In Chondropterygians (Fig. [55]) the stomach is divided into a cardiac and pyloric portion, the former frequently terminating in a blind sac, and the latter varying in length. The pyloric portion is bent at its origin and end, and separated from the short duodenum (called Bursa entiana in these fishes) by a valve; the ductus hepaticus and pancreaticus enter the duodenum. This is succeeded by the straight intestine provided with the spiral valve, the coils of which may be either longitudinal and wound vertically about the axis of the intestine, as in Carcharias, Galeocerdo, Thalassorhinus, and Zygœna, or they may be transverse to that axis, as in the other genera. The number of gyrations in the latter case varies: there may be as many as forty. The short rectum passes into a cloaca, which contains also the orifices of the urogenital ducts. Only the commencement and end of the intestinal tract are fixed by mesenterial folds.
In the Holocephali and Dipnoi, the intestinal tract is short, straight, and wide, without stomachic dilatation, a pyloric valve, close to which the ductus choledochus enters, indicating the boundary of the intestine proper (Fig. [57], p). The spiral valve is perfect, and makes from three (Chimæra) to nine (Ceratodus) gyrations. A cloaca is present, as in Chondropterygians. A mesentery fixing the dorsal side of the intestine is absent.
Fig. 55.—Siphonal stomach and spiral valve of Basking-Shark (Selache). (After Home and Owen.)
a, Œsophagus; b, Cardiac portion of stomach; c, pyloric portion; d, pouch intermediate between stomach and duodenum, with circular valves at both ends; e, Duodenum; f, Valve of intestine; g, Ductus hepaticus; h, Spleen.
The other Ganoids resemble again more the Chondropterygians in the structure of their intestinal tract. The stomach has always a distinct pyloric portion, and has a still more complicated structure in Acipenser. The duodenal portion receives the contents of Appendices pyloricæ, which are confluent into a gland-like mass in Acipenser, but separate in Polyodon, and numerous and short in Lepidosteus, whilst Polypterus possesses one such appendage only. A spiral valve is developed in the Sturgeons and Polypterus, but in Amia, in which the intestine performs several convolutions, the four gyrations of the valve are situated far back towards the end of the intestine. In Lepidosteus the valve is rudimentary, and indicated only by three raised lines crossing the terminal portion of the intestine. In all these Ganoids the rectum has a separate opening, without cloaca.
The structure of the intestinal tract of Teleosteous fishes is subject to so numerous modifications that we should go beyond the limits of the present work if we would attempt to enter into details. Great differences in this respect may be found even in groups of the same natural families. Frequently the intestinal tract remains of nearly the same width throughout its course, and only the entrance of the various ducts serves as a guide for the distinction of its divisions. An intestine of such uniform width may be straight and short, as in Scombresocidæ, Symbranchidæ, or it may be more or less convoluted and long, as in many Cyprinidæ, Doradina, etc. On the whole, carnivorous fishes have a much shorter and simpler intestinal tract than herbivorous.
In the majority of Teleosteans, however, œsophagus, stomach, duodenum, small intestine and rectum, can be more or less distinctly, even externally distinguished.
There are two predominant forms of the stomach, intermediate forms being, however, numerous. In the first, the siphonal, it presents the form of a bent tube or canal, one-half of the horse-shoe being the cardiac, the other the pyloric portion. In the second, the cæcal, the cardiac division is prolonged into a long descending blind sac, the cardiac and pyloric openings of the stomach lying close together (Clupea, Scomber, Thynnus, etc.)
