Morphology of Organs. 151 



therefore, of a coelomate animal is that of two tubes, one sur- 

 rounding the other, enclosing between them a space, the 

 coelom. 



The first appearance of the alimentary canal in all coelomates 

 is the archenteron. The archenteron is the equivalent of the 

 gastric cavity of Hydra. But the alimentary canal of the 

 adult coelomate is not the exact equivalent of the gastric cavity 

 of the hydra. For it has been found that the archenteron 

 gives rise, without doubt, in a few animals, to pouches, from 

 which the coelom and its walls are formed ; while in others, if 

 this does not actually occur, it is in the opinion of many a 

 modification of the same process of developipent which gives 

 rise to the coelom and the walls of mesoblast ; and theory 

 altogether apart, the hypoblast of the embryonic archenteron 

 does give rise to, at any rate, a part of the mesoblast. Further- 

 more, in the coelomate animals, two additional portions are 

 added to the archenteron {iiiinus the mesoblastic portion), viz. 

 the stomodseum anteriorly and the proctodaeum posteriorly, 

 both of which are subsequent ingrowths of epiblast. These 

 various layers of cells, however, only give rise to the actual 

 linihg epithelium of the alimentary tract and the lining epi- 

 thelium of the glands, such as the liver, appended to it. The 

 alimentary tract has, besides this epithelial lining, a wall com- 

 posed of connective tissue and muscle. These outer layers 

 are derived from the splanchnopleuric layer of the mesoblast. 

 It is clear, therefore, that the alimentary tract of a coelomate 

 animal is not the precise homologue of the alimentary tract of 

 Hydra. 



There is no doubt that the alimentary tracts of all the 

 higher animals are equivalent structures. They show, however, 

 considerable variation in structure. The proportions of the 

 stomadseum and proctodaeum to the mesenteron vary greatly. 

 The term " mesenteron," it should be remarked, applies to that 

 portion of the gut whose epithelial lining is derived from the 

 archenteron only. Thus in the crayfish all is stomodseum to 

 the end of the stomach, while the proctodseum commences very 

 shortly afterwards, the mesenteron being limited to a very short 

 tract into which the liver opens. In the earthworm, on the 



