Chap. IV.] IXTESTINE OF VfRTEBRATES. l6l 



some Fishes and the higher Mammalia, opens into an 

 epiblastic pit, the cloaca, into which there also open 

 the ducts of the renal and generative glands. (See page 

 263.) Primitively, as in Amphioxus, this tract is quite 

 straight, and has no definite outgrowths along any 

 parts of its course ; later on it undergoes considerable 

 modifications ; its anterior region gives rise to an 

 outgrowth, which serves at first as an air bladder, 

 and later on becomes converted into a pair of lungs, 

 which serve as the definite respiratory organs of all 

 higher vertebrates (see page 236) ; the stomach ceases to 

 have its long axis parallel to the long axis of the body, 

 and at the same time becomes enlarged and more or 

 less complicated ; the intestine buds off two glands of 

 high physiological importance, the liver (only feebly, 

 if really, represented by a caecum in Amphioxus) and 

 the pancreas ; the intestine becomes narrower an- 

 teriorly than it is posteriorly, so that one may distin- 

 guish a small and a large intestine ; between 

 these a blind outgrowth, or caecum, which in Birds 

 is often double, is nearly always developed ; while 

 the inner face of the walls of part of the intestine is 

 thrown into folds, whereby the extent of the absorbing 

 surface of this region is very greatly increased. 



So far as the intricacy of the tract is concerned, 

 we find it to be a very general rule, not only in Ver- 

 tebrates, but in all animals, that carnivorous, as 

 compared with herbivorous, forms have a simpler 

 enteric tract ; thus, the gar-pike (Belone) has the 

 intestine short and straight, and the herbivorous tad- 



o * 



pole has a more complexly coiled intestine than the 

 insectivorous frog ; the proportion of the length of 

 the intestine in the cat is to the body as from 3 to 

 5 times to 1, while in the pig the proportion is as 

 12 to 1 ; so, too, the milk-fed calf has a much less 

 complex stomach than the grass-eating and ruminating 

 cow. 



L 16 



