TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



121 



Fig. 9.3. Capture and ingestion of a small water animal, cyclops, by hydra. The 

 animal is first ensnared by the tentacles and stung into insensibility by the nematocysts. 



Then it is forced into the mouth. 



an amoeba engulfs food particles. Within the cells digestion is com- 

 pleted by intracellular enzymes. This is an advance over the con- 

 dition found in the sponges, which had only intracellular digestion, 

 but is still more primitive than more advanced forms of animals which 

 have only extracellular digestion. This partial extracellular digestion 

 does have its advantages, however. It permits the hydra to ingest 

 large pieces of food which can then be broken down into pieces small 

 enough for the cells to engulf. The sponges can utilize only those 

 food particles small enough for the cells to engulf without benefit of this 



