STRUCTUEE OF TENTACLES. 19 



in little clusters between the inner or basal ends 

 of the large ectoderm cells. 

 iii. The nematocysts or ' thread cells ' are sharply 

 defined oval capsules, filled with fluid and en- 

 closing a long hollow spirally wound filament, 

 formed by doubling in of the wall of the capsule 

 at one pole. They develop within certain of 

 the interstitial cells called cnidoblasts, and 

 lie, when mature, in and between the large 

 ectoderm cells, causing marked projections on 

 the surface. 



The cnidoblast, or cell within which the 

 nematocyst is developed, persists as an envelop- 

 ing capsule, and is produced at its outer or free 

 surface into a small process, the cnidocil. 



The nematocysts can be discharged or ex- 

 ploded, the contained hollow filament being 

 shot out with great force and velocity. In this 

 process of discharge, which is brought about by 

 contraction of the cell in which the nematocyst 

 lies, the basal part of the thread is discharged 

 first, and, being provided with barbs, of which 

 there are three principal and several smaller 

 ones, fixes itself at once in the body against 

 which the thread is shot, and into which the 

 remainder of the filament is then propelled. 



The discharge of these filaments appears to 

 be under the control of the Hydra : they exert a 

 marked numbing or paralysing influence on ani- 

 mals into which they are shot, and are the means 

 by which the Hydra captures its prey. A ne- 

 matocyst once discharged cannot be used again, 

 and is cast off. 



Besides these large nematocysts, much 

 smaller ones, with short thick threads devoid of 

 barbs, occur in large numbers in the tentacles. 

 Their function is unknown. 



Nematocysts of both the larger and the 



c 2 



