152 Elementary Zoology. 



other hand, the stomodaeum ends with the pharynx/ and the 

 proctodaeum is an exceedingly short tract at the opposite end 

 of the intestine. 



Combined with these essential points of similarity in the 

 alimentary tract of coelomates are many points of difference. 

 The canal itself is differently specialized in all the types con- 

 sidered in this volume, and the glands appended to it can only 

 be said to generally correspond. It is impossible, for instance, 

 to definitely compare the " liver " of Astaats with the liver of 

 Ratta. 



CCELOM. 



All the animals above the hydra which have been described 

 in the foregoing pages, possess a cavity, or system of cavities, 

 known as the coelom. This space lies in the mesoblast, and is 

 bounded by its own walls ; it is not, that is to say, simply the 

 interspace between various organs. The term " body-cavity " 

 is sometimes applied to this system of spaces ; but the term is 

 ill-advised as applicable to any cavities in the body, as, for 

 example, the quite spacious cavities in the cockroach, which are 

 really a part of the haemocoel. In representatives of three or 

 four groups of animals it has been conclusively proved that the 

 coelom arises as a pair, or more than a pair, of diverticula of 

 the embryonal gut (archenteron). These pouches grow out, 

 and then meet to become fused, at least in part, and so surround 

 the gut, whence they were derived. In the majority of animals, 

 however, e.g. in the earthworm, the coelom does not arise in this 

 way. 



It appears as a splitting of the mesoblast, the cells imme- 

 diately abutting upon the cavity thus formed becoming the 

 proper wall of the coelom,. the peritoneum. To a coelom 

 which arises in the former of these two ways the term " entero- 

 ckI" has been applied, and " schizocxel" to a ccelom arising in 

 the second way. Thus Amphioxus possesses an enterocoel, 

 Lumbricus, a schizocoel. The extent to which the coelom is 

 developed varies greatly among coelomate animals. It is 



' Possibly with the buccal cavity. 



