iv] MODES OF NUTRITION 113 



remain thin because it is protected by its situation. 

 Such an internal cavity we find in Hydra and in coelen- 

 terates generally. Let the cavity open at both ends 

 a mouth and an anus and we have an alimentary 

 canal. Even then the surface may be insufficient, 

 for as the animal increases in size its movements 

 through the -water become slower when compared 

 with its size ; but if it should begin to swallow water 

 and pass this through its alimentary canal the ab- 

 sorptive power would still further be increased. If 

 it should still be insufficient, a further increase in 

 absorptive surface may be attained by the outfolding 

 of the body- wall so as to form tentacles, gills, plumes, 

 etc., structures which have a large surface compared 

 with their volume, and the walls of which may remain 

 thin if they are protected from mechanical injury in 

 some way as in the case of the gills of a fish. Now 

 let the animal still further increase in size and two 

 new devices may come into operation : (1) a circulation 

 of blood or other internal fluid is initiated, and this 

 is most abundant in the outfoldings of the body- walls 

 and round the wall of the internal cavity. Since the 

 absorbing fluid is thus continually being changed, 

 obviously more food-stuff can be taken in. (2) a 

 circulation of water is established in relation to the 

 gills, or outfoldings, by means of cilia, or other 

 mechanisms, and thus the sea-water is caused to 

 flow more rapidly over the absorptive surface. Since 



