COELENTERATA 



57 



Ingestion and Digestion.— Yiydrz. feeds on those minute animals 

 that it can seize with its tentacles. It attacks them with nemato- 

 cysts and propels them to its mouth by tentacles. Muscular 

 contraction of the body walls forces the food into the lower part 

 of the coelenteric chamber. Some of the endoderm cells have 

 ^ro]ect\ng pseudopodia or flagella, while others are glandular. 



Digestion. — i. The secretory cells of the endoderm 

 furnish the digestive fluid which acts on the contents 

 of the gastrovascular cavity. 2. The digestive cells 

 with pseudopodia engulf some of the food. 3. The 

 absorptive cells take it in. 



Nervous Systejn. — Ectodermal nerve fibers and 

 cells form a plexus. There are superficial 

 cells. Some nerve cells connected with epithc 

 muscular cells are motor. There are a few endo- 

 dermal nerve cells also. 



Behavior. — Hydra attach, swing and 

 feed, and sometimes loop or somersault. 

 In response to mechanical stimula- 

 tion, hydra contracts at 

 first but, becoming ha- 

 bituated, gives no fur- 

 ther reaction, except as 

 it may at times move 

 away from the occupied 

 region. The righting 

 position is not deter- 

 mined in hydra by grav- 

 ity. It reacts to light, 

 temperature and chemi- 

 cal stimuli. I n other 

 coelenterates, particu- 

 larlv the sea anemones, one finds remarkable response to tactile 

 and photic stimuli. 



Food. — The food of hydra consists of aquatic forms, including 

 an occasional mosquito larva and rarely the eggs and fry of fishes. 



Ene?nies. — The chief enemies of hydra are bacteria, saprolegnia, 

 aquatic insects and of course fishes and crayfish. 



Economic Importance. — Hydra is to some extent beneficial in 

 that it captures an occasional mosquito larva. Since it also devours 



Fig. 225. Hydra stinging cells. (After Dahlgren 

 and Kepner. Courtesy of The Macmillan Co.) 



