Alimentary Canal.—The alimentary canal forms a tube traversing the body from mouth to cloacal opening. Corresponding with structural and functional differences it is for descriptive 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 inpushing 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.
In certain cases (e.g. Lepidosiren)[9] 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 cleft is known as the hyomandibular cleft, the next one as hyobranchial. In common usage the hyomandibular cleft is called the spiracle, and the series of clefts behind it the branchial 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 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 Teleostomes—including 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 Heterotis)[10] the gill lamellae 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 backgrowth 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 branchial 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 or closed 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 organ of problematical function.[11]