92 



HYDRA. PHYLUM CCELENTERATA 



spiral thread and reduced spines, and two other small kinds with a 

 straight thread and a narrower sac than the others, one of which 

 has spines while the other has not. The broad end of each cnido- 

 blast is anchored into the body by a process which runs inward 

 to the structureless lamella. The tentacles are covered with a 



©0 



m.s.c 



n.c. 



Fig. 59. — The histology of Hydra. 



The figures are diagrammatic and not drawn to scale. 



A, Musculo-epithelial cells ; B, a nerve cell ; C, a cnidoblast ; D, nematocysts o f three 



kinds ; E, zoochlorellae ; F, a sense cell. 



cnb, Cnidoblast ; cnb', interstitial cell which will become a cnidoblast, with vacuole for nematocyst ; 

 cnc., cnidocil ; fib., fibre ; int.c, interstitial cell ; m.prs., contractile process ; m.s.c, musculo-epithelial 

 cell ; n.c, nerve cell ; ntc, nematocvst ; nu., nucleus ; prs., basal process of the cnidoblast; s.c, sense 

 cell. 



number of wart-like knobs, each consisting of a large musculo- 

 epithelial cell, in which is sunk a battery of cnidoblasts consisting 

 of one or two of the large kind with several of the smaller kinds 

 around them. Each of the kinds of nematocysts has a function of 

 its own. The large, barbed nematocysts are weapons of offence and 

 perhaps also of defence. When they are discharged, their barbs 

 emerge first and make a wound in the tissues of the prey, into 



