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 
Fic. 231.—Spiral valve of Raia, after Mayer. 
one or two blind pouches or ceca at their junction or by a circular 
fold or a pair of internal folds of the lining, constituting an ileo-colic 
(ileo-czecal) valve, both valve and ceca coexisting in many cases. 
Both large and small intestines may be subdivided, chiefly by differ- 
ences in their walls. Thus in the small intestine 
there may be recognized in different groups a 
jejunum, a spiral valve region and an ileum, 
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- 
Fic. 232.—Diagram i i : : 
of spiral valve of Carcha- testine which pursues a slightly spiral course, 
ios 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- 
