SECRETION : GLAND CELLS 227 



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

 a nettle. 



The nematocysts are formed in special interstitial cells 

 called cnidoblasts (B, cnbl} 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 (A, ntc' and F). 



There are sometimes found in connection with the cnido- 

 blast small irregular cells with large nuclei : they are called 

 nerve-cells, and constitute a rudimentary nervous system, the 

 nature of which will be more conveniently discussed in the 

 next lesson (p. 241). 



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

 of the body in being very granular (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 metabolism : 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 discharged 

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

 performing the function is known as a gland-cell. 



Q 2 



