XXI NEMATOCYSTS 227 



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

 a nettle. 



The nematocysts are formed in special interstitial cells 

 called cnidoblasts (Fig. 50, b, cnhl and Fig. 51), 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 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. 50, a, titc and f). 



There are sometimes found in connection with the cnido- 

 blasts small irregular cells with large nuclei : they are called 

 nerve-cells (Fig. 51, c, nv. c\ and constitute a rudimentary 

 nervous system, the nature of which will be more con- 

 veniently discussed in the next lesson (p. 242). 



The ectoderm cells of the foot differ from those of the rest 

 of the body in being very granular (Fig. 50 a). The 

 granules are probably the material of the adhesive substance 

 by which the Hydra fixes itself, and are to be looked upon as 

 products of destructive nietabolism : i.e. as being formed by 

 conversion of the protoplasm in something the same way as 

 starch granules (p. 33). This process of formation in a cell 

 of a definite product which accumulates and is finally dis- 

 charged at the free surface of the cell is called secretion, 



Q 2 



