18 HYDRA 



closing a long hollow spirally- wound filament, 

 formed by doubling in of the wall of the cap- 

 sule at one pole. They develop within certain 

 of the interstitial cells, and are placed when 

 mature in and between the large ectoderm 

 cells, causing marked projections on the 

 surface. 



The ceU within which the nematocyst is 

 developed persists as a capsule surrounding 

 it, and is produced at its outer or free surface 

 into a small process, the cnidocil. 



These nematocysts can be discharged or 

 exploded, the contained hollow filament being 

 shot out with great force and velocity. In 

 this process of discharge, which is probably 

 brought about by contraction of the cell in 

 which the nematocyst lies, the basal part of 

 the thread is discharged first, and, being pro- 

 vided with three barbs, fixes itself at once in 

 the body against which the thread is shot, 

 and into which the remainder of the filament 

 penetrates with great rapidity. 



The discharge of these filaments appears 

 to be under the control of the Hydra : they 

 exert a very marked numbing or paralysing 

 action on animals into which they are shot, 

 and are the means by which the Hydra cap- 

 tures its prey. A nematocyst 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 



smaller kinds occur in the body-wall, though 



less abundantly than in the tentacles. 



iv. The nerve-cells. Certain small stellate cells 



found in the ectoderm are described by Jickeli 



