DIGESTION AND ASSIMILATION 229 



When a water-flea or other minute organism is swallowed 

 by a Hydra, it undergoes a gradual process of disintegration. 

 The process is probably begun by a partial solution of the 

 soft parts due to the action of a digestive fluid secreted by 

 the gland-cells of the endoderm : it is certainly 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 pseudo- 

 pods 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 partly infra-cellular^ i.e., 

 takes place in the interior of the cells themselves, as in 

 Amoeba or Paramcecium : and partly extra-cellular or enteric, 

 i.e., is performed in a special digestive cavity lined by cells. 



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 endo- 

 derm, 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 osmosis from the endoderm. 



It will be evident from the preceding description that 

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



