THE STOMACH IN MOLLUSCA. 589 



of this organ with the use of a vegetable diet. The existence of a gizzard in 

 some Reptiles, is one of the indications of the relations of this Class with 

 Birds. 



Amphibia. The oesophagus of the Amphibia is short, dilatable, and mus- 

 cular. The stomach is fish-like, being tubular, wider at the cardiac than at 

 the pyloric end, and placed transversely, or curved upon itself. The intestine 

 in the toad and frog is readily distinguishable into small and large, the former 

 opening into the side of the latter ; the ileo-csecal valve is indistinct or absent. 

 In the more fish-like Batrachia, the division into small and large intestine is 

 imperceptible. The latter ends in a cloaca, which receives the ducts of the 

 urinary and reproductive organs. The relation between the length of the in- 

 testinal canal and the nature of the food, is illustrated in the long and coiled 

 intestine of the young vegetable-feeding tadpole, as compared with the short 

 intestine of the insectivorous adult frog and toad. 



Fishes. In Fishes, the alimentary canal presents its most simple vertebrate 

 form, being wide, and, in relation to the body, short. The oesophagus, short, 

 wide, and muscular, sometimes passes so evenly into the stomach that the 

 structure of the mucous membrane alone distinguishes them ; in the former, 

 it is pale and longitudinally plicated ; in the latter, it is softer, redder, and 

 full of gastric tubuli. In the Cyclostomata, it forms only a dilated portion of 

 the nearly straight canal. In the Osseous fishes especially, it varies in size, 

 but is usually tubular, bent once upon itself, and narrower towards the py- 

 lorus ; sometimes, by protrusion of the convex border, and shortening of the 

 concave border, it becomes flask-shaped or globular, with its cardiac and py- 

 loric openings placed near together. The cardiac orifice, large, and some- 

 times provided with a valvular fold, not only readily permits the swallowing 

 of the prey whole, but sometimes allows of regurgitation and rumination, the 

 food being remasticated by the teeth, or by the pharyngeal bones, as seen in 

 the carp. The pyloric part is sometimes so muscular as to resemble an im- 

 perfectly developed gizzard, having thick walls and a dense squamous epithe- 

 lial lining. A pyloric valve nearly always exists. 



The intestine is relatively short and wide, of nearly uniform diameter 

 throughout, has few convolutions, and is distinguished into a large and small 

 intestine, by a slight constriction only ; there is no distinct ileo-colic valve, 

 but sometimes a short caecum exists. The small intestine has usually con- 

 nected with it, immediately below the pylorus, the so-called appendices pylor- 

 icce, which have been compared with the pancreas. The large intestine is 

 often, as in the sharks, provided with internal folds or a spiral valve, by 

 which its surface is much increased. It is also generally thrown into rugae, 

 which augment its surface. In some species, the intestine is unusually long ; 

 it is rarely supported upon a mesentery, excepting at a few points. The peri- 

 toneal cavity presents the unusual condition of opening directly on the exte- 

 rior. In the singular amphioxus, the alimentary canal is short and nearly 

 straight, the stomach being scarcely dilated ; the intestine, as well as the 

 mouth and sides of the pharynx, is provided throughout with cilia, which 

 assist in moving on the fluids in the alimentary canal. 



Mollusca. In these animals, the alimentary canal, though simpler than in 

 the Vertebrata, presents, as in them, many gradations, from a very complex 

 form in the Cephalopods, to that of a slightly convoluted canal, with a simple 

 dilatation for the stomach in the Lamellibranchiata. No distinction exists 

 into small and large intestine. 



In the Cephalopods, the oesophagus, which perforates the cephalic cartilage, 

 is long, very dilatable, and ends in a strong gizzard, roundish or elongated in 

 shape, lined with a hard epithelium, provided with two digastric muscles 

 radiating from two lateral tendons, and having its cardiac and pyloric orifices 

 near together. Sometimes, before entering the gizzard, the oesophagus ex- 

 pands into a crop. Below the pylorus, the intestine dilates to form a spherical, 

 triangular, elongated, or spiral cavity, having a follicular mucous membrane ; 

 this has also been regarded as a stomach, but the ducts of the liver enter it 

 through a sort of sac. Lower down, the intestine forms a simple, more or less 

 curved, tube, which bends up, and opens into the branchial chamber, at the 



