. Elements of comparative anatomy. Anatomy, Comparative. 562 COMPARATIVE ANATOMY. piece (Fig. 313, r; 316, G c). In the Selachii it is provided with a special glandular appendage (Fig. 316, Ox). It is only in the Amphibia that, owing to its greater length and width, it becomes of some im- portance, but in them, as in the Reptilia, it retains its straight course in correspondence with its shortness. In consequence of this straight course it has got the name of " ; It is generally separated from the mid-gut by a transverse fold or valve. Many Reptiles are provided with a cascal
. Elements of comparative anatomy. Anatomy, Comparative. 562 COMPARATIVE ANATOMY. piece (Fig. 313, r; 316, G c). In the Selachii it is provided with a special glandular appendage (Fig. 316, Ox). It is only in the Amphibia that, owing to its greater length and width, it becomes of some im- portance, but in them, as in the Reptilia, it retains its straight course in correspondence with its shortness. In consequence of this straight course it has got the name of " ; It is generally separated from the mid-gut by a transverse fold or valve. Many Reptiles are provided with a cascal appendage, which, in the Ophidii is feebly, and in the Saurii is better developed. The ca3ca in Birds are much more independent. In this group, also, the hind-gut is short and straight (Fig. 320). The cascum is generally paired, and is absent in a few families only ( Woodpecker, Psittacus, etc.). They vary greatly in the extent to which they are developed, so that they may form short papilliform appendages, or very long tubes (Apteryx, Gallinas, Anseres). The hind-gut is longest in the Mammalia, where it forms the large intestine, and is distinguished, as such, from the mid-gut, or small intestine. Owing to its greater length it is arranged in coils, so that the terminal portion, only, has the straight course taken by the hind-gut of other Vertebrata. The anterior portion ordinarily forms a loop which bends from the right side of the abdominal cavity forwards, and then to the left, and then again backwards to be continued into the rectum. This loop is sometimes broken up into secondary loops. At the boundary between it and the small intestine crecal struc- tures are likewise developed, but these are rarely arranged in pairs (Fig. 318, c d), and are commonly single. The size of this caecum may be shown to depend on the food. In the Carnivora it is short, and sometimes com- pletely absent (Ursina, Muste- lina): it is very large in the Herbivora, where its length is compensate
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