144 BIOLOGY 



is not shot away from the animal, but only protruded to its 

 length. If any small animal with thin skin comes in contact 

 with the Hydra, some of these threads are discharged, the 

 animal is hit by them, paralyzed by the poison, and then 

 transferred to the Hydra's mouth by means of its tentacles. 

 In Hydra these cells are so small that they cannot pierce the 

 human skin and their sting cannot be felt; in some allied animals, 

 like the jellyfishes or sea nettles, these cells, although the same 

 in structure as those of the Hydra, are much larger and may pro- 

 duce a severe sting. After the thread is once discharged, it can- 

 not be withdrawn again into its sac; the cell thus becomes use- 

 less. It is necessary, therefore, for Hydra to be constantly 

 replacing them, and new nematocysts are constantly growing 

 from the old interstitial cells. The special cell that produces 

 the nematocyst is known as the cnidoblast. This is simply one 

 of the interstitial cells which has for its function the production 

 of these stinging sacs. 



In the brown Hydra there are three kinds of nematocysts. 

 The larger one, G, is somewhat pear-shaped, and when its thread 

 is protruded it has, close to the base of the thread, two or three 

 slender barbs projecting backwards. When the thread is dis- 

 charged from the cell these barbs are ejected first. It is thought 

 that their function is to pierce the skin of the animal into which 

 the poison is to be ejected. Close to the base of the thread is a 

 minute little organ called the cnidocil (Gr. cnide = thistle) whose 

 function is unknown ; Fig. G, en. It has been supposed that it 

 helps to discharge the cell as a trigger does a gun. This is doubt- 

 ful, for it is known that the cell is most easily discharged by 

 changing the internal pressure, rather than by any mechanical 

 touch upon this cnidocil. The second of the nematocysts in 

 the Hydra, H, is smaller but more elongated. The thread when 

 discharged is very different in shape, lacks the projecting barb, 

 and, relative to the size of the sac, is much longer. The third 

 cell is smaller still, I, oval in shape, and contains a thread that 

 when discharged always coils up in a spiral form. It is thought 



