IV NEMATOCYSTS 297 



project the barbs. The whole space between the wall of 

 the sac and the contained pouch and thread is tensely filled 

 with fluid. When pressure is brought to bear on the outside 

 of the sac the whole apparatus goes off like a harpoon-gun 

 (b), the compression of the fluid forcing out first the barbed 

 pouch and then the filament, until finally both are turned 

 inside out. 



It is by means of the nematocysts — the resemblance of 

 which to the trichocysts of Paramoecium (p. 267) should be 

 noted — that the Hydra is enabled to paralyse its prey. Prob- 

 ably some specific poison is formed and ejected into the 

 wound with the thread : in the larger members of the group 

 to which Hydra belongs, such as jelly fishes, the nematocysts 

 produce an effect on the human skin quite like the sting of 

 a nettle. 



The nematocysts are formed in special interstitial cells 

 called cnidoblasts (Figs. 74, b, and 75, cnbt), and are thus 

 in the first instance at a distance from the surface. But the 

 cnidoblasts migrate outwards, and so come to lie quite 

 superficially either in or between the large ectoderm cells. 

 On its free surface the cnidoblast is produced into a delicate 

 pointed process, the aiidocil or " trigger-hair " (cnc). In all 

 probability the slightest touch of the cnidocil causes con- 

 traction of the cnidoblast, and the nematocyst, thus com- 

 pressed, instantly explodes. 



Nematocysts are found in the distal part of the body, but 

 are absent from the foot or proximal end, where also there 

 are no interstitial cells. They are especially abundant in the 

 tentacles, on the knob-like elevations of which — due to little 

 heaps of interstitial cells — they are found in great numbers. 

 Amongst these occur small nematocysts with short threads 

 and devoid of barbs (Fig. 74, a, ntd and f). 



In connection with the cnidoblasts small irregular cells 



