HYDRA 



iii. The nematoeysts 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 cap- 

 sule 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 a capsule 

 surrounding it, and is produced at its outer 

 or free surface into a small process, the 

 cnidocil. 



These nematoeysts can be discharged or 

 exploded, the contained hollow filament being 

 shot out with great forde 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 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 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 nematoeysts, much 

 smaller ones, with short thick threads devoid of 

 barbs, occur in large numbers in the tentacles. 

 Their function is unknown. 



Nematoeysts of both the larger and the 



