xxi DIGESTION 229 



laria (p. 154). In H. fusca bodies resembling these chromato- 

 phores are present, but are of an orange or brown colour, and 

 devoid of chlorophyll. Brown and black granules occurring 

 in the cells (B) seem to be due in part to the degeneration of 

 the chromatophores, and in part to be products of excretion. 



Muscle-processes exist in connection with the endoderm 

 cells, and they are said to take a transverse or circular 

 direction, i.e., at right angles to the similar processes of 

 the ectoderm cells. 



When a water-flea or other minute organism is swallowed 

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

 The process is begun by a solution of the soft parts due to 

 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 at least 

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

 themselves, as in Amoeba or Paramcecium : it is however 

 mainly 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. 



