TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



119 



variety of bodily activity with the quiescent condition of scypha, it is evi- 

 dent that this animal has something that seems to tie the cells together so 

 that hydra reacts as a unit. This is a nervous system. It cannot be 

 seen in the living hydra, but by treating it with a stain known to be ab- 

 sorbed by nerve tissue, it is possible to bring out a complete network of 

 nerves running all over the hydra's body, being a little more concen- 

 trated at the oral end which is where most of the activity occurs. 



Fig. 9.1. 



Photo by P. S. Tice 

 Living hydra with bud attached. 



There seems to be no nerve center which could be compared to a brain 

 of higher animals. 



Hydra does not aggressively seek out its food, but, when hungry, 

 extends itself and its tentacles to the maximum length and waits for its 

 food to come to it. Eventually some small water animal, such as a water 

 flea, will swim within reach and touch one of the tentacles. Immediately 

 the flea will be showered with dozens of poisonous arrows shot from 

 stinging capsules, called nematocysts, which cover the tentacles and are 

 found in smaller numbers on the body. At the same time the tentacles. 



