on the ventral surface at the root of the tail, between the
pelvic fins, is the opening leading into the cloaca, or chamber
forming the common outlet for the intestine and the renal and
reproductive organs. A pair of small depressions, the abdominal
pores, situated behind the cloacal opening, lead into narrow passages
opening into the abdominal cavity.
The skeleton is composed entirely of cartilage, with, in certain
places, depositions of calcareous salts. Asin Vertebrates in general,
we distinguish two sets of elements in the skeleton—the axial set
and the appendicular, the former comprising the skull and spinal
column, the latter the limbs and their arches.
The spinal column is distinguishable into two regions—the
region of the trunk and the region of the tail. Inthe trunk region
each vertebra (Fig. 767, A) consists of a centrum (cent.), neural arch
(neur.), and transverse processes (¢7.). In the caudal region there
are no transverse processes, but inferior or hemal arches (B, haem.)
take their place. The centra ofall the vertebre are deeply biconcave
or amphicelous, having deep conical concavities on their anterior
and posterior surfaces. Through the series of centra runs the noto-
chord, greatly constricted in the centrum itself, dilated in the large
spaces formed by the apposition of the amphiccelous centra of
adjoining vertebrae, where it forms a pulpy mass. The concave
anterior and posterior surfaces of the centra are covered by a dense
XIII PHYLUM CHORDATA 137
calcified layer, and eight radiating lamelle of bone (C) run longi-
tudinally through the substance of the centrum itself, The centra.
unhke those of the higher forms, are developed as chondrifications
of the sheath of the
notochord imto which B sp
cells of the skeletogen- Wieur
ous layer have migrated €
(p. 66). Each neural
arch consists of a pair haem
ot rod-like neural pro-
cesses, Which torm the
sides, and two pairs of
compressed newral plates
(one placed opposite the
centrum, the other or
intercalary cartilage, Op- Fic. 767.—Chiloscyllium, vertebra. 4, end view of
posite the interval be- funk vertebns. "nr. "coftrums; "new, neoral pat
tween adjoming centra ) transverse processes. B, lateral "view of the same.
Fig. 768), which form PEG mouateata rd Glee aerarae e
5 J 2 ) 8 8
bone.
the roof of the arch,
together with usually
two nodules—the representatives of newral spines (sp.)—which form
the keystones. The transverse processes are very short: connected
with each of them is a cartilaginous rudimentary rib (r.) about
half an inch in length.
The cranium (Fig. 768) is a cartilaginous case, the wall of which
is continuous throughout, and not composed, like the skulls of
higher Vertebrates, of a number of distinct elements (bones) fitting
in together. At the anterior end is a rostrwm, consisting in Scyllium
of three cartilaginous rods converging as they extend forwards and
meeting at their anterior ends. At the sides of the base of this are
the olfactory capsules (olf.)—thin rounded cartilaginous sacs opening
widely below—the cavities of the two capsules being separated
from one another by a thin septum. The part of the roof of the
cranial cavity behind and between the olfactory capsules is formed,
not of cartilage, but of a tough fibrous membrane, and the space
thus filled in is termed the anterior fontanelle: in contact with the
lower surface of the membrane is the pineal body, to be afterwards
mentioned in the account of the brain. Each side-wall of this
part of the skull presents a deep concavity—the orbit—over which
is a ridge-like prominence, the supra-orbital crest, terminating
anteriorly and posteriorly in obscure processes termed respectively
the pre-orlital and post-orbital processes. Below the orbit is a
longitudinal infra-orbital ridge.
Behind the orbit is the auditory region of the skull
cartilage im which the parts of the membranous labyrinth of the
internal ear are embedded. On the upper surface of this posterior
a mass of
138 ZOOLOGY SECT.
portion of the skull are two small apertures situated na mesial
depression. These are the openings of the agueductiis vestibuli
(endolymphatic ducts), leading into the vestibule of the membranous
labyrinth. Behind this again is the occipital region, forming the
posterior boundary of the cranial cavity, and having in the middle
a large rounded aperture—the foramen magnum—through which
the spinal cord contained in the neural canal and protected by the
neural arches of the vertebra, becomes continuous with the brain,
lodged in the cranial cavity. On either side of this is an articular
surface—the occipital condyle—for articulation with the spinal
column.
A number of smaller apertures, or foramina, chiefly for the
passage of nerves, perforate the wall of the skull. Behind and to
J Eri
A. Ss
SUK oelas ophs Z P eur tnlerc
3 a
is ‘a edly VOY GOREII IDS.
fi UO tr
ony
Fic. 768.—Chiloscyllium, lateral view of skull with visceral arches and anterior part of spinal
column; the branchial rays are not represented. The skull and hyoid arch are somewhat
drawn downwards, so that the hyoid and first branchial arch are not exactly in their natural
relations. b7.1—br.5 branchial arches ; cer. hy. cerato-hyal; ep. br. epibre vials ; gl. aperture
for glosso-pharyngeal nerve; b. hy. basi-hyal; hy. mn. hyo-mandibular ; interc. intercalary
plates ; Ick. Meckel’s cartilage ; neur. neural processes ; olf. olfactory capsule ; oc. foramen
for oculo-motor ; opt. optic foramen ; pal. g. palato-quadrate ; path. foramen for 4th nerve ;
ph. tr first pharyngo-branchial ; ph. br.5 fifth pharyngo-branchial; sp. neural spines ; tr.
transverse processes and ribs ; tri. foramen for trigeminal nerve.
the outer side of the anterior fontanelle is the aperture for the
ophthalmic branch of the fifth, or trigeminal, nerve. Piercing the
inner wall of the orbit are foramina through which the optic nerves,
or second pair of cranial nerves (opt.); the oculo-motor (ac.), or third :
the pathetic, or fourth (path.); the trigeminal, or fifth; the abducent,
or sixth; and the facial, or seventh, gain an exit from the interior of
the cranial cavity. Just behind the auditory region is the foramen
for the glosso-pharyngeal, and in the posterior wall of the skull, near
the foramen magnum, is the foramen for the vagus.
In close connection with the cranium are a number of cartilages
XIIL PHYLUM CHORDATA 139
Sees the visceral arches ( Figs. 768 and 769). These are in-
complete hoops of cartilage pce segmented, which lie in the
sides and floor of the mouth- cavity or phi wynx. The first of these
forms the upper and lower jaws. The upper jaw, or palato-quadrate
( pal. q-)s consists of two stout rods of ¢: artilage firmly bound to-
gether in the middle line ae bearme the upper (or anterior) series
of teeth. The lower jaw, or IMechel’s cartilage (Mch.), likewise con-
sists of two stout ee rods firmly united aaa im the
middle line, the union being termed the symp YSis. At their outer
Cre
cerhy
cer brs
ax ep or 2
cerbr2
cer6r3
ak
ph. brs
Fic. 769. —Chiloscyllium, ventral view of the visceral arches. Letters as in preceding figure.
In addition,'b. br. basi-branchial plate ; cer. br. cerato-branchials ; hyp. br. hypo-branchials.
ends the upper and lower jaws articulate with one another by a
movable joint. In front the upper jaw is connected by a ligament
with the base of the skull.
Immediately behind the lower jaw is the hyoid arch. This con-
sists of two cartilage »s on each side, and a mesial one in the middle
below. The uppe rmost cartilage is the hyo-mandibular (hy. mn.) :
this articulates by its proximal end with a distinct articular facet
on the auditory region of the skull; distally it is connected
by ligamentous fibres with the outer ends of the palato-quadrate
140 ZOOLOGY SECT.
and Meckel’s cartilage. The lower lateral cartilage is the cerato-
hyal (cer.hy.). Both the hyo-mandibular and cerato-hyal bear a
number of slender cartilaginous rods—the branchial rays of the
hyoid arch. The mesial element, or basi-hyal (0. hy.), lies in
the floor of the pharynx. Behind the hyoid arch follow the
branchial arches, which are five in number. Each branchial arch,
with exceptions to be presently noted, consists of four cartilages.
The uppermost of these—pharyngo-branchial (ph. br.—ph. br.)—lie
in the dorsal wall of the pharynx, not far from the spinal column ;
the pharyngo-branchials of the last two arches are fused together.
The next in order—the epibranchials (ep. br.)—with the exception
of those of the last arch, bear a number of slender cartilaginous
rods—the branchial rays—which support the walls of the gill-sacs ;
and the next—the cerato-branchials (cer. br.)—are, with the same
exception, similarly provided. The hypo-branchials (hyp. br.), which
succeed these, are absent in the case of the first and fifth arches.
In the middle line on the floor of the pharyngeal cavity is a mesial
cartilage—the basi-branchial (Fig. 769, b. br.)\—which 1s connected
with the ventral ends of the third, fourth, and fifth arches. Three
pairs of slender curved rods—the eatra-branchials—lie superficial
to the second, third, and fourth branchial arches, along the borders
of the corresponding branchial clefts.
Two pairs of delicate labial cartilages lie at the sides of the
mouth, and a couple at the margins of the openings of the olfactory
capsules.
The skeleton of all the fins—paired and unpaired—presents a
considerable degree of uniformity. The main part of the expanse
of the fin is supported by a series of flattened segmented rods, the
pterygtophores or cartilaginous fin rays, which lie in close apposition :
in the case of the dorsal fins these are calcified along their axes. At
the outer ends of these are one or more rows of polygonal plates of
cartilage. On each side of the rays and polygonal cartilages are a
number of slender horny fibres of dermal origin. In the smaller
median fins there may be an elongated rod of cartilage constituting
the skeleton, or cartilage may be entirely absent. In the pectoral fin
(Fig. 770) the fin rays are supported on three basal cartilages articu-
lating with the pectoral arch. The latter (pect.) is a strong hoop
of cartilage incomplete dorsally, situated immediately behind the
last of the branchial arches. it consists of a dorsal, or scapular,
and a ventral, or coracoid portion, the coracoid portions of oppo-
site sides being completely continuous across the middle line,
while the scapular are separated by a wide gap in which the
spinal column lies. Between the two portions are the three arti-
cular surfaces for the three basal cartilages. The coracoid portions
are produced forwards in the middle line into a flattened process
supporting the floor of the pericardial cavity in which the heart
is lodged. The three basal cartilages of the fin are named,
ATIT
PHYLUM CHORDATA 141
respectively, the anterior, pro-pterygium (pre.), the middle, meso-
pterygium (meso.),
and the posterior,
metapterygrum
(meta.).
the
the
only
Tay ;
two
or
rently arranged in
the two genera.
The pelvic fin
(Fig.
only a single basal
cartilage
articulating with
the
with
one or two of the Fu.
rays
hn
first is
smallest, and the
last the largest:
frst bears
one large
the other
bear twelve
more, ditte-
tf velvic
Of these
is the
771) has
(meta.)
arch,
which also
horn
pectoral ar
articu-
late directly. The
pelvic arch (pelv.) is a nearly straight bar of cartilage which
runs transversely across the ventral surface of the body, just
pelv
Fic. 771.—Chiloscyllium, pelvic arch and pelvic
fin.
pylorus (pyl.)j—the point wh
intestine—is a slight constriction, fol
meta. metapterygium ; pelv. pelvic arch.
770.—Chiloscyllium, pectoral arch and fin.
3 meso. mesopt i
h ; pro. propterygium.
d. 7. dermal
yzium ; meta. metapterygium ; pect.
in front of the cloacal
opening.
Enteric canal (Fig. 772).
—The mouth leads into a
very wide cavity, the pharyna,
into which open at the sides
the internal apertures of the
branchial clefts and of the
spiracle. From this runs
backwards a short wide tube
—the esophagus (@s.)—which
passes behind into the
stomach. The stomach 1s
a U-shaped organ, with a
long left limb continuous
with the cesophagus, and a
short right passing into
the intestine. At the
here the stomach passes into the
The
lowed by a thickening.
142 ZOOLOGY SECT. XIIL
intestine consists of two parts—small intestine or duodenum, and
large intestine. The former is very short, only an inch or two in
length. The latter is longer and very wide; it is divisible into
two portions—the colon (col.) in front and the rectwm (rect.)
behind. The former is very wide and is characterised by the
presence in its interior of a spiral valve, a fold of the mucous mem-
brane which runs spirally round its interior, and both retards the
too rapid passage of the food and affords a more extensive surface
for absorption. The rectum differs from the colon in_ being
narrower and in the absence of the spiral valve; it opens behind
into the cloaca.
There is a large liver (liv.) consisting of two elongated lobes. A
rounded sac—the gall-bladder (g. bl.)\—lies embedded in the left
lobe at its anterior end. The duct of the liver—the bile-duet (b. det.)
—runs from the liver to the intestine. Proximally it is connected
with the gall-bladder, and by branch-ducts with the nght and left.
lobes of the liver. It opens into the commencement of the colon.
The pancreas (pancr.) is a light-coloured compressed gland con-
sisting of two main lobes with a broad connecting isthmus, lying in
the angle between the right-hand limb of the stomach and the
small intestine. Its duct enters the wall of the small intestine
and runs in it for about half an inch, opening eventually at the
point where the small intestine passes into the colon.
Connected with the rectum on its dorsal aspect is an oval gland
—the rectal gland (rect. gl.)—about three-quarters of an inch in
length.
The spleen (spi.) is a dark-red or purple body attached to the con-
vexity of the U-shaped stomach and sending a narrow lobe along
the right-hand limb.
The organs of respiration in the Dog-fish are the gills, situated
in the five gzll-pouches. Each gill-pouch (Fig. 773) is an antero-
posteriorly compressed cavity opening internally into the pharynx
and externally by the gill-slit. The walls of the pouches are sup-
ported by the branchial and hyoid arches with their rays, the first
pouch being situated between the hyoid and first branchialarches,the
last between the fourth and fifth branchial arches. On the anterior
and posterior walls of the pouches are the gil/s, each hemibranch
consisting of a series of close-set parallel folds or plaits of highly
vascular mucous membrane. Separating adjoining gill-pouches,
and supporting the gills, are a series of broad interbranchial septa,
each containing the corresponding branchial arch with its con-
nected branchial rays. The most anterior hemibranch is borne on
the posterior surface of the hyoid arch. The last gill-pouch differs
from the rest in having gill-plaits on its anterior wall only. On
the anterior wall of the spiracle is a rudimentary gill—the pseudo-
branch or spiracular gill—in the form of a few slight ridges.
Blood system.—The heart is situated in the pericardial cavity,
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IS
144 ZOOLOGY SECT.
on the ventral aspect of the body, in front of the pectoral arch, and
between the two series of branchial pouches. Its dorsal wall is
supported by the basi-branchial cartilage. Placing it in communi
cation with the abdominal cavity is a canal—the pericardzo-pert-
toneal canal. The heart (Fig. 772) consists of four chambers—sinus
venosus (sin. ven.), auricle (aur.), ventricle (vent.), and conus arteriosus
(con.), through which the blood passes in the order given. The sinus
venosus is a thin-walled, transverse, tubular chamber, into the ends
of which the great veins open. It opens into the auricle by an
aperture, the sinu-auricular aperture. The auricle 1s a large, tri-
angular, thin-walled chamber, situated in front of the sinus veno-
sus and dorsal to the ventricle. Its apex 1s directed forwards, and
its lateral angles project at the sides of the ventricle : it commu-
nicates with the ventricle by a slit-lke aperture guarded bya two-
lipped valve. The ventricle as a thick-
walled, globular chamber, forming the most
conspicuous part of the heart when looked
at from the ventral surface. From it the
conus arteriosus runs forwards as a median
stout tube to the anterior end of the peri-
cardial cavity, where it gives off the ventral
aorta. It contains two transverse rows of
valves, anterior and posterior, the former
consisting of three, the latter of three or
four. The ventral aorta (Fig. 774) gives
origin to a series of paired afferent branchial
SCR Sheena cerned arteries (br.v.), one for each branchial pouch.
srvanchial sac exposec PO" = P d s
the outside. In Scyllium the two most posterior arise
close together near the beginning of the
ventral aorta, the third pair a little further forwards. The
ventral aorta then runs forwards a little distance and bifurcates
to form the two znnominate arteries, right and left, each of which
in turn bifureates to form the first and second afferent vessels of
its side. In Chiloscylium (Fig. 774) the arrangement is some-
what different.
From the gills the blood passes by means of the efferent branchial
arteries. These efferent vessels form a series of loops, one running
around the margin of each of the first four internal branchial
clefts: a single vessel runs along the anterior border of the fifth
branchial cleft and opens into the fourth loop. The four main efferent
branchial vessels run inwards and backwards from the loops under
cover of the mucous membrane of the roof of the mouth to unite
in a large median trunk—the dorsal aorta. From the first efferent
vessel, that from the first or hyoidean gill, arises the carotid
artery, Which runs forwards and bifureates to form the dnternal
and eaternal carotid arteries, supplying the head with arterial
blood. A hyoidean artery arises further out from the same vessel,
XIII
PHYLUM CHORDATA 145
and, after giving off branches to the pseudobranch, passes into the
cranial cavity and joins the internal carotid of the opposite side
The dorsal werta runs backwards throughout
body cavity, giving off
numerous branches,
and is continued as
the caudal artery,
which runs in the
canal enclosed by the
inferior arches of the
caudal vertebree. The
first pair of branches
are the subclavian, for
the supply of the
pectoral fins; these
are given off between
the third and fourth
pairs of efferent ar-
teries. The next large
branch is
the un-
paired caliac (Fig.
772, cel.): this runs
im the mesentery and
divides into branches
for the supply of the
stomach and liver, the
first part of the in-
testine, and the pan-
creas. The
mesenteric artery, also
median, supplies the
rest of the intestine
and gives off branches
to the reproductive
organs. he Jieno-
gastrie supplies part
of the stomach, the
spleen,and part of the
pancreas. The pos-
terior mesenteric 18 a
small vessel mainly
supplying the rectal
anterior
gland. A pair of
small renal arteries
carry a small quantity
the length of the
Fic. 774.—Chiloscyllium,. Diagrammatic representation of
the ventral aorta and afferent branchial arteries, and of the
chief veins. a/v. alimentary canal; br. v.1-br. v.5 afferent
branchial arteries; caud. v. caudal vein; deft. c. ductus
Cuvieri; At. heart ; hep. port. v. hepatic portal vein ; hep. 8.
hepatic sinus; ij. guy. v. inferior jugular vein or sinus ;
jug. jugular vein or sinus ; Jat. v. lateral vein; liv. liver ;
1. card. 8. left cardinal sinus ; /. port. v. left renal portal vein ;
neph. kidney; r. right posterior cardinal sinus ;
r. port. v. right renal portal vein,
card. 8.
of arterial blood to the kidneys, and a pair of iliac arteries,
likewise of small size, supply the pelvic fins. In addition to
VOL, II
L
~ 146 ZOOLOGY SECT.
these a number of small arteries, the parietal, supplying the
wall of the body, are given off throughout the length of the
aorta.
The veins are very thin-walled, and the larger trunks are re-
markable for their dilated character, from which they have
obtained the name of sinuses, though they are true vessels and
not sinuses in the sense in which the word is used in dealing
with the Invertebrates (¢f. p. 87). ;
The venous blood is brought back from the head by a pair of
jugular or anterior cardinal sinuses (Fig. 774, jug. v.), and from the
trunk by a pair of posterior cardinal sinuses. At the level of the
sinus venosus the anterior and posterior cardinals of each side unite
to form a short, nearly transverse sinus, the precaval sinus or ductus
Cuvieri (Fig. 774,dct.c.), which is continued into the lateral extremity
of the sinus venosus. Into the ductus Cuvieri,about its middle, opens
an inferior jugular sinus (inf. jug. v.) which brings back the blood
from the floor of the mouth and about the branchial region of the
ventral surface. The two posterior cardinal sinuses extend back-
wards throughout the length of the body cavity; in front they are
enormously dilated, behind they le between the kidneys. Ante-
riorly each receives the corresponding subclavian vein, bringin
the blood from the pectoral fin and adjacent parts of the body-
wall. The /Jateral vein (v.), instead of joining with the sub-
clavian (p. 87), opens separately into the precaval. The genital
sinus discharges into the posterior cardinal sinus.
There are two portal systems of veins, the renal portal and the he-
patie portal (hep. port.v.), by which the kidneys and liver, respectively,
are supplied with venous blood. The caudal vein, which brings back
the blood from the tail, running, along with the caudal artery,
through the inferior arches of the vertebra, divides on entering
the abdominal cavity into right and left renal portal veins, which
end in a number of afferent renal veins supplying the kidneys.
The hepatic portal vein (h. port. v.) is formed by the confluence of
veins derived from the intestine, stomach, pancreas, and spleen,
and runs forwards to enter the liver a little to the right of the
middle line. In Chiloscyllium a large branch connects the genital
sinus with the intestinal tributaries of the hepatic portal system.
The blood from the liver enters the sinus venosus by two hepatic
sinuses placed close together.
Nervous System.—The fore-brain consists of a rounded,
smooth prosencephalon (Fig. 775, V.H.), divided into two lateral
parts by a very shallow median longitudinal groove. From its
antero-lateral region each half gives off a thick cord, which dilates
into a large mass of nerve-matter, the olfactory lobe (Z.0l.), closely
applied to the posterior surface of the corresponding olfactory
capsule. The diencephalon (ZH) is comparatively small; its roof
is very thin, while the floor is composed of two thickish masses
XU PHYLUM CHORDATA 147
—the optie thalami. Attached to the roof is a slender tube,
the epiphysis cerebrt or pineal body (Gp.), which runs forwards and
A B
P
H VL ASH
Sv
Fic. 775.—Brain of Scyllium canicula, A, dorsal view; B, ventral view; C, lateral view.
F. rho. fossa rhomboidalis (fourth ventricle); @p, epiphysis; HI, cerebellum ; /{S. A, hypo-
physis; Z. ol. olfactory lobe; MH, mid-brain ; NH, medulla oblongata ; Sv, saccus vasculosus ;
Tro, olfactory peduncle; UL, lobi inferiores; Vi, prosencephalon; Z/, diencephalon ;
ITI, optic nerves; J//, oculomotor; JV, pathetic; V, trigeminal; VJ, abducent; V’//, facial ;
VIL, auditory ; LX, glossopharyngeal; X, vagus. (From Wiedersheim.)
terminates in a slightly dilated extremity fixed to the membranous
part of the roof of the skull. Projecting downwards from its
Teme
148 ZOOLOGY SECT.
floor are two rounded bodies, the lobi inferiores (UL), which are
dilated portions of the infundibulum ; andattached to this, behind,
is a thin-walled sac—the pituitary body or hypophysis cerebri (HS),
having a pair of thin-walled vascular lateral diverticula the
sacet vasculosi (S.v.), and having on its ventral surface a .median
tubular body attached at its posterior end to the floor of the skull.
In front of the infundibulum, and also on the lower surface of the
diencephalon, is the optie chiasma, formed by the decussation of
the fibres of the two optic nerves. The mid-brain (J/H) consists
of a pair of oval optic lobes dorsally, and ventrally of a band of
longitudinal nerve-fibres corresponding to the erura cerebri of the
higher vertebrate brain. The cerebellum (H/) is elongated in
the antero-posterior direction, its anterior portion overlapping the
optic lobes, and its posterior the medulla oblongata. Its surface
is marked with a few fine grooves. The medulla oblongata (V/),
~ broad in front, narrows posteriorly to
pass into the spinal cord. The fourth
ventricle (F. rho.) 1s a shallow space on
the dorsal aspect of the medulla ob-
longata covered over only by a thin
vascular membrane, the choroid plerus:
it is wide in front and gradually nar-
rows posteriorly. At the sides of the
anterior part of the fourth ventricle
are a pair of folded ear-shaped lobes,
the corpora restiformua.
The fourth ventricle (Fig. 776,
meta.) 18 continuous behind with the
central canal of the spimal cord. In
front 1t 1s continuous with a narrow
passage, the der (iter.), which opens
anteriorly into a wider space, the
diacele or third ventricle (dia.) occupy-
ing the interior of the diencephalon.
eS eee rene re: |) Tome this opens mmmon beat median
brain viewed from the dorsal side, / 1 < 1 2
the roofs of the various ventricles yrosacale. w eh olives r* air of nara-
removed so as to show the relations Hf ee nc giv ee. oft et p a of pai in
of the cavities(semi-diagrammatie). ca@les (para.) extending into the two
cer, dilatation from which the epi- lateral porti s of tl ann p
cole is given off; dia. diaccle, ateral portions of the prosencephalon.
pointing to the opening leading into Men he ¢ eee aes rs ery
the infundibulum ; iter. iter or From the anterior enlargements of
passage between the diaccele and the olfactory lobes already mentioned
the metaceele ; meta. metaccele ; opt. Nee wate a ae - :
optocwle; para, paracwele; pro, SPYING Numerous fibres which consti-
prosoccele ; rh, rhinoccele. tute the first pair of cranial nerves
and enter the olfactory capsules,
From the optic chiasma the two optic nerves (Fig. 775, ZZ) ran
outwards through the optic foramina into the orbits, each per-
forating the sclerotic of the corresponding eye and terminating
in the retina. The third, fourth, and sixth pairs of nerves have
XIII PHYLUM CHORDATA 149
the general origin and distribution which has already been
described as universal in the Craniata (p. 97).
The trigeminal (Fig. 775, V) arises in close relation to the
facial. As it passes into the orbit it swells into a ganglion—the
Gasserian. Its chief branches are three in number. The first
given off is the superficial ophthalmic (Fig. 777, oph. V), which runs
forwards through the orbit above the origin of the recti muscles,
and in very close relation with the ophthalmic branch of the facial.
Anteriorily it breaks up into branches for the supply of the mucous
canals of the dorsal surface of the snout. The main trunk of the
nerve then runs forwards and outwards across the floor of the
orbit, and divides into two branches the maxillary and mandibular
or second and third divisions of the trigeminal. The former
(mx.V) supplies the mucous canals of the ventral surface of the
snout ; the latter (mnd.V) supplies the muscles of the lower jaw.
A nerve of considerable size—the ophthalmicus profundus—
arises in front of the root of the trigeminal, with which it is
in close communication. After leaving the cranial cavity it
enlarges into a small ganglion, and runs forwards over the external
rectus muscle and under the superior rectus, and perforates the
pre-orbital process to end in the integument of the snout. Among
other branches it gives off ciliary branches to the iris: these are
joined by the ciliary branches of the oculomotor.
Of the branches of the facial, the superficial ophthalmic runs
through the orbit in close relation to the superficial ophthalmic
branch of the trigeminal, and is distributed to the ampulle and
mucous canals of the snout region; the buccal runs forwards in
intimate relation with the maxillary division of the trigeminal,
and breaks up into branches which are mainly distributed to the
ampulle and canals of the region of the snout; the palatine
(pl. VII) runs to the roof of the mouth; the main body of the
nerve—hyomandibular nerve (hy. mnd. VIJ)}—then runs outwards
close to the edge of the hyomandibular cartilage and behind the
spiracle, eventually becoming distributed to the muscles between
the spiracle and the first branchial cleft.
The eighth or auditory nerve passes directly into the internal
ear, and breaks up into branches for the supply of its various
parts. The glossopharyngeal (gl. ph.) perforates the posterior part
of the auditory region of the skull, and, after it reaches the
exterior, passes to the first branchial cleft, where it bifurcates, one
branch passing to the anterior, and the other to the posterior
wall of the cleft. The last nerve of the series—the pnewmogastric
or vagus (vag.)—is a large nerve which emerges from the skull by
an aperture situated between the auditory region and the foramen
magnum. It first gives off a series of four branchial branches, cach
of which bifurcates to supply the anterior and posterior borders of
the last four branchial clefts. It then gives off a lateral nerve
150 ZOOLOGY SECT.
(lat. vag.), which runs along beneath the lateral line to the posterior
end of the body. The rest of the nerve runs backwards to divide
into cardiac branches for the heart and gastric branches for the
stomach. ’
The spinal cord is a cylindrical cord which extends from the
foramen magnum, where it is continuous with the hind brain,
backwards throughout the length of the neural canal enclosed by
ERA eRe
ASSERT
%
Q
9
lal.vag
a. 777.—Seyllium catulus.—Dissection of the brain and spinal nerves from the dorsal
rface. The right eye has been removed. The cut surfaces of the cartilaginous skull and
spinal column are dotted. The ophthalmicus profundus and the buccal branch of the facial
are not represented ; c/.j—cl.5, branchial clefts; ep. epiphysis; ert. rect. external rectus
muscle of the eye-ball; gl. ph. glossopharyngeal ; hor. cwn. horizontal semicircular canal ;
hy. mnd. VII. ; byomandibular portion of the f. ; inf. old. inferior oblique muscle ; int.
rect. internal rectus muscle ; /at. vag. lateral branch of vagus; mx. V. maxillary division of
the trigeminal; o/f. cps. olfactory capsule; olf. s. olfactory sac; oph. V. VII. superficial
ophthalmic branches of trigeminal and facial ; path, fourth nerve ; pl. VII. palatine branch of
facial; sp. co. spinal cord; spir. spiracle; s. rect. superior rectus muscle; s. of). superior
oblique; vag. vagus; vest. vestibule. (From Marshall and Hurst.) \
the neural arches of the vertebre. As in the Craniata in general
(see p. 92),1t has dorsal and ventral longitudinal fissures and
a narrow central canal, and gives origin to a large number of
paired spinal nerves, each arising from it by two roots.
Organs of Special Sense.—The olfactory organs are rounded
chambers enclosed by the cartilage of the olfactory capsules of the
skull, and opening on the exterior by the external nares on the
ventral surface of the head. The interior has its liming membrane
raised up into a number of close-set ridges running out from
XII PHYLUM CHORDATA 151
a median septum. The fibres of the olfactory nerves terminate in
cells of the epithelium covering the surface of these ridges.
The eye has the general structure already described as char-
acterising the Craniata in general (p. 103). The sclerotic is
cartilaginous, the choroid has a shining metallic internal layer or
tapetuim, and the lens is spherical. The eyeball is attached to the
inner wall of the orbit by a cartilaginous stalk. There .are the
usual eye-muscles, the two obligues situated anteriorly, the four
rect? posteriorly. There are no eyelids.
The ear consists only of the membranous labyrinth equivalent to
the internal ear of higher Craniata, the middle ear and the outer ear
being absent. The membranous labyrinth consists of the vestibule
and three semieireular canals. The former, which is divided into
two parts by a constriction, communicates by a narrow passage—
the agueductus vestibuliwith the exterior, in the position already
mentioned. Of the three semicircular canals, the anterior and
posterior are vertical and the external horizontal, as in Craniata in
general. Each has an ampulla, that of the anterior and external
canals situated at their anterior ends, and that of the posterior
canal, which is the largest of the three, and forms an almost
complete circle, at its posterior end. In the fluid (endolymph)
in the interior of the vestibule are suspended, in a mass of
gelatinous connective tissue, numerous minute calcareous particles
or otoliths, giving it a milky character.
The mucous canals of the integument contain special nerve-
endings, and doubtless function as organs of some special sense.
The same probably holds good of a number of minute canals
situated on the anterior portion of the trunk, and on the head,
being particularly numerous in the neighbourhood of the snout.
These are dilated internally into vesicles, the ampullw, provided
with special nerve-endings.
Urino-genital Organs.—In the female there is a single
ovary (Fig. 773, ov.), an elongated, soft, lobulated body, lying
a little to the right of the middle line of the abdominal
cavity, attached by a fold of peritoneum, the mesoarium. On
its surface are rounded elevations of various sizes, the Graafian
follicles, each containing an ovum of a bright yellow colour. There
are two oviducts (Miillerian ducts) entirely unconnected with the
ovaries. Each oviduct (Fig. 773, ovd.; Fig. 778) is a greatly
elongated tube extending throughout the entire length of the
abdominal cavity. In front the two unite behind the pericardium
to open into the abdominal cavity by a wide median aperture
(abd. ap.). At about the point of junction of the middle and
anterior thirds is a slight swelling marking the position
of the shell-gland (sh. gld.). The posterior part dilates to form
a wide chamber, and in Scyllium the two unite to open into
the cloaca by a common aperture situated just behind the
152
ZOOLOGY SECT.
opening of the rectum, while in Chiloscyllium they remain distinct
and have separate cloacal openings. Each kidney consists of two
+ a : Tt roof, an z
parts, anterior and posterior. The former (Fig. 773, 7- meson.) 18
abd ap
urap
Vic. 778.—Chiloscyllium.
Oviducts. abd. ap, common
abdominal aperture of ovi-
ducts ; clo. ap, cloacal aper-
ture; sh. gid, shell-gland ;
wr. ap, urinary papilla.
a long narrow ribbon of soft reddish sub-
stance, which runs along throughout a
great part of the length of the body-cavity
at the side of the vertebral column,
covered by the peritoneum. The posterior
portion (7. metan.) is a compact, lobulated,
dark-red body, lying at the side, of the
cloaca, continuous with the anterior por-
tion; like the latter, it is covered over by
the peritoneum. Both portions have their
ducts. Those of the anterior are narrow
tubes, which run over its ventral surface
and become dilated behind to form a pair
of elongated chambers, the wrinary sinuses
(Fig. 779, wr. sin), which unite into a
median sinus (med. wr. sinus.), opening into
the cloaca by a median aperture situated
on a papilla, the wrinary papilla. The
duets of the posterior portion, the wreters,
which are usually from four to six in num-
ber, open into the urinary sinuses.
In the male there are two elongated,
soft, lobulated testes, each attached to the
wall of the abdominal cavity by a fold of
peritoneum—the mesorchium. From each
testis efferent ducts pass to the anterior
end of a long, narrow, strap-shaped body,
which corresponds to the anterior portion of
the kidney in the female. This is the
epididymis, the duct of which is a convo-
luted tube running along the entire length
of the mesonephros, and where it leaves
the latter posteriorly becoming a _ wide
tube—the vas deferens or spermiduct—
which opens into a special median com-
partment of the cloaca, the w7rino-genital
sinus. Posteriorly the spermiduct dilates
to form a wide thin-walled sac, the vesi-
cula seminalis. Closely applied to the
latter is a thin-walled elongated sac, the
sperm-sac. Anteriorly the sperm-sac narrows to a blind extremity.
Posteriorly the right and left sperm-sacs combine to form the
urino-genital sinus.
The posterior part of the kidney has the
same character as in the female ; its ducts, usually five in number
XIII PHYLUM CHORDATA 153
on each side, open into the urino-genital sinus. The latter has
a median aperture into the general cavity of the cloaca situated
on the summit of a prominent wrino-genital papilla. The oviduets
(Mullerian ducts) of the female are represented im the male by
rudiments of their anterior portions. The entire kidney is some-
times regarded as a mesonephros, but the posterior portion, de-
veloped entirely behind the part which is converted in the male
into the epididymis, and having its own ducts, is sometimes
eee
SS
a
~—>)
|
Ur Sin
/
fs
\ & Ur sire
1
. 750.—Dog-fish, egg-case. (After
bic. 779.—Chiloscyllium. Right kidney BiG
and urinary sinus of female. med. wr. sinus, Dean.)
median urinary sinus; wneph, kidney;
vr. sinus, Yight urinary sinus.
looked ‘upon as corresponding to the metanephros of the higher
Vertebrates.
The ripe ovum, rupturing the wall of its Graafian follicle,
escapes into the abdominal cavity, whence it reaches the interior
of one of the oviducts; there it becomes fertilised by sperms
received from the male in the act of copulation, and then becomes
enclosed in a chitinoid case or shell (Fig. 780) secreted by the
shell-gland.
1b4 ZOOLOGY SECT.
2.—DISTINCTIVE CHARACTERS AND CLASSIFICATION.
The Elasmobranchii are Pisces in which the cartilaginous
cranium is never ossified by cartilage-bones, and in which mem-
brane-bones are not developed in connection either with the
cranium or the pectoral arch. The skull is hyostylic, except in
some of the Protoselachii, in which it is amphistylic. The dermal
fin-rays are horny; they are supported by cartilaginous pterygio-
phores which are never very numerous. The pelvic arch is a dis-
tinct cartilage. There is nearly always an exoskeleton, which, when
present, is of the placoid type. The intestine has a spiral or a scroll-
like valve. There is a cloaca into which both the rectum and the
ducts of the urinary and reproductive systems open. There is
never an operculum in recent Elasmobranchs, and only rarely in
fossil forms. The inter-branchial septa are of considerable breadth,
and the gill-filaments are attached to them throughout their entire
extent. A spiracular gill is only exceptionally present as a fully
developed organ ; it is represented usually by a vestige (pseudo-
branch). A conus arteriosus is always developed ; it is rhythmically
contractile, and in its interior are several transverse rows of valves.
The optic nerves forma chiasma. The ova are very large: they are
usually fertilised internally. The oviducts are not continuous
with the ovaries, but open by wide mouths into the body-cavity.
Dy --- ~~ ---~--~--> +
yp eae ae
Ss eres
Oa
Fic, 781.—Restoration of Cladoselache fyleri, lateral and ventral views. (Restored, after
Dean.)
ORDER 1.—CLADOSELACHEA.
Extinct Shark-like Elasmobranchs in which both pectoral and
pelvic fins had much wider bases of attachment than in existing
XIIL PHYLUM CHORDATA 155
forms. The notochord was persistent: there were no intercalary
eartilages. The caudal fin was
strongly heterocercal. — Claspers
were absent. The gill-openings
were apparently protected by a
fold of skin. The teeth were of
the nature of placoid denticles.
The lateral Ine was represented
by an open groove.
This order comprises only one
known representative—Cladose-
lache—trom the lower Carbon-
iferous rocks of America.
" Meckel’s ci
€
. radial fin-cartilages ; A
ORDER 2.—PLEURACANTHEA.
M.
Extinet Shark-like Elasmo-
branchs in which the skeleton
of the pectoral fin was con-
structed on the type of the
al fin-cartilages ; D, dermal
archipterygium, tc. consisted of Sa
an elongated segmented central ae
axis bearing two rows of jointed Boe
rays. The notochord was _per- ays
sistent, but intercalary cartilages ee
were present. The caudal fin ae
was diphycercal. Claspers were eS
present. There was no opercular Bas
fold, and the teeth resembled BS
those of other Elasmobranchs. Zag =e
a BBB Se
There were no placoid scales, but Za et
A
ne
Pe
the skull was protected by roofing
dermal bones.
This order, like the last, in-
cludes only one satisfactorily
known genus—Plewracanthus—
\n
a
1 It is possible that this structure may be a coprolite or fossilized mass of feecal matter.
pine ; H. A. hemal arches
neural process and spine ;
82.—Pleuracanthus ducheni, res
of Carboniferous and Permian a
g
age. ea
a
2
SAS
ORDER 3.—ACANTHODEA.
Fic.
Extinct Elasmobranchs having
the anterior margin of each fin
supported by a stout spine. The tail was heterocercal. There
were probably membrane-bones on the roof of the skull. The
156 ZOOLOGY ee
teeth were few and large. The lateral line was 11 the form of an
open groove.
Fic. 783.—Acanthodes wardi. (Restored, after Dean.)
ORDER 4.—SELACHIL.
Living and extinct Elasmobranchs in which the skeleton of the
paired fins is never of the nature of an archipterygium. The
notochord is more or less completely replaced by vertebrz, and there
is a series of intercalary cartilages. The caudal fin is nearly
always heterocercal. Claspers are always developed.
and yellows, spots or stripes of gold or silver, are common, and,
although the combination of tints may sometimes seem to our
Pa 2
212 ZOOLOGY SECT.
eye rather crude and glaring, they appear to be distinctly pro-
tective, harmonising with the brilliant hues of the Coral Polypes
and other members of the reef fauna. Pelagic fishes, such as the
Mackerel and Herring, are usually steely-blue above, white
beneath.
Many deep-sea Teleostei are phosphorescent: in some of these
definite Jwminous organs (Fig. 834) are arranged in longitudinal
Fic. 834.—Stomias boa. The white dots are the luminous organs. (From Hickson., after
Filhol.)
rows along the body, each provided with a lens, hke that of the
eye, the whole organ having thus the characters of a minute
bull’s-eye lantern. Some species of the same order, such as the
Weaver (T'rachinus), possess poison-glands, opening either on one
of the dorsal spines, or on a spinous process of the operculum, or,
as in the Cat-fishes (Siluridee), on the spine of the pectoral fin.
Exoskeleton.—In many Teleostomi, such as Polyodon and
the Eels, the skin is devoid of hard parts, but in most cases a
dermal exoskeleton is present. In Amia and in the majority of
Teleostei this takes the form, as in the Trout, of scales, rounded
plates of bone imbedded in pouches of the derm and overlapping
one another from behind for-
wards. When the free border
a
of the scales presents an even &
curve, as in Amia and most a
Physostomiand Anacanthini, de
they are called cycloid scales ce
(Fig. 808); when, as in most (ge
Acanthopteri, the free edge ass
is produced into small spines if
(Fig. 835, A) they are dis-
tinguished as ctenoid scales,
In exceptional cases the
scales may be so large and
strong as to form a rigid armour.
there 1s a strong armour, formed of stout bony plates, or scutes,
Fic, 835.—A, ctenoid scale; B, ganoid scales.
(After Giinther.)
In the Sturgeon (Fig. 824)
XU PHYLUM CHORDATA 213
produced into enamelled spines and articulating with one another
by suture. Seutes are also found in many Siluroids (Fig. 827)
and in Lophobranchii (Fig. 832) and some Plectognathi (Fig. 831),
while in the Plectognathi the exoskeleton takes the form, as in the
File-fishes, of minute spines like the shagreen of Sharks, or, as in
many Globe-tishes, of long, outstanding, bony spines. Lastly, in
Polypterus and Lepidosteus are found rhomboid or ganoid scales
(Fig. 835, B), in the form of thick, close-set, rhomboidal plates
formed of bone, covered externally by a layer of enamel or ganotin,
and joined together by pegs and sockets. In many Ganoids the
anterior fin-rays of both median and paired fins bear a row of
spine-like seales called Sulera (Fig. 825, 72.).
Endoskeleton.—In the Sturgeon the vertebral column (Fig. 837
IVS.) consists of a persistent notochord with cartilaginous arches
Fic. $36.—Anterior end of yertebral column of Polypterus. PS. parasphenoid ; Rk. /—V, dorsal
ribs ; WK, centra; +. ventral ribs. (From Wiedersheim’s Comparative Anatomy.)
and is fused anteriorly with the cranium. In the remaiming
orders bony vertebrze are present ; the centra are biconcave, except
in some Eels, in which the anterior face is flat or even convex,
and in Lepidosteus, in which the anterior face is distinctly convex.
Vertebree of this form, i.e. having the centrum convex in front and
concave behind, are called opisthocelous. Ribs are usually present:
im Polypterus each vertebra has two pairs, a dorsal pair (Fig. 836,
R, I—TV ) of considerable length, running between the dorsal and
ventral muscles, and a short ventral pair (+) between the muscles
and the peritoneum: the former answer to the ribs of Elasmo-
branchs, the latter to the ribs of the remaining Teleostomi, which
214 ZOOLOGY SECT.
are always placed immediately beneath the peritoneum. There
may be one or more sets of intermuscular bones, attached either
to the neural arch (epinewrals), to the centrum (epicentrals), or to
the ribs (epiplewrals). The posterior end of the vertebral column
is turned up in the Sturgeons, Lepidosteus, and Amia, resulting
in a heterocercal tail-fin: in Amia, however, the fin-rays are so
disposed that the fin appears almost symmetrical. Among
Teleostei the tail-fn is rarely as obviously unsymmetrical as in
the Trout: usually in the adult the development of the large, fan-
shaped, posterior heemal arches completely hides the upturned end
of the notochord, and in some cases the spmal column ends simply
in a somewhat compressed centrum around which the fin-rays
Fic. 837.—Skull of Sturgeon, with the membrane bones removed. a, pharyugo-branchial =
AF, antorbital process ; AR. articular ; b. epibranchial ; c. cerato-branchial ;°C, notochord ; Cop.
basi-branchials ; d, hypobranchial; De. dentary ; GA, auditory capsule ; HI. hyomandibular ;
hy. hyoid cornu; /h. inter-hyal; Md. mandible; Na. nasal capsule; Gb, neural arches ;
PF, post-orbital process ; PQ. palato-quadrate ; Ps. Ps’. Ps’’. phenoid ; Psp. neural spines :
Qu. quadrate; R. rostrum; Az. ribs; Sp. N. foramina for spinal nerves ; Sy. symplectic; IWS
vertebral column; 7, vagus foramen; /—V, branchial arches. (From Wiedersheii’s Com-
ourative Anatomy.)
are symmetrically disposed; such truly symmetrical tail-fins are
called diphycercal.
In the structure of the skull the Chondrostei make the nearest
approach to Elasmobranchs. The cranium (Fig. 837) is an un-
divided mass of cartilage with a few isolated cartilage bones.
The roofing membrane bones he in the dermis, so as to be practi-
cally superficial, and behind pass insensibly into the scutes
covering the trunk: the fact that these bones (parietals, frontals,
&c.) are exoskeletal structures is here perfectly obvious. The
same 1s the case in Polypterus (Fig. 838), in which, however, the
cartilage bones are better developed. In Lepidosteus and Amuia,
and especially the latter, the skull resembles that of the Trout in
all essential respects, the main differences consisting in the
absence of certaim bones, such as the supra-oecipital, and in the
presence of additional membrane bones. Among Teleostei it is
only in the Physostomi that the membrane bones remain separable
XIII PHYLUM CHORDATA 215
from the chondroeranium in the adult: in the remaining orders,
eg. m the Cod, Haddock, or
Perch, they become grafted on to the
chondrocranium and so closcly united with the cartilage bones that
they can be removed only by pulling the whole skull to pieces ;
most of the orginal cartilage frequently disappears in. the adult,
and the cranium thus be-
comes a firm bony mass in
which no distinction be-
tween cartilage and mem-
brane bones is discernible,
The varying size of the
gape, which is so noticeable
a feature in the Teleostomi
depends upon the inclina-
tion of the suspensorium ;
m wide-mouthed Fishes
(Fig. $28) the axis of the
hyomandibular and suspen-
sorium is nearly vertical or
even inclined backwards:
m small-mouthed forms
(Fig. 831) it is strongly in-
chned forwards and the
length of the jaws is pro-
portionately reduced. In
the branchial arches the
pharyngo-branchials of each
side are very commonly
fused, and constitute what
are called the superior
pharyngeal bones: the re-
duced fifth branchial bars,
or wfpertor pharyngeal bones,
bite against them. The
Pharyngognathi are dis-
tinguished by having the
inferior pharyngeal bones
united into a single bony
mass of characteristic form
(Fig. 830, B). The gill-
rakers are often very highly
developed, and may form
microscopic organisms.
1
Fic. $38.—Skull of Polypterus, from above. Ff.
frontal; W. maxilla; NA. nasal; Na. nostril; Op.
opercular; Orb. orbit P. parietal. The remaining
letters point to less important membrane bones.
The arrow is passed into the spiracle. (From
Wiedersheim's Comparative Anatomy.)
a mesh capable of retaining even
In the shoulder-girdle, as in the skull, the Chondroste1 approach
the Elasmobranchs. There is a primary shoulder-girdle con-
sisting of large paired cartilages, not united in the middle ventral
eens | 3 i 7 pb aoe SC ae
line, and unossified: each is covered externally by a large scute
216 ZOOLOGY SECT.
like membrane-bone, the c/avicle. In the remaining Ganoids and
in Teleostei, the prunary shoulder-
girdle is reduced in size and is
usually ossified by two bones, a
dorsal scapula and a ventral cora-
cold: sometimes, as in the Trout,
there may be an ‘additional ossifica-
tion, the meso-coracoid. Additional
membrane bones—supra-clavicle,
post-clavicle, &e.—are added, and
one of them, the post-temporal,
serves to articulate the shoulder-
girdle with the skull (Fig. 815).
In the skeleton of the ‘pectoral fin
it is the Crossopterygii which ap-
proach mostnearly to Klasmobranchs.
In Polypterus (Fig. 839) the ee
lobe of the fin is supported by :
rod-like ossified propterygium (Pr),
Da ais a broad cartilaginous mesoptery-
Fig. $3).—Pectoral fin of Polypterus. olum (JZS), and an ossified meta-
gium 3 MT. peop ane pterygium (JZ7'); to these, two
NOREEN eh: SO rows of elongated radials (Aa, fa?)
du. first radials; Aa’, second radials. aye articulated fan-wise, and these
(From Wiedersheim’s Comparativn 2 3 4 3
Anatomy.) in their turn give attachment to
the fin-rays (#8). In all the re-
maining orders the basalia (pro-, meso-, ‘and me tapterygium) are
absent, nd the endoskeleton of the fin consists only of a single
or double row of radials (Fig. 815).
In Polypterus there is avestigial
pelvr ve girdle (Fig. 839 bis, BP) m
the form of a small rhomboidal
cartilage to which the anterior
ends of the basaha (Bas!) are at-
tached: thus in the structure of
the posterior extremities also, the
Crossopterygil are the most ae
tive of the Teleostomi. In all the
remaining orders the pe Avie girdle
is atrophied. The pelvic fin is ae My rit
ported by a single bone of variable
form (Fig. 816,
BSTG) and re-
ap es < 3 is Rad
presenting a basale, vc. a structure - $39 bis.—Pelvie fin of young Poly-
arising from the fasion of proxi- “pterns, = Wa (OH WERENGS ire,
basale ; BP. pelvic cartilages (fused in
mal pte rygiophores. Between its adult); Cep. epipubis ; Aad. radials.
From Wieders!
posterior end and the dermal Ce ed eee)
vays irregular nodules, representing radials, may be interposed.
XI PHYLUM CHORDATA 217
The distinction between hard or unjoited fin-rays, or spines,
and soft or jointed fin-rays has already been referred to. The
first ray of the dorsal and pectoral fins sometimes, e.g. in Siluroids
(Fig. 827), has the form of a very strong spine articulated by a
bolt-and-shackle joint, ie. by the interlocking of two rings. In
some cases the first dorsal spine springs from the skull.
The texture of the bones is subject to wide variation: in some
Acanthopteri they are very thick and strong, in some places
almost like ivory, while in the Lump-fish (Cyclopterus), the huge
Suntish (Orthagoris-
eus), and in many
deep-sea forms, such
as the Ribbon-fishes
(Regalecus and Trac-
hypterus), the amount
of mineral matter is
so small that the
bones are easily eut
with a knife and
weigh astonishingly
little when dry.
Electric organs.
Two genera of Teleo-
stomi possess electric
organs, the Electric
Cat-tish (Malapter-
urus), one of the Sil-
unidx, found in the
fresh waters of tropi-
cal Africa, and the
Electric Eel (Gyin-
notus), a2 Physostome
occurring in Brazil
and the Guyanas.
Fic. $40.—Gymnotas electricus, showing the extent of
ale EU » 2 electric organ (2). F/, ventral fin. B, small portion of
Hs Malapter ae the a cara OE DM.’ dorsal muscles; 2. EH. electric
electric organ ex- organ Fi, ventral fin; HW, skin; ZH, caudal canal ; Sep.
te | ere } 7 I 91 x fibrous septum ; VM. VM’. ventral muscles ; WSs, W S’,
ends over the whole vertebral column, with spinal nerves. (From Wiedersheim’s
body. beneath the Comparative Anatomy.)
skin; in Gymnotus
(Fig. 840) there are two pairs of batteries in the ventral half of
the greatly elongated tail. ‘
Digestive organs.—Some Teleostomi are toothless: but in
most instances teeth are present, and may be developed on the
premaxilla, maxilla, palatine, pterygoid, vomer, dentary, basi-
hyal, and superior and inferior pharyngeal bones. It is character-
istic of most Teleostei, with the exception of Physostomi, that the
maxilla is edentulous (Fig. 829) and does not enter into the gape.
218 ZOOLOGY SECT.
In a large majority of species the teeth are small, conical, and
recurved, suitable for preventing the struggling prey from shpping
out of the mouth, but quite unfitted for either tearing or crushing.
In some Fishes, such as the Pike, the teeth are hinged backwards
so as to offer no resistance to the passage of the prey towards the
gullet, but effectually barring any movement in the other direc-
tion. In many deep-sea Fishes (Fig. 834) the teeth are of immense
size and constitute a very formidable armature to the Jaws.
Many instances occur in which there is a marked differentiation
of the teeth, those in the front of the jaws (Fig. 841) being pointed
or chisel-edged, and adapted for
seizing, while the back teeth have
spherical surfaces adapted _ for
crushing. In the Wrasses (Fig.
830, B) strong crushing teeth are
developed on the pharyngeal bones.
In the Globe-fishes the teeth are
apparently reduced to one or two
in each jaw, but each “tooth” in
this case really consists of numer-
ous calcified plates fused together.
Fic. $41.—Premaxille of Sargus, The teeth may be either simply
nope aise eee Cen) imbedded in the mucous mem-
brane so as to be detached when the
bones are macerated or boiled, or they may be implanted in sockets
of the bone, or ankylosed to it. They are formed of some variety
of dentine, and are often capped with enamel. Their succession
is perpetual, ic. injured or worn-out teeth are replaced at all ages.
In some species the enteric canal shows little differentiation ito
regions, but, as a rule, gullet, stomach, duodenum, ileum, and
rectum are more or less clearly distinguishable. The stomach is
generally V-shaped, but its cardiac region may be prolonged into
a blind pouch; it is often very distensible, allowing some of the
deep-sea Teleostei to swallow Fishes as large as themselves. In the
Globe-fishes the animal can inflate the gullet with air, when it floats
upside down on the surface of the water. The Ganoids have a spiral
valve in the intestine, which is very well developed in Polypterus
and the Sturgeon, vestigial in Lepidosteus (Fig. 848, sp. v.) and
Amia : it is absent in all Teleostei, except possibly in Chirocentrus,
one of the Physostomi. The liver is usually large; a pancreas
may be present as a compact gland, as in Elasmobranchs, or may
be widely diffused between the layers of the mesentery. Pyloric
cwea are commonly present, and vary in number from a single one
to two hundred. The anus is always distinct from, and in front
of, the urino-genital aperture.
Respiratory organs.— The gills are usually comb-like, as in
the Trout, the branchial filaments being free, owing to the atrophy
NUL PHYLUM CHORDATA 219
ot the interbranchial septa. In the Sturgeon, however, the septa
are fairly well developed, reaching half-way up the filaments, so
that the latter are free only in their distal portions ; this arrange-
ment is obviously intermediate between the Elasmobranch and
Teleostean conditions. The most striking deviation from the
normal structure oceurs in Lophobranchit, in which the gill-
filaments are replaced by curious tufted processes (Fig. 832, B, y.)
As a rule gills (holobranchs) are developed on the first four
branchial arches, but the fourth is frequently reduced to a hemi-
branch, and further reduction takes place in some cases. The
Pic. $42.—A, Anabas scandens (Climbing Perch).
B, dissection of head, showing accessory respiratory
organ. (A, after Cuvier; B, after Giinther.)
pseudobranch or vestigial hyoidean gill nay either retain the
characteristic comb-like structure, as in the Trout, or may be
reduced, as in the Cod, to a gland-like organ formed of a plexus
of blood vessels and called a vaso-ganglion or rete mirabile.
In addition to the gills some Teleostei possess accessory organs
of respiration. In Amphipnous, an Indian Physostome, the gills
are poorly developed and are functionally replaced by a vascular
sac occurring on each side of the body and opening in front into
the first (hyo-branchial) gill-cleft. Such sacs are physiologically,
though not morphologically, lungs. In the Climbing Perch
(Anabas) of the Oriental Region (Fig. 842) the superior pharyngeal
bones are developed into folded plates (B) covered with vascular
220
ZOOLOGY SECT.
mucous membrane and capable of retaining water for a consider-
able period: the Fish is able to traverse the land, and is even said
Ww tp
I
Fic. 843.—Digestive organs and air-
bladder of Lepidosteus. «a.
anus ; 7. b. air-bladder ; a.b’. its
aperture in the phe xa Donc
aperture of bile-duet; ¢. pyloric
ceca; 7. b. gall-bladder ; hp. d.
hepatic duct ; 7. liver; py. pylo-
ric valve ; s. spleen; sp. v. spiral
valve ; sf. stomach. (From Wieder-
sheim's Comparative Anatomy.)
absorbed and replaced by
to climb trees, holding on alternately
by the spines of its pre-operculum and
of its ventral fins. It has become so
thoroughly a land animal that it is
drowned if immersed in water. In the
little armoured Siluroid Callichthys, anal
respiration takes place, air being drawn
into and expelled from the rectum.
And, lastly, in the curious little goggle-
eyed Periophthalmus of the Indian and
Pacific Oceans the tail-fin seems to
serve as a respiratory organ, being kept
in the water while the Fish perches on
a rock.
The air-bladder retains its connec-
tion with the gullet in Ganoids and
Physostomes; in the other Teleostei
the pneumatic duct atrophies in the
adult, and the bladder becomes a shut
sac. The pneumatic duct is always
connected with the dorsal wall of the
gullet except in Polypterus, in which
the aperture is ventral, and in some
Physostomes, such as the Herring, in
which it is connected with the stomach.
The bladder is sometimes divided into
compartments or produced into lateral
offshoots: in Amia, Lepidosteus (Fig.
843, «. b.), and Polypterus its wall is
sacculated or raised into anastomosing
ridges, enclosing more or less well-
marked chambers and thus resembling
a lung. In Polypterus its lung-like
character is enhanced by its division
into two compartments by a longitu-
dinal partition, as well as by the ven-
tral position of the opening of the
pheumatic duct.
The air-bladder seems able to act as
a sort of accessory respiratory organ ;
it has been found that in a Perch,
asphyxiated in stagnant water, the
oxygen in the bladder, which normally
amounts to 20 or 25 per cent., is entirely
vnitrogen and carbonic acid. Its normal
XII PHYLUM CHORDATA 221
function, however, is hydrostatic, Zc. it serves to keep the Fish of the
same specific gravity as the water. Variations in pressure as the
Fish ascends or descends are regulated by absorption or secretion of
gas, often by means of vaso-gangha or red glands (Fig. 844, vs. gn.)
in the walls. These are elevations of the wall of the bladder.
abundantly supplied with blood, and having tubular elands which
open into the cavity of the bladder and secrete a fluid of unknown
function. In Fishes with a pneumatic duct the red glands are
Fic. 844.—Horizontal section of posterior portion of head and anterior end of air-bladder in
Pseudophycis bachus, one of the Gadide or Cods (semi-diagrammatic). «, thickened
portion of air-bladder fitting into fenestra in posterior wall of auditory capsule ; «. bl. air-
bladder; au. cp. outer wall of auditory capsule; au. cp.’ inner (membranous) wall; 6,
hollow offshoots of air-bladder; cp. str. corpora striata; erb. cerebellum ; memb. lab. mem-
branous labyrinth; olf. /. olfactory lobes; olf. p. olfactory peduncles ; op. operculum ; opt. (.
optic lobes ; vs. yn. vaso-ganglia.
absent, but in Eels their place is taken by red bodies of similar appear-
ance but with non-glandular epithelium. In some forms with closed
air-bladder the anterior end of the organ is forked,and each branch
(a) fits closely against a membranous space in the posterior wall ot
the auditory capsule, while laterally it extends outwards in the
region of the shoulder-girdle, and comes to lie immediately beneath
the skin; in this way varying pressures on the surface of the body
are transmitted through the air in the bladder to the auditory
222, ZOOLOGY SECT.
organ. In the Carps and Siluroids a chain of bones connects the
ur-bladder with the auditory organ, forming the MWeberian
apparatus, the function of which, as of the simpler arrangement
described above, is probably “ to bring directly to the consciousness
of the Fish the varying tensions of the gaseous contents of the air-
bladder, due to the incidence of varying hydrostatic pressures.”
The structure of the heart forms one of the most striking
(ifferences between the three Ganoid orders and the Teleoste1. In
Ganoids there is a muscular conus arteriosus with rows of valves,
as in Elasmobranchs: in Teleostei a vestige of the conus containing
two rows of valves has been found in Albula, one of the Herring
family, but in no other member of the order. On the other hand,
Teleostei always have a large bulbus aortie, formed as a dilatation
ot the base of the ventral aorta.
In the brain the cerebellum and optic lobes are large; the
diencephalon is well developed in Ganoids, almost obsolete in Tele-
ostel. In Ganoids there is an unpaired
prosencephalon, which may be produced
Wa
yi - : =
if eae into lobes (Fig. 845, prs.) and has a non-
/ 7 nervous roof, giving off anteriorly a pair
of cerebral hemispheres (¢.h.) into which
the prosoccele is continued as a pair of
lateral ventricles or paracceles; thus the
fore-brain of Ganoids presents many re-
semblances to that of the Lamprey. In
Teleoste1 (Fig. 818) there are no cerebrad
hemispheres, but only an undivided pro-
sencephalon with a non-nervous roof or
pallium, and with its floor raised into large
rounded corpora striata. The Ganoids
agree with Elasmobranchs in the fact that
the optic nerves form a chiasma, while in
Teleoste1 they simply cross one another or
decussate. Here also, however, the dis-
Fic. $45.—Brain of Lepi-
dosteus, dorsal view. tinction 1s not quite absolute, since in the
chi. cereb 5c. jv. cere- ey - on A = p ys Be 2
Pil hemispheres. dcdiex. Herring and some other Physostomes one
cephalon; im. 0. medulla arve asses throue e Jat 7 she i.
Saeent: wie otene nerve passes through a slit in the other.
lobes; opt. 1. optic lobes; In some cases the olfactory lobes spring
Deis Balfour and directly from the prosencephalon, as in
Parker.) the Trout ; in others they are borne on long
; olfactory peduncles (Fig. 844, olf. p.), as in
the Cod. In some Plectognaths the spinal cord undergoes a re-
markable shortening: in a Sun-fish 24 metres in length and
weighing a ton and a half, the cord is only 15 millimetres long,
being actually shorter than the brain.
Urino-genital Organs.—The kidney (Fig. 817, kd.) is formed
from the mesonephros of the embryo and usually attains a great
NUL PHYLUM CHORDATA 223
size; the pronephros usually atrophies. The wreter (ar.) is the
undivided segmental duct = it unites with its fellow of the opposite
side before opening either chreectly on to the exterior or into a
urino-genital sinus. A urinary bladder is formed as a single or
double dilatation of the ureter, The right and left kidneys undergo
more or less fusion, and their anterior :
ends are usually converted into adenoid
or lymphatic tissue (Ad), so that, while
resembling the rest of the organ In ex-
ternal appearance, they do not discharge
a renal function.
The male organs of Lepidosteus may
be taken as an example of those of
Ganoids. The testis (Fig. 846, ts.) is a
paired lobulated organ, the secretion of
Which is carried by a large number of
vasa efterentia (v. ef.) into a longitudinal
canal (/.c.) lying alongside the ureter
(wr.). From this canal tubes are given
off which communicate with the urinary
tubules of the kidney, so that the
seminal fluid has to traverse these
tubules in order to reach the urinary
bladder (4/.) and make its escape by the
common urinogenital aperture (uw. g. Up).
In Teleostei there are no vasa etferentia,
but the posterior end of the testis is
directly continued into a duct (Fig. 817,
vr. d.) which unites with its fellow of the
opposite side and opens either into a
urino-genital sinus, as in the Trout, or,
as in the Cod, directly on to the exterior,
between the anus and the urinary aper-
ture. In the Eels the seminal fluid
escapes into the ccelome and is dis-
charged by genital pores.
In most Ganoids the ovidaets (Fig. Fie. 843.—Mole organs of Lepi-
> . ze ee dosteus. J/. bladder; /. ¢.
847, B, ovd.) have funnel-like anterior longitudinal canal; ts. testis:
ends (ov7”) opening into the coolome, 0,1, siege porte!
while posteriorly (ord.’) they discharge (After Balfour and Parker.)
into the dilated ureters (4/.). A similar
arrangement occurs in the Smelt, one of the Physostomi, in which
the eggs are discharged from the outer or lateral face of the ovary
into the open end of the oviduct. But in most Teleostei and in
Lepidosteus (Fig. 847, A) the ovary (ovy.) 1s a hollow sae continued
posteriorly into the oviduct (ovd.); the eggs are set free into its
cavity from the folds into which its inner surface is produced, and
224 ZOOLOGY SECT.
so pass directly into the oviduct without previously entering the
celome. An ovary of this kind reminds us of the state of things
in Arthropods, in which also the ovary is a hollow organ discharg-
ing its products into its internal cavity, whence they pass directly
into the continuous oviduct. It was pointed out that the lumen
of the ovary in this
case was to be
looked upon as a
shut-off portion of
the celome : this is
certainly the case
in Lepidosteus and
Teleostei. In the
embryo a longitu-
dinal fold grows
from the ventral
edge of the then
sohd ovary, and
turns upwards along
the lateral face of
the organ : it is met
by a descending told
of peritoneum from
the dorsal wall of
the abdomen, and
by the union of the
two folds a cavity
is enclosed, which 1s
the lumen of the
ovary. The oviduct
is developed as a
backward continua-
tion of these folds
of peritoneum, and
appears to be quite
eee eee ar ae ere one ea
kidney ; ovd. oviduct ; ord./ aperture of oviduct into bladder ; the embryonic ne-
ovd.”” peritoneal aperture; ovy. ovary; p. peritoneum ; *4: =
u.g. ap. urino-genital aperture ; vr. ureter. (A, after Balfour phridial system, and
and Parker; B, after Huxley.) therefore not to be
homologous with
the oviducts of Elasmobranchs and Holocephali, which, as we
have seen, are Miillerian ducts. In the Salmonide and the
Eels oviducts are absent, and the ova are discharged by genital
pores, which are probably to be looked upon as degenerate
oviducts. True abdominal pores are present in Ganoids and in
some Physostomi. Most Teleostomi are dicecious, but Serranus,
one of the Perch family, is hermaphrodite and self-impregnating,
&
L
&
"i
\
Se eee
—— LL Gf. ap
XIII PHYLUM CHORDATA 225
and there are many well-known species, such as the Cod and the
Herring, which exhibit the hermaphrodite condition as an occa-
sional variation,
Reproduction and Development.—Most Teleostomi are
oviparous, the eggs being impregnated after they are laid, but
in some Teleostei, such as the Viviparous Blenny (Zoarces), internal
impregnation takes place; the young are developed in the hollow
ovary and are brought forth alive. Many instances of parental
care of the young are known, the most familiar being that of the
male Stickleback (Gasterosteus), which constructs a nest of weeds,
fastened together by a glutinous secretion of the ladneys, and
jealously guards the developing young. In the Sea-horse (Lippo-
campus) and the Pipe-tish (Syngnathus) the young are developed
in a pouch (Fig. 832, bra. p.) on the abdomen of the male. In
the Siluroid Aspredo the eggs are pressed into the soft spongy
skin of the belly and thus carried about by the parent. The
ova are always small as compared with those of Elasmobranchs,
never exceeding 5 to 10mm. in diameter, and bemg usually
much smaller. They are rarely protected by an ege-shell, They
are produced in immense numbers, a single female sometimes
laying several millions: in such cases the mortality among the
unprotected embryos and young is immense. The eges may be
pelagic, ae. so light as to float when laid, as in the Cod, Haddock,
Turbot, Sole, &., or demersal, 7.e. so heavy as to sink to the
bottom, as in the Herring, Salmon, Trout, &c. In some cases
they become cemented to the surface of a rock.
In all the Ganoids hitherto investigated segmentation is com-
plete, but very unequal (Fig. 848): the megameres are immense
as compared with the micromeres, and
the process may be said to be inter-
mediate between the holoblastic and
meroblastic types. In Teleostei, on
the other hand, segmentation is al-
ways partial and discoidal. The general
features of development are much the
same as in the Trout, except that in
the Sturgeon there is an open medul-
lary groove. There is frequently a
metamorphosis : in Lepidosteus, for in-
stance, the newly hatched OTS is Fic. 845.,—Segmentation in Lepi-
provided with a sucking-dise, and the dosteus. (After Balfour and
proportions of the head are quite dif- a
ferent from those of the adult. In the
larval Sturgeon provisional teeth are present, and in many
Teleostei the young differ from the adult in the presence of large
spines, which probably, ike the spines in the zowa-stage of some
Crustacea, serve a defensive purpose. The larvee of Eels are
VOL. II Q
226 ZOOLOGY SECT.
strongly compressed, perfectly transparent, and have colourless
blood. They are sometimes known as “ Glass-fish,” and were
formerly placed in the genus Leptocephalus, their real nature beimg
unknown. The Crossopterygii (or at least Polypterus) are unique
Fic. $49.—Polypterus bichir. Head of advanced larva; E. G. external gill. (From Dean,
after Steimdachner.)
in the sub-class in possessing, on each side, a single external gill,
as in Dipnoi and Amphibia (vide infra).
The Geographical Distribution of the Ganoid Teleostomi is
curiously limited: they are all essentially fresh-water forms—
although some Sturgeons are found in the sea—and are almost
exclusively inhabitants of the Northern Hemisphere, and especially
of the Holaretic Region. The Chondrostei occur in the rivers of
Europe, Asia, and North America: one genus of Sturgeons
(Scaphirhynchus) lives in the Mississippi and in the rivers of
Central Asia, but not in the intermediate regions: in the same
way Polyodon is found only in the Mississippi, while the closely-
alhed Psephurus is found in the Yangtse-kiang and Hoangho—a
striking instance of discontinuous distribution. Amia is found
in the fresh waters of the United States; Lepidosteus extends
also into Central America and Cuba. Polypterus lives in the
Upper Nile and some other tropical African rivers; Calamoichthys
in the Old Calabar River.
Among Teleostei the Physostomi are largely, though not ex-
clusively, fresh-water Fish; the Carps, Eels, Salmonoids, and
Siluroids are important examples. The Acanthopteri, Pharyngo-
enathi, and Anacanthini are mostly marine, some being in-
habitants of the shores, some pelagic, some abyssal, extending
to a depth of nearly 3,000 fathoms. As we have seen, many
species are practically terrestrial. All the sub-orders are uni-
versally distributed, so that we have to descend to families before
meeting with any important facts in geographical distribution.
The Distribution in Time of the Teleostomi is interesting
as showing the gradual replacement of the lower or more
generalised members of a group by the higher or more specialised
forms. During the whole of the Palzozoic and the greater part
of the Mesozoic era the three orders of Ganoids, to-day small
and isolated groups, formed the whole of the Teleostomian fauna,
XU PHYLUM CHORDATA 227
and it is not until the Cretaceous period that the Teleostei, the
present dominant order, make their appearance. From the Cre-
taceous onwards the Ganoids undergo a progressive diminution
in numbers, genus after genus and family after family becomine
extinct, while a corresponding increase takes place in all the sub-
orders of Teleostei 5;
The Crossopterygii make their first appearance in the Devonian
period, and, between that period and the Cretaceous, include six
famihes and a large number of genera and species. They exhibit
(Fig. 850) a very considerable range of variation in external
] ME ////
MUMIA, Oy
Fic. 850.—A, restoration of Glyptolepis (Devonian); B, Macropoma mantelli (Cretaceous)
a. bl. ified air-bladder ; d. 7.1, d. 7. 2, dorsal fins; h. a. hzemal rches ; jug. pl. jugular plates ;
n. a. neural arches; nach. position of notochord ; pet. f. pectoral fin; pv. f. pelvic fin; v. f.
ventral fin. (From Nicholson and Lydekker.)
and internal characters. There are usually two dorsal fins, the
tail may be diphycereal or heterocercal, the scales rhomboid or
cycloid. In some genera, also, there was a persistent notochord
(LB. neh.), the fossils showing well-preserved neural and hemal
arches, but no signs of centra. In many cases the interspinous
bones or proximal pterygiophores of the dorsal fins are fused into
a single basal bone. All agree in the possession of lobed fins.
the basal lobe is sometimes so long as to approach the type of
structure we shall find to characterise the Dipnoi (vide infra).
The Chondrostei are also largely represented, from the Devonian
upwards, and include a great variety of forms, many of which,
Q 2
228 ZOOLOGY SECT.
apart from the heterocercal tail, have a strong external re-
semblance to Teleostei (Fig. 851). Some have the characteristic
spindle-form of strong-swimming Fishes (A), others the high
compressed form of such shore-fishes as_ the Reef-fishes (B).
Scutes are present in some species, rhomboid scales 1n others,
and in one genus the greater part of the body 1s covered by
BE RN
i i
TT HESS
HS
Fic. $51.—A Paleeoniscus macropomus (Permian); B,Platysomus striatus (Permian).
(From Nicholson and Lydekker.)
cycloid scales, while rhomboid scales occur in the upper part of
the tail.
The Holostei first make their appearance in the Triassic rocks
and are abundant in secondary and lower tertiary strata. They
also (Fig. 852) show a wide diversity in form and structure. The
body may be spindle-shaped or high and compressed; the scales
may be rhomboid or cycloid, or may exhibit every gradation from
rhomboid to cycloid in passing from the trunk to the tail of one
and the same Fish; the teeth may be sharp and conical, or blunt,
rounded, and adapted for crushing. A persistent notochord is
present in some species, a well-ossified vertebral column in others.
XIII PHYLUM CHORDATA 229
MW e see, then, that all the orders of Ganoids, during the period
of their prime, branched out into diverse forms, adapted to
different environments, and often resembling, in a remarkable
manner, the divergent forms of Teleostei which il similar
positions at the present day.
ml pit ed ae i
The Teleostei first appear in the Cretaceous rocks, where many
existing families are represented, From this period onwards the
three Ganoid orders undergo a progressive diminution in the
=
ss
eeaeel
66
él
asee
aE cee
eee cere reciente w ETRE TE CEGCE CECE
aad seats ceat tee CELE, 666 bbeGe ee
Saag ttesecc eee g apy ee
Retcacci cde bat CbkEE RR EEE
secrete PP MMLLELE GLEE leuk
bj RS be
fa ic ictalsl
KR
NG
Sa AAQYy
SMHVMBio9w!
Fic. 852.—A, Lepidotus maximus (Jurassic). s. scale; ¢. teeth. B, Caturus furcatus
(Jurassic). (From Nicholson and Lydekker.)
number of families, genera, and species, their places being taken
by the more highly differentiated Teleostei, until, at the present
day, as we have seen, they are reduced to a few scattered forms,
mostly confined to fresh waters.
Sub-class V.—The Dipnoi.
The Dipnoi or Lung-fishes, comprising as their living repre-
sentatives only the Queensland Ceratodus or “ Burnett Salmon,”
and the Mud-fishes (Protopterus and Lepidosiren) of certain South
African and South American rivers, are fishes of such well-marked
and special features that by some zoologists they ave separated
trom the true Fishes and regarded as constituting a separate class
230
of Vertebrates.
is fish-like
is covered with very large imbricated cycloid scales, somewhat
(Fig.
ZOOLOGY SECT.
One of their peculiar features 1s indicated by the
853.—Ceratodus Forsteri. Reduced. (From Dean, after Gimther.)
Fic.
853) with
name Dipnoi. Not only do these ani-
mals breathe by means of gills, like
tae y Fishes, ‘but they fae a highly
dev ela apparatus for ‘the respiration
of lung or lungs—w ith an
arrangement OF the Greulaion co-
ordinated with this. They have bony
seales and dermal fin-rays, but the
paired fins, unlike those of any other
fishes, with the exception of certain
extinct Elasmobranchs, are constructed
on the type of the archipterygium
(see p. 155).
1. EXAMPLE OF THE CLASS—Cer-
todus Forsteri.
The Ceratodus or “ Burnett Salmon”
(Fig. 853) is by far the largest of the
Dipnoi, attaining a length sometimes
of four or five feet. es occurs at the
present day only in the Burnett and
Mary Rivers in Queensland, but fossil
teeth referred to he same or nearly re-
lated genera have been found ina bund-
ance in Paleozoic and Mesozoic beds
in Europe, America, the East Indies,
Africa, and Australia. It lives in still
pools in which the water in the dry
season becomes extremely stagnant and
overladen with decomposing vegetable
matter; and itis only by rising to the
surface occasionally, and taking air into
its lung, that it is enabled to obtain
sufficient oxygen for purposes of re-
spiration. Its food consists of such
small annals as live among the water-
plants and decaying eee and in
order to obtain a sufficient amount of
such food it swallows relatively large
quantities of vegetable matter, which
passes with little or no alteration
through its enteric canal.
External Characters.—The lod
1a diphycercal caudal fin. The surface
XI PHYLUM CHORDATA 231
smaller towards the tail. The limbs have a characteristic shape,
bemg in the form of two pars of elongated, leaf-lke, pointed
paddles. The mouth is situated on the ventral surface of the
head, close to the anterior extremity of the snout. The external
nares differ from those of other Vertebrates in being situated
immediately outside the aperture of the mouth, enclosed within
the upper lp. A paw of internal nares opens not far behind
them into the anterior part of the mouth cavity. At the root of
the tail is the cloacal aperture. There is an operculum similar
to that of the Teleostomi, with a single slit-like branchial aperture
behind it.
Endoskeleton.—The spinal column (Fig. 854) is represented by
a persistent notochord enclosed in a sheath without any trace of
5 oo lam
ce |_———, suprise /
— ~ « ! 7 ff
suborb
dent—~ p
Fic. 854.—Ceratodus Forsteri. Lateral view of the anterior portion of the skeleton.
A, anterior median membrane-hone of the roof of the skull. 8, posterior median membrane-
bone. bas. basal cartilage of the pectoral fin; 67. branchial arches; iat. inter-operculum ;
lum. plate overhanging®* branchial region ; ick, Meckel’s cartilage 5 occ. 7b. occipital rib; op.
operculum ; pal. palato-quadrate ; pet. pectoral arch; bs. ribs; sub. orb. sub-orbital bones ;
8g. squamosal ; supra-se. supra-scapula.
separate vertebre, except im the caudal region, the segmentation
being indicated by the metamerically arranged neural arches and
ribs. Each neural arch, composed partly of cartilage, partly of
bone, bears on its summit a slender rod composed of three segments
representing a neural spine, a basal cartilage, and a radial cartilage
—the two last extending into the unpaired fin. In the caudal
region the hemal arches present a similar arrangement. The
most anterior of the vertebree are coalescent with one another and
with the skull. At the sides of the pree-caudal region are a series
of rod-like cartilages of the nature of ribs.
The skull (Figs. 854, 855 and 856) consists of an undivided mass
of cartilage, narrowest between the orbits and broadening before and
behind ; posteriorly it is prolonged into a plate (/am.) overhanging
232
the branchial region
Imbedded in the ecar-
tilage of the posterior
part are a small pair
of exoccipital ossifica-
tions. On the upper
surface two unpaired
(4 and #) and four
paired (Cand Sq) mem-
brane-bones overlie the
cartilage; and on the
ventral surface is a
large membrane-bone
(Fig. 856, P. sph.) re-
presenting the para-
sphenoid of the Tele-
ostomi. Rudimentary
vomers (Vo.) support
the vomerine teeth In
front is a pair of small
upper labial cartilages.
Apalatoquadrate carti-
lage (pal.) firmly fixed
to the side-wall gives suppo
ZOOLOGY
SECT.
art
5b.—Ceratodus Forsteri,
s 4,anterior median membrane-bone ; art. articular
surface for second fin- Tay; B, posterior median membrane-
bone; C, inner lateral membra e-bone; lab. labia car-
tilages 5 /ai. process projectix over gills; op. oper-
eulum; pr.orb. pree-orbital ocess of chondrocranium ;
sg. squamosal. (After Huxley.)
Dorsal view of the
sb. orb, sulb-orbital bones
rt to the mandible, and seems to contain
representatives, not only of the palatine, pterygoid and quadrate,
¢. 856. —Ceratodus Forsteri.
Vv
y of the skull.
palatine teeth; d’
anterior and posteri vy nares
region of palato-pteryg: P.sph.
sphenoid ; Pt. pterygoid; Qu. quac
region; Vo. vomer. (From Dean,
Giinther.)
¢, occipital rib; d,
, vomerine teeth; na.
P. pal: “ie
but of the hyomandibular and
symplectic of the Teleostomi.
In front it contains a palato-
pterygoid ossification. Behind
it a small cartilage immovably
fixed to the Brae wall of the
skull is probably the opercular
cartilage. One of the two
lateral | membrane-bones (Sq)
situated over the palato-quad-
rate 1s the squamosal. Opereu-
lar (op.) and inte sropercular (at. )
bones support the operculum.
The hyoid (hy.) and branchial
arches (br. )are cartilaginous. Of
the latter, four are completely
developed, each consisting of a
dorsal and a ventral cartilage ; a
fifth is represented by a aad
ment attached to the fourth.
The pectoral arch (Fig. 854,
pet.) is va stout cartilage with a
entral
ates
XII PHYLUM CHORDATA 233
pair of investing bones. The skeleton of the pectoral fin consists
of a stout basal cartilage (bas.), an elongated tapering central axis
made up of a number of short cartilaginous segments, and two rows
of jointed cartilaginous rays extending out on either side of the
AXIS so as to support the expanse of the fin.” The pelvic arch is
a single cartilage, produced forwards into an elongated rod-like
process. The skeleton of the pelvic fin (Fig. 857) is similar to
that of the pectoral.
Digestive Organs.—The fecth (Fig. 856) are of a remark-
able and characteristic shape. There are two pairs of large
compound teeth of similar character, one pair (d.) on the
roof of the mouth (palato-pterygoid region) and the other
on the lower jaw. Each is a curved plate with the convex
border, which is directed inwards and somewhat backwards,
entire, while the concave border presents a series of six or seven
Fic. 857.—Ceratodus Forsteri. Pelvic arch and skeleton of pelvic fin. (After Giinther.
bluntly pointed projections or cusps. In addition to these there
are, in front of the palatine pair, a pair of much smaller, simple,
somewhat chisel-like vomerine teeth (d’) placed close together and
directed vertically.
In the enteric canal the chief feature of special interest is the
presence, throughout the length of the intestine, of a spiral valve
similar to that of the Elasmobranchs and Ganoids. The. rectum
opens into a small cloaca. A pair of abdominal pores open just
behind this.
Organs of Respiration.—Ceratodus combines aquatic respira-
tion by means of gills similar to those of true fishes, with aerial
respiration by means of a lung.
There are four pairs of gil/s, each consisting of a double row of
gill-filaments supported on the branchial arches. A rudimentary
hyoidean gill or psewdobranch is present as well. The lung (Fig. 858)
is an elongated median sac connected with the ventral wall of the
234 ZOOLOGY SECT.
pharynx by a slit-like aperture, the glottis. Its internal surtace 1s
sacculated, and a regularly-arranged series of blind pouches open
out of the main central cavity. This dwg of Ceratodus corresponds
morphologically to the air-bladder
of Ganoids and Teleosts, but differs
from it in its blood-supply, and con-
sequently in its function, being sup-
plied with venous blood by a special
pulmonary artery and acting as an
important organ of respiration.
Blood Vascular System.—Co-
ordinated with the existence of a lung
and distinct pulmonary circulation is
a complication in the structure ot
the heart. The sinus venosus 1s 1m-
perfectly divided into two parts, and
the cavity of the auricle is divided
into two by an incomplete septum
in the form of a ridge. The venous
blood enters the right-hand division
of the sinus venosus and passes
thence through the right-hand divi-
sion of the auricle to the ventricle ;
the pulmonary vein, by which the
blood is returned from the lung,
Fic. 898.—Ceratodus Forsteri. opens into the left-hand division of
Ae teal wall GEL ease eetubeseinus, and! ats blood teachecuthe
show theinterior. (After Gunther.) ventricle through the left-hand divi-
sion of the auricle. There are no
auriculo-ventricular valves guarding the opening between the
auricle and the ventricle. A contractile conus arteriosus 1s
present, and has a remarkable spirally-twisted form; im its in-
terior are eight transverse rows of valves, and its cavity is divided
imperfectly by means of an incomplete longitudinal septum.
The blood-vessels (Fig. 859) present an arrangement which is
intermediate im some respects between that which has been already
described as observable in the Elasmobranchs and that which will
be found to characterise the Amphibia. The four afferent
branchial arteries (aff.) take their origin close together, immediately
in front of the conus, so that a ventral aorta can hardly be
said to exist. Each branchial arch has two efferent branchial
arteries (¢p7.). A hyoid artery (hy. art.) is connected dorsally and
ventrally with the most anterior of these. The eight efferent
vessels unite in pairs to form four epibranchial arteries. The
latter unite dorsally to form a main trunk, which combines with
the corresponding trunk of the opposite side to form the median
dorsal aorta (d. a.). The head is supplied by carotid branches
XU PHYLUM CHORDATA 235
given off from the first epibranchial (2. ané. car. and 7. ant. car.)
and from the hyoidean arteries (/. post, car. and 7, post. car.), and
the latter also gives off a lingual artery to the tongue. From
the last (fourth) epibranchial artery arises the pulmonary artery
(2. pul. art. and 7, pul. art.), carrying blood to the lung. ~ ;
ee Cpostcar Lantcar
rbrvu~
4
ECU.
La
59.—Ceratodus Forsteri. Diagrammatic view of the heart and main blood-vessel,
as seen from the ventral surface. af. 1,2, 3,4, afferent vessels ; 1 br, 2 br, 3 br, 4 br, position of
gills ; c. a. conus arteriosus; 7. w. dorsal aorta; d.c. ductus Cuvieri; epi. 1, epi. 2, epi. 3, epi. 4,
efferent branchial arteries; hy. wrt. hyoidean artery ; 7, v, c, post-caval vein; /. ant. car. left
anterior carotid artery; /. avi. left auricle ; 1. br. v. left branchial vein ; /. jug. v. left jugular
vein ; /. post. car. left posterior carotid artery; (. post. card. left posterior cardinal vein
l.:pul. art. left pulmonary artery ; /. v. left sub-scapular vein; 7. ant. car. right anterior
carotid artery; 7. aur. right auric . br. v. vight brachial vein; 7. jug. v. right jugular
vein; 7. post. car. right posterior carotid; 7. pul. art. right pulmonary artery; 7. se. ¢
right sub-scapular vein ; vent. ventricle. (After Baldwin Spencer.)
rt (d.c.),as in the Dog-fish (p. 146).
There are two ductiis Cur
The right ductus is formed by the union of jugular (7. jug. v. and
rv. jug. v.), brachial (1. br. v. and 7. br. v.), and subscapular veins
(1.se.v. and 7. sc.v.). The left receives in addition a left posterior
cardinal vein (1. post. card.). A large lateral cutaneous ven running
superficially along the side of the body opens into the subscapular.
236 ZOOLOGY SEUT. XIII
A large post-caval vein (i. v. c.) brings back the greater portion
of the blood from the posterior parts of the body ; it is situated
somewhat to the right of the middle line, and opens into the sinus
venosus between the two hepatic veins. The post caval is present
in no other Fishes, but is universal in all the higher classes. Pos-
teriorly the posterior cardinal and the post-caval are formed by the
bifurcation of a median caudal vein ; close to its origin each re-
celves the efferent renal veins bringing back the blood from the kid-
ney. The blood from the pelvic fin is brought back by an 7/ae vein
which divides into two branches. One of these, running forwards and
inwards, unites mesially with the corresponding vessel of the op-
posite side to form a median abdominal vein—a vessel not present in
the Fishes, but universal in the Amphibia ; it opens into the sinus
venosus. The other branch is the renal portal vein; after receiving
tributaries from the posterior region of
the body it passes to the corresponding
kidney.?
Brain.—The whole brain (Fig. 860)
is enclosed in a tough and thick mem-
brane, which becomes glandular in two
positions—on the roof of the diaccele
and on that of the metaccele. In the
former position this glandular deve-
lopment of the enclosing membrane or
choroid plexus passes downwards into the
diaccele and is developed into a spongy
mass which is prolonged forwards to the
anterior end of the prosencephalon.
The prosencephalon (pros.) presents two
elongated hemispheres, which are com-
pletely separated except posteriorly,
where they are united by a narrow com-
missure. The contained cavity is divided
into two bythe prolongation of the choroid
plexus already referred to. The nervous
‘Ig, S60—Brain of Ceratodus wall of the hemisphere is very thin and
Forsteri, dorsal view. aud. : J,
auditory nérve; ebl. cerebellum; 18 incomplete dorsally and internally.
ay eeeal ied aoe @ Lherenis van ipatrcor large olfactory lobes
mes. mesencephalon; (7), each with its cavity or rhinoccele.
nerve. (Chiefly after Sanders.) SUMMmit of a conical membranous cap
on the roof of the third ventricle. The
, infundibulum developes a pair of lobi
inferiores. The mesencephalon (ieso.) is bilobed, but the division
is not strongly pronounced. The cerebellum (cbl.) is very small,
' How far this arrangement combines fish-like and amphibian characters will
be best understood at a later stage.
I fF —coel. Af 2
Fic. 861.—Ceratodus Forsteri. Reproductive organs of female; the inner surface of the
right and the outer surface of the left ovary shown. co/. ap. coelomic aperture of oviduct ;
lic. portion of the liver ; /. ov. leftovary ; /. ov’. its posterior termination ; /. ovd. left oviduct ;
rv. ov. vight ovary ; 7. ov’, its posterior termination ; 7, ovd. right oviduct. (After Giinther.)
to
38 ZOOLOGY SECT.
being little more than a transverse bridge of nerve-matter over
the anterior end of the fourth ventricle. The medulla (med.) is of
relatively large size.
Urinogenital Organs.—The kidneys are short, being confined
to the posterior portion of the body-cavity, and are firmly attached
to the ovaries or testes. Each has a thick-walled ureter which
joins its fellow, the passages, however, remaining distinct to
near the opening into the urinogenital division of the cloaca,
when the right opens into the left.
There are two elongated ovaries (Fig. 861, ov.) which remain