HYDRA AND OBELIA. POLYPS AND MEDVSM 165 



thread or in that of the sac. This fluid has also a 

 temporary numbing action upon the prey, but the main 



Fig. 104. — The histology of Hydra. 



A, Two musculo-epithelial cells ; B, a nerve cell ; C, a cnidoblast ; 

 D, nematocysts of three kinds ; E, Zoochlorella?. 



cnc, Cnidocil ; ntc, nematocyst ; /rj., basal process of the cnidoblast ; 

 vac., vacuoles in musculo-epithelial cells. 



purpose of the nematocysts is not to kill but to hold the 

 prey until it is swallowed. In this the spiral nematocysts 



