PRIMARY FUNCTIONS OF THE ORGANISM 43 



in general becomes restricted first to a layer of cells 

 (in Hydra) and then to localized regions of this 

 layer (digestive portion of the alimentary canal), 

 the specific secretions for different kinds of food 

 eventually being segregated in higher animals in 

 different areas of the canal (stomach, duodenum, 



FIG. 17. Transverse section of Hydra, showing the coelenteric cavity 

 and the two layers of the body wall. (Shipley and McBride.) 



etc.)- Such an adaptation makes for much greater 

 efficiency. However, in some parasitic worms, such 

 as the tapeworm, which lies in the alimentary canal 

 of its host and, so to speak, does not have to exert 

 itself to get or digest food, the localized apparatus 

 for alimentation has disappeared, and the digested 

 food supplied by the host is absorbed directly through 

 the body- wall of the worm. 



In connection with the specific character of the 



