302 HYDRA CHAP. 



whole space between the wall of the sac and the con- 

 tained 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 Paramcecium (p. 267) 

 should be noted that the Hydra is enabled to paralyse 

 its prey. Probably 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. 76, B, and 77, cnb), 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 cnidocil or 

 "trigger-hair" (cnc). In all probability the slightest 

 touch of the cnidocil causes contraction of the cnidoblast, 

 and the nematocyst, thus compressed, 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. 76, A, ntc', and F). 



In connection with the cnidoblasts small irregular 

 cells with large nuclei occur (Fig. 77, C, nv. c) ; they are 



