DIGESTION IN HYDRA 91 



threads anchor it to the captor. Other tentacles turn to it, 

 and it is passively drawn to the mouth and swallowed. It 

 enters the large sac-like enteron where digestion takes place. 

 This is accomplished by a combination of methods, of higher 

 animals and of protozoa. As in the stomach of a higher animal 

 digestive ferments are poured into the digestive cavity from the 

 gland cells. The slime cells, as shown above, are limited to the 

 oral region, and no gland cells are on the pedal disc. The prey, 

 apparently in accord with this distribution, is kept in the upper 

 region of the enteron and in the vicinity of the mouth. There 

 is some uncertainty as to the nature of the ferment in Hydra 

 but in distant related coelenterates (Actinians) its reactions are 

 similar to those of trypsin. Starch-dissolving ferments are 

 absent, and starch grains fed to Hydra are thrown out unaltered. 



The result of ferment activity is the dissolution of the prey 

 into soluble substances and fine particles of solid matter, and it 

 is in connection with the latter that the protozoon method of 

 digestion is involved. The small solid particles are seized by 

 pseudopodia formed by the nutritive muscle cells and drawn 

 into the protoplasm of these cells. Here intra-cellular digestion 

 takes place exactly as in an Amoeba or allied form. Such seizure 

 and intra-cellular digestion is called phagocytosis and the cells, 

 because of this function, are known as phagocytes. Similar 

 functions are performed by white blood cells (phagocytes) of 

 higher animals, but not in connection with metabolism (see 

 p. 200). Nothing is known about the process of absorption of 

 the digested food; presumably it takes place by absorption from 

 cell to cell since there is no blood system to carry the products 

 of digestion to different parts of the organism. 



The undigested food substances like starch, cellulose, chitin, 

 etc., are defecated through the mouth, so this organ acts both 

 as mouth and anus. In this respect many of the protozoa 

 (ciliates) are more specialized than Hydra in having a definite 

 anal opening quite as distinct as the mouth and, in some cases, 

 with special cilia to aid in defecation. 



In animals higher in the scale than Hydra the digestive sys- 

 tem gradually becomes more and more perfected. In the round 



