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ANATOMY]
253
ICHTHYOLOGY


purposes divided into the following regions~(1) Buccal cavity or mouth cavity, (2) Pharynx, (3) Oesophagus or gullet, (4) Stomach, (5) Intestine, and (6) Cloaca. The buccal cavity or mouth cavity is-morphologically a stomodaeum, i.e. it represents an in pushing of the external surface. Its opening to the exterior is wide and gaping in the embryo in certain groups (Selachians and Crossopterygians), and even in the adult among the Cyclostomata, but in the adult Gnathostome it can be voluntarily opened and shut in correlation

with the presence of a hinged

jaw apparatus. ' The mouth

opening is less or more ventral

in position in Cyclostomes and

Selachians, while in Dipnoans

and Teleostomes it is usually

terminal.

P-Q.

fr ggi

ag ii

f bc

% M

From Bridge, Cambridge Natural llistory, vol. vii., “Fishes, &c.” (by permission of Macmillan & Co., Ltd.). After Boas, Lehrbuc/1 der Zoologie (by permission of Gustav Fischer).

F IG. 8.-Diagrams to illustrate

the relations of bronchial clefts and pharynx in an Elasmobranch (A)

and a Teleost (B); 1, 2, &c., Branchial septa.

b.c, Opercular cavity.

b.l, Respiratory lamellae.

c, Coelom.

e.b.a, Opercular opening.

hy.a., Hyoid arch.

Hyobranchial cleft.

Valvular outer edge of gill

septum.

11, Nasal aperture.

Oesophagus.

hyf,

1.5,

aes,

op, Operculum.

Palato quadrate cartilage.

In certain cases (ag. Lepidosiren)l the buccal cavity arises

by secondary excavation without

any actual pushing in of

ectoderm.

It is highly characteristic

of the vertebrata that the

pharynx-the portion of the

alimentary canal immediately

behind the buccal cavity communicates with the exterior

by a series of paired

clefts associated with the

function of respiration and

known as the visceral clefts.

It is especially characteristic

of fishes that a number of

these clefts remain open as

functional breathing organs in

the adult.

The visceral clefts arise as

hollow pouches (or at first solid projections) of the endoderm.

Each pouch fuses with the

ectoderm at its outer end and

then becomes perforated so as

to form a free communication

between pharynx and exterior.

The mesenchymatous packing

tissue between consecutive

clefts forms the visceral

arches, and local condensation

within each gives rise to important skeletal elements-to

which the name visceral arches

is often restricted. From the

particular skeletal structures

which develop in the visceral

arches bounding it the anterior

Ph' lih?“'Y“x' cleft is known as the hyoman-Sp spiracle' dibular cleft, the next one as hyobranchial. In common usage the hyomandibular cleft is called the spiracle, and the series of clefts behind it the bronchial clefts.

The typical functional gill cleft forms a vertical slit, having on each side a gill septum which separates it from its neighbours in the series. The lining of the gill cleft possesses over a less or greater extent of its area a richly developed network of capillary blood-vessels, through the thin covering of which the respiratory exchange takes place between the blood and the water which washes through the gill cleft. The area of respiratory surface tends to become increased by the development of outgrowths. Frequently these take the form of regular plate-like structures known as gill lamellae. In the Selachians these lamellae are strap-like structures (Elasmobranch) attached. along nearly their 1). Graham Kerr, Quart. Journ. Micr. Sci. xlvi. 423. whole length to the gill septum as shown in fig. 8, A. In the Holocephali and in the sturgeon the outer portions of the gill septa have disappeared and this leads to the condition seen in the higher Teleostomes (fig. 8, B), where the whole of the septum has disappeared except its thick inner edge containing the skeletal arch. It follows that in these higher Teleostomesincluding the ordinary Teleosts-the gill lamellae are attached only at their extreme inner end.

In the young of Selachians and certain Teleosts (e.g. Gymnarchus and Heterutis)'3 the gill lamellac are prolonged as filaments which project freely to the exterior. These must not be confused with true external gills.

The partial atrophy of the gill septa in the Teleostomes produces an important change in their appearance. Whereas in the Selachian a series of separate gill clefts is seen in external view each covered by a soft valvular back growth of its anterior lip, in the Teleostean fish, on the other hand, a single large opening is seen on each side (opercular opening) covered over by the enormously enlarged valvular flap belonging to the anterior lip of the hyobranchial cleft. This flap, an outgrowth of the hyoid arch, is known as the operculum.

In the Teleostomi there are usually five functional clefts, but these are the survivors of a formerly greater number. Evidence of reduction is seen at both ends of the series. In front of the first functional cleft (the hyobranchial) there is laid down in the embryo the rudiment of a spiracular cleft. -In the less highly organized fishes this survives in many cases as an open cleft.

In many sharks and in sturgeons the spiracle forms a conspicuous opening just behind the eye. In rays and skates, which are modified in correlation with their ground feeding habit, the spiracle is a large opening which during the great widening out of the body during development comes to be situated on the dorsal side, while the bronchial clefts come to be ventral in position. In existing Crossopterygians the spiracle is a slit-like opening on the dorsal side of the head which can be opened orclosed at will. In Dipneusti, as in the higher Teleostomes, the spiracle is found as an embryonic rudiment but in this case it gives rise in the adult to a remarkable sense orgarl of problematical function?

Traces of what appear to be pre-spiracular clefts exist in the embryos of various forms. Perhaps the most remarkable of these is to be found in the larval Crossopterygian# and apparently also in Amiaft at least, amongst the other ganoids, where a pair of entodermal pouches become cut off from the main entoderm and, establishing an opening to the exterior, give rise to the lining of the cement organs of the larva. Posteriorily there is evidence that the extension backwards of the series of gill clefts was much greater in the primitive fishes. In the surviving sharks (Chlamydoselachus and N otidfmus cinereus), there still exist in the adult respectively six and seven bronchial clefts, while in embryonic Selachians there are frequently to be seen pouch-like outgrowths of entoderm apparently representing rudimentary gill pouches but which never develop. Further evidence of the progressive reduction in the series of clefts is seen in the reduction of their functional activity at the two ends of the series. The spiracle, even where persisting in the adult, has lost its gill lamellae either entirely or excepting a few vestigial lamellae forming a “pseudo branch” on its anterior wall (Selachians, sturgeons). A similar reduction affects the lamellae on the anterior wall of the hyobranchial cleft (except in Selachians) and on the posterior wall of the last bronchial cleft.

A pseudo branch is frequently present i11 Teleostomes on the antcrior wall of the hyobranchial cleft, Le. on the inner or posterior-face of the operculurn. It IS believed by some morphologists to belong really to the cleft in front.“ °

Phylogeny.-"l'lie phylogeny of the gill clefts or pouches is uncertain. The only organs of vertebrates comparable with them morphologically are the enterocoelic pouches of the entoderm which 2 J. S. Budgett, op. nit.

“ W. E. Agar, Anat. Anz., 1905, S. 298. 4 j. Graham Kerr, The Budgett Jllemorial Volume. 5 J. Phelps, Science, vol. NS. ix. p. 366; j. Eycleshymer and Wilson, Amer. Journ. Anal., v. 1906, p. 154.

6 F. Maurer, Morphol. Ja/1rb.ix., 1884,5.22'),21I”l£lXlV., 1888.5 175.