134 



ZOOLOGY 



SECT. 



stinging-capsules or nematocysts (Figs. 97 99, ntc), organs closely 

 resembling those of Epistylis unibellaria (p. 95), and, like them, 

 serving as weapons of offence. Each consists (Fig. 98, A) of a tough 

 ovoid capsule, full of fluid, and invaginated at one end in the form 

 of a hollow process continued into a long, coiled, hollow thread. 

 The whole apparatus is developed into an interstitial cell called a 

 cnidoblast (cnb), which, as it approaches maturity, migrates towards 



98. Nematocysts of Hydra. A, undischarged ; B, discharged ; C, nerve-supply ; cnb. 

 cnidoblast ; cnc. cnidocil ; nu. nucleus ; ntc. nema'tocyst ; nv.c. nerve-cell. (From 

 Parker's Biology, after Schneider.) 



the surface and becomes embedded in one of the large ectoderm 

 cells. At one point of its surface the cnidoblast is produced into 

 a delicate protoplasmic process, the cnidocil or trigger-hair (cnc) : 

 when this is touched for instance by some small organism 



i i * ^ 



brought into contact with the waving tentacle the cnidoblast 

 undergoes a sudden contraction, and the pressure upon the stinging 

 capsule causes an instantaneous eversion of the thread (B), at the 



