and physiology* 

 Fig, 198. 



195 



Digestive Apparatus of Unicellular Animals. 



356. The Hydra.— Fig. 199, an unicellular animal, is an 

 example where the simplest sort of digestive organs is ex- 

 hibited. The animal is little ^. ^ 

 else than a gelatinous sac, 

 and if it be turned inside out 

 digestion will still go on. 

 Thus, " notwithstanding the 

 simplicity of its structure, 

 this creature feeds not merely 

 upon algae, but upon young 

 • active animalcules, the young of 

 crustaceans, etc. ; any one of 

 these, when they happen to 

 came in contact with one of 

 the tentacular filaments, be- 

 usually retained by 

 As this 

 itself, all 



the surviving filaments ap- 

 ply themselves to this captive 



particle, so that it becomes gradually inclosed, and grad' 



mg 



adkesion to it. 

 filament shortens 



Hydra or Fresh Water Polyp, 

 trance and Exit to Stomach. 



a. En' 



356. What is the process of digestion in the hydra ? 



