THE HYDRAS, FRESH WATER JELLYFISHES 



food which they can reach. Its other end is a kind of foot 

 supplied with a sticky secretion, enabHng the hydra to hold 

 itself to submerged twigs or even to the under side of the 

 surface film of water. Clinging by its sticky foot it can glide 

 along so slowly that it is hard to realize that it moves at all. 

 Nearly all over its body, especially on the tentacles, there are 

 peculiar stinging cells containing poison sacs. There are 



sbervno.r( 



o\/iar( 



*ASiS> 



Fig. 88. — Diagrams of hydras (H. oligactis) show- 

 ing bud and reproductive organs. 



usually two or three sizes of these and on the tentacles they are 

 arranged in batteries from which their poison sacs are explod- 

 ed at the slightest touch or invitation. These sacs are to 

 some extent defensive weapons, but they are probably of 

 more value in capturing some of the lively organisms upon 

 which hydras feed. 



When feeding, a hydra hangs attached by the foot, swinging 

 its tentacles nonchalantly about in the water until one of them 

 touches something (Fig. 87). If it is a Daphnia (p. 165) or 

 something equally appetizing the tentacle will instantly pull 

 it toward the mouth. The other tentacles then contract and 



113 



