V DIGESTION 305 



the action of a digestive fluid secreted by the gland-cells of 

 the endoderm ; it is apparently completed by the endoderm 

 cells seizing minute particles with their pseudopods and 

 engulfing them quite after the manner of Amoebae. It is 

 often found that the protrusion of pseudopods during 

 digestion results in the almost complete obliteration of the 

 enteric cavity. 



It would seem, therefore, that in Hydra the process of 

 digestion or solution of the food is to some extent intra- 

 cellular, i.e., takes place in the interior of the cells them- 

 selves, as e.g., in Amoeba or Paramcecium : it is, however, 

 largely extra-cellular or enteric, i.e., is performed in a special 

 digestive cavity lined by cells (pp. 67 and 131). 



The ectoderm-cells do not take in food directly, but are 

 nourished entirely by diffusion from the endoderm. Thus 

 the two layers have different functions : the ectoderm is pro- 

 tective and sensory ; it forms the external covering of the 

 animal, and receives impressions from without ; the endoderm, 

 removed from direct communication with the outer world, 

 performs a nutrient function, its cells alone having the power 

 of digesting food. 



The essential difference between digestion and assimilation 

 is here plainly seen : all the cells of Hydra assimilate, all 

 are constantly undergoing waste, and all must therefore form 

 new protoplasm to make good the loss. But it is the endo- 

 derm-cells alone which can make use of raw or undigested 

 food : the ectoderm has to depend upon various products 

 of digestion received by diffusion or osmosis from the 

 endoderm. 



It will be evident from the preceding description that 

 Hydra is comparable to a colony of Amoebae in which par- 

 ticular functions are made over to particular individuals 



PRACT. ZOOL. X 



