STRUCTURE OF THE VERTEBRATES 



19: 



comes thrown into wide folds or ruffles, while the attachment 

 along the dorsal median line remains only the length of the 

 animal's body. The growth of glands further complicates its 

 structure. Each, as it grows out, pulls the mesentery- with it, so 

 that the adult has numerous small supporting mesenteric folds. 

 In the mammals a ventral pocket of the mesentery grows pos- 

 teriorly from the stomach, forming an apron which covers the 

 viscera. This is the fat laden omentum. 



Fig. 112. Mesentery of 

 a Cat. As the intestine 

 grows and coils the ven- 

 tral margin of the mesen- 

 tery is thrown into folds. 



C. Modifications of the Digestive Tract 



The digestive tract as a whole has undergone so many modi- 

 fications during the course of evolution that recent animals give 

 slight evidence of the evolutionary history. The functions of the 

 different regions of the tract remain very constant, but the struc- 

 tural variations are almost limitless. Within a family or order 

 the specializations are usually not great; but there is little pro- 

 gressive evolution from class to class. 



There is an excellent, although not perfect, correlation between 

 the food habits of the animal and the length and shape of the 

 intestine. The herbivores have a complex stomach, a long in- 

 testine, and usually a large caecum. In the carnivores the 

 stomach and intestine are simpler in structure, the tract being 

 decidedly shorter. This correlation is also found in a single 

 order (for example the bats), in which the fruit eating varieties 

 have longer intestines than the carnivorous ones. The blood 

 eating bats have the shortest intestine known among the mam- 

 mals. Even in a species there may be variations in correlation 



