228 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The length of the intestine is roughly related to the food, being 

 longer in the plant-eating than in the carnivorous species. This is 

 strikingly shown in the frogs, where the tadpole (larva) has a very 

 long intestine, correlated with the vegetable food, while the adult 

 flesh-eating frog has a canal hardly longer than that of the tadpole of 

 half the size. 



In the intestine there are two divisions, an anterior small intestine 

 and a posterior large intestine, terms adapted from the digestive tract 

 of man, though not always appropriate in the lower groups. The 

 line between the two may be marked externally by the development of 



FIG. 231. Spiral valve of Raia, after Mayer. 



one or two blind pouches or caeca at their junction or by a circular 

 fold or a pair of internal folds of the lining, constituting an ileo-colic 

 (ileo-caecal) valve, both valve and caeca coexisting in many cases. 

 Both large and small intestines may be subdivided, chiefly by differ- 

 ences in their walls. Thus in the small intestine 

 IH^ there may be recognized in different groups a 

 H jejunum, a spiral valve region and an ileum, 



lil 



while the large intestine may furnish a colon, a 



rectum and a cloaca. 



In the cyclostomes but two regions occur, 

 the intestine and the rectum, differentiated ex- 

 ternally by the larger size of the latter. In the 

 petromyzonts there is an internal fold of the in- 



FIG. 232. Diagram ' . , . , .. , Al . , 



of spiral valve of Carcha- testine which pursues a slightly spiral course, 

 constituting a spiral valve, a structure which 

 reaches its highest development in the elasmobranchs. 



In the elasmobranchs the intestine is nearly straight, but its dif- 

 ferentiation has proceeded farther. At the junction of small and large 

 intestine is a dorsal blind sac, the rectal gland. Its function is un- 



