THE HYDRA AS A SIMPLE METAZOAN 261 



We have seen that certain gland cells of the endoderni appear to be 

 the source of such enzymes. Hence, there is a digestive process in 

 hydra which occurs outside the cells and is therefore called extra- 

 cellular. It appears also that during the latter stages of digestion 

 the large endoderm cells ingest small particles in much the same 

 way as does an amoeba, and that such particles are then digested 

 within food vacuoles by an intracellular process. If this is true, 

 hydra carries on the type of digestive process characteristic of 

 higher animals and also that of protozoa. This is not so strange, 

 since white blood cells and certain others, even in our own bodies, 

 exhibit intracellular digestion of ingested particles. 



Since the hydra has no circulatory system, there can be no activ- 

 ities strictly comparable with " absorption " and " circulation " 

 in an animal with a well-developed blood system. Something 

 analogous to circulation is, however, accomplished by the move- 

 ment of food within the enteron, as when material in the later 

 stages of digestion passes from the cavity of the body out to 

 the cavities of the tentacles, to be taken up by the cells of these 

 regions. The enteron of the coelenterate is sometimes called the 

 (/astro-vascular cavity on the ground that it possesses this circula- 

 tory function in addition to that of digestion. In the jellyfish and 

 some other larger members of the group, this seems to be true, 

 V)ut in hydra there appears to be no special arrangement other 

 than the shifting of the gut contents incidental to the various bodily 

 movements and the beating of the flagella at the free ends of the 

 large endoderm cells. It appears, however, that such transfer of 

 nutrient material as exists must take place mainly within the 

 enteron, since there is evidence that the lateral diffusion from cell 

 to cell in endoderm and ectoderm is very Umited. The step that 

 follows digestion, either within the enteron or in the vacuoles of 

 digestion, is therefore comparable with assimilation in a higher 

 animal, since it consists of the incorporation and utilization, by 

 cells of the endoderm, of the products of digestion, and their dif- 

 fusion, as necessary, to the ectodermal layer. 



It is supposed that the same metabolic changes occur within the 

 cells as in those of higher animals, with the resultant excretory 

 products. Excretion, in an animal like hydra, may take place by 

 diffusion over the entire external surface, and perhaps, to a limited 

 extent, into the enteron. There is some indication that granules 

 of insoluble excretory matter may at times accumulate in the 



