The (esophagus is large, dilatable, and mus cular. Its mucous membrane is generally shnple, sometimes involuted or glandular ; and offers a remarkable contrast to the redder and more vascular membrane of the stomach at the point of their junction. As the dia meter of the tube rarely undergoes any great and sudden increase in this situation the above contrast of structure is often the only distinc tion between the two cavities.
The stomach varies greatly in size and shape. Usually, however, it forms a curved tube like a siphon. The obliteration of the concave side of this tube converts it, by many gradations in different genera, into the shape of a two-necked flask, or of a blind tube with a double orifice at one end. In other instances it is dilated, or almost globular. Where tubu lar, it generally tapers away towards the pylo rus. And this end of the stomach, which is usually more muscular than the cardiac, some times approaches the structure of a gizzard, having constricted extremities, a thick muscu lar coat, and a scaly epithelium on its mucous 'membrane. The valve itself is almost always present, as a circular ridge of muscular fibre, covered by a fold of mucous membrane.
The intestine of the fish is short and wide : and generally consists of two portions, which are separated from each other by a slight con striction into a small and large intestine. The first receives the bile-duct, and the follicles which form the rudimentary pancreas. The latter also occasionally receives a cmcal tube. The intestine has the usual three coats— serous, muscular, and mucous. The serous membrane is often pigmentary, and its cavity communicates by apertures with the exterior. It rarely forms a continuous mesentery : — the attachment of the intestine being generally ligamentous or filamentous, or even, as in one instance t, by means of a mass of areolar tissue that involves the whole tube. The muscular coat is of unstriped fibres t, which form two layers, the circular generally ex ternal.,§ The mucous membrane is variously folded : it sometimes contains ductless glands : rarely it is ciliated.11 The chief deviations from these the ordinary characters of the intestine are, either an increase of length, which is some times accompanied by a diminution in width ; or an equally real increase of active surface, which is due to the development of folds, such as the spiral valve of the Shark.
The appendices pyloricce, or pancreatic follicles., are absent in many fishes. They vary in number from one to two hundred.
In structure they range from simple, short canals, to elaborate branches, which are united by areolar tissue and vessels, and are enclosed in a muscular tunic.
The alimentary canal of Reptiles preserves much of the simplicity, width, and shortness, seen in that of Fishes. But it offers important differences in many' respects. The thick, serni-transparent, gelatinous-looking intes tinal parietes generally possessed by the Fish, are strongly contrasted with the thinner and more condensed and opaque tube present in the Reptile. Such a comparison seems to indicate a great advance in the develop ment of the Reptilian digestive canal. This advance, though no doubt correlative with that of the tissues generally, probably depends chiefly on the increased efficiency of the respi ratory function.
The (esophagus varies greatly in size and appearance. It is usually large and dilatable. In the Ophidians this width and laxity are so greatly increased, that it forms a tube which can be distended to almost any extent ; and the parietes of which are so thin, that they may be regarded as supplanted by the muscular parietes of the belly itself.
The stomach rarely possesses any well-marked cardiac constriction. Hence the characters of its mucous niembrane are the chief means by which it can be distinguished from the ceso phagus. Its form, in the Chelonian and Ba trachian divisions, somewhat resembles that seen in many fishes. Beginning by a dilated cardiac pouch, it tapers away towards the py lorus, taking a curve like a siphon. In the Crocodiles, the stomach may be regarded as consisting of two portions. Of these, the first is a gizzard : which resembles the form and appearance of that of the Cuttle-fish ; and con sists, like it, of a plane of muscular fibres, that radiate from a central tendon on each side of the organ. The second is a small pyloric pouch or diverticulum, which passes out of the gizzard at its upper part, close to where it receives the dilated cesophagus. In many Serpents the pyloric extremity is so narrow and muscular, that the organ has been distin guished into two parts :—an upper, or cardiac, which is thin and saccular ; and a lower, or pyloric, which is narrow, strong, and tubular. The pyloric valve varies in development. But even where best marked, it never approaches the distinctness seen in man and the higher Mammalia. It consists, as usual, of a pro jection, which is formed by the circular muscular fibres, and is covered by a fold of mucous membrane.