DIGESTIVE ORGANS. 



229 



known, but it apparently corresponds to the caeca of the higher groups. 

 In the ' small ' intestine is the spiral valve which has two forms, both 

 leading to increase of surface. In most species a fold, carrying blood- 

 and lymph-vessels, arises in a spiral line from the wall of the tube, and 

 its free edge projects into the lumen like a spiral stairway (fig. 231). 

 In a few forms (Carchariidae, Galeocerdo) the 

 line of origin of the fold is straight and its free 

 margin is coiled like a roll of paper (fig. 232). 

 In the large intestine rectum and cloaca are 

 recognized, the cloaca being that part which 

 receives the ends of the excretory and repro- 

 ductive ducts and thus is both digestive and 

 urogenital in character. 



Ganoids and dipnoi (figs. 230, 233) also have the 

 intestine nearly straight and a spiral valve, least 

 developed in Lepidosteus. In the teleosts the canal 

 may be straight (fig. 227) or may make more or fewer 

 coils, the predaceous species being simplest, while in 

 the mullet (Mugil} there may be 13 or 14 turns. In 

 the teleosts the line between small and large intestine 

 is often marked by an ileo-colic valve and a few species 

 have a caecum or rectal gland. A spiral valve rarely 

 occurs in teleosts and a cloaca is never found. In a 

 few teleosts, in correlation with the translation of the 

 ventral fins, the anus may lie in front of the pectoral 

 girdle. 



The intestine is straight in the caecilians, has a 

 few coils in the perennibranchs and more in the sala- 

 manders, while the anura have a greatly convoluted 

 intestine. (Reference has already been made to the 

 differences between the intestines of the larval and 

 adult frogs (p. 228). The line between small and 

 large intestine is frequently marked in the amphi- 

 bians by an ileo-colic valve and in a few forms 

 (Rana, Salamandra) there is a rudimentary caecum. 

 The rectum is larger than the rest of the intestine and a cloaca is always present 

 in the amphibia. 



The reptiles have the intestine coiled (nearly straight in amphisbaenans) and 

 usually of about the same diameter throughout. Small and large intestine are 

 separated by an ileo-colic valve, and except in crocodiles a caecum is usually present, 

 while a cloaca constantly occurs. The spirally twisted coprolites of the ichthyo- 

 saurs have been supposed to indicate the existence of a spiral valve, but since in 

 other groups the faeces are formed in the rectum, this is not conclusive. 



FIG. 233. Digestive tract 

 of scup (Stenostomus chrysops 

 Princeton 296). bd, bile 

 duct; gb, gall bladder; /, liv- 

 er; li, ' large intestine; PC, 

 pyloric caeca; si, small in- 

 testine. 



