. Elements of Comparative Anatomy. FORE-GUT OF VERTEBRATA. 557 once into the stomacli, whicli can only be distinguislied from it by the differences in the characters of its mucous membrane. As a rule, the stomach (Fig. 313) forms a csecal sac, which is directed back- wards, and from which a narrow portion (pyloric tube) which bends forwards, can be distinguished; this leads to the mid-gut (i). This is the case in all Selachii and Ganoi'dei, and in many Teleostei, while the rest vary greatly in the absence, or the great development backwards, of the caecal sac. Among the Amphibia we find a lowe
. Elements of Comparative Anatomy. FORE-GUT OF VERTEBRATA. 557 once into the stomacli, whicli can only be distinguislied from it by the differences in the characters of its mucous membrane. As a rule, the stomach (Fig. 313) forms a csecal sac, which is directed back- wards, and from which a narrow portion (pyloric tube) which bends forwards, can be distinguished; this leads to the mid-gut (i). This is the case in all Selachii and Ganoi'dei, and in many Teleostei, while the rest vary greatly in the absence, or the great development backwards, of the caecal sac. Among the Amphibia we find a lower stage in Proteus, for the enteric tube, which has a perfectly straight course, has no stomachal enlargement at all. In the other Urodela, however, the stomach forms a wider portion of the enteron; and this is the case also in the Anura, where the stomach is sometimes, indeed, placed transversely (Bufo). Among the Reptilia, the fore-gut is of a lower stage in the Ophidii and Saurii, owing to the greater width of the oeso- phagus and the straight course of the stomach. However, there is an arrange- ment in the Saurii which calls to mind the pyloric tube of the Selachii, and from this the stomach might gradually acquire a transverse position. In the Chelonii and Crocodilini the oesophagus is more sharply separated from the stomach, which in the former has a large and a small curvature, owing to the great elevation of the pyloric portion. Owing to the approximation of the cardiac end of the stomach to the pylorus, this portion is rounded in the Crocodile, and is also distinguished by a tendinous disc on each face of its muscular wall; in this point it resembles the stomach of Birds. In the fore-gut of Birds there is a greater division of labour. The influence of adaptation to the mode of life, and here especially to the mode of nutrition, is most clearly shown by the varia- tions in the different arrangements. The oesophagus, which is of the same length as the neck, is either of
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