172 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



has inherited this structure, for it is found also in all 

 the anthropoid apes, while its wider distribution 

 among the ancestors of the present Mammalia is 

 spoken to by the presence of a vermiform prolongation 

 to the caecum in the rabbit. An appendix which is 

 physiologically, even if it be not morphologically com- 

 parable to that of a man, is to be seen in the wombat 

 among the Marsupialia. 



Though it is not correct to say that the caecum is 

 largest in herbivorous mammals, for it is absent in the 

 bears, who are the most herbivorous of the Carnivora, 

 yet in many cases there is a certain relation between 

 the size of the caecum and that of the stomach ; for 

 example, in the rabbit the stomach is simple, while 

 the caecum is enormous (more than 18 inches in length), 

 and its absorbent surface is increased by the develop- 

 ment in its interior of a large spiral valve ; similarly, 

 the horse has a very large caecum ; on the other hand, 

 the Ruminantia, in which the stomach is so complexly 

 arranged, have a simple caecum. It would, therefore, 

 appear that when the absorbing surface of the stomach 

 is not greatly increased by adaptive modifications, a 

 special region of the intestine is entrusted with the 

 functions that the stomach cannot perform. 



In Hyrax there are two caeca. 



In all Vertebrates the liver appears to arise as a 

 bud or outgrowth from the wall of the enteron, and 

 this embryonic condition is that which is seen in the 

 lancelet, where, probably, the organ has no definite 

 function ; in most of the higher forms the secretion 

 from the cells is stored up in a receptacle connected 

 with the bile ducts ; this is known as the gall 

 bladder, and it is generally, though not invariably, 

 attached to the right lobe. The primitive bud veiy 

 early becomes double, though this is effected in differ- 

 ent ways ; in the chick the two diverticula are at first 

 unequal, in sharks and Amphibians the primary 



