v DIGESTION 305 



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 

 intracellular, i.e., takes place in the interior of the cells 

 themselves, 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 ecto- 

 derm is protective and sensory it forms the external 

 covering of the animal, and receives impressions from 

 without ; the endoderm, removed from direct communi- 

 cation with the outer w r orld, performs a nutrient 

 function, its cells alone having the power of digesting 

 Ifood. 



The essential difference between digestion and assimi- 

 lation (p. 234) is here plainly seen : all the cells of Hydra 

 ssimilate, all are constantly undergoing waste, and all 

 mst therefore form new protoplasm to make good the 

 lioss. But it is the endoderm-cells alone which can make 

 of raw or undigested food : the ectoderm has to 



lepend upon various products of digestion received by 



liffusion or osmosis from the endoderm. 



It will be evident from the preceding description that 

 lydra is comparable to a colony of Amoebae in which 

 articular functions are made over to particular 



lividuals just as in a civilised community the 



I PRACT. ZOOL. X 



