II HYDRA 91 



The nematocyst is a powerful offensive organ. If any small organism 

 swimming through the water blunders up against the cnidocil this acts 

 like a trigger and causes the nematocyst to discharge, the thin tube 

 being violently everted so as to pierce the body of the organism, an 

 opening being made for it by the three blades as they swing outwards. 

 The movements of the animal pierced by the nematocyst, at least if 

 it be small, are paralysed — and it is assumed that a virulent poison is 

 injected through the tube. 



As regards the mechanism by which the explosion is brought about, 

 a hint is got from the fact that nematocysts which have been freed from 

 their cnidoblast commonly explode instantly when they come in contact 

 with water. This suggests that the fluid within the nematocyst is of 

 such a nature as to cause very rapid diffusion of water inwards through 

 the nematocyst wall if this is in contact with water, the increased 

 pressure so set up bringing about the discharge of the nematocyst. 

 Possibly what happens is that on the cnidocil being touched the proto- 

 plasm of the cnidoblast shrinks back and exposes the outer surface of 

 the nematocyst to the action of the water. 



The cells which constitute the gonad of Hydra are also derived from 

 interstitial cells. The gametes show well-marked sexual differentiation ; 

 the portions of gonad which give rise to the small actively motile micro- 

 gametes are termed testes, those which give rise to the large non-motile 

 macrogametes or eggs are termed ovaries. The testes when present 

 are in the form of conical or rounded thickenings of the ectoderm, 

 varying in number and most usually situated towards the upper end 

 of the Hydra : the ovaries, fewer in number, are spherical in shape and 

 are situated rather towards the basal end. Hydra is hermaphrodite, i.e. 

 the same individual may develop both ovaries and testes, but as a rule 

 the ovaries develop later than the testes. 



The study of sections shows that the young gonad is formed by a 

 heap of actively multiplying interstitial cells. In the case of the testes 

 (Fig. 33, C) they keep on multiplying until there are formed enormous 

 numbers of minute round cells each of which becomes a slender micro- 

 gamete with a long vibratile tail and a small rounded " head." 



The earliest stages in the development of the ovary are precisely 

 like those of the testis but presently a few cells begin to grow rapidly 

 in size at the expense of the others. One of these (Fig. 33, B, E) eventually 

 shoots ahead of the others, ingesting the bodies of its neighbours in 

 Amoeba fashion, and not merely grows to a relatively enormous size 

 but in later stages loads up its cytoplasm with reserve food material or 

 yolk. This cell after it has undergone a process of maturation by the 



