54 



COELENTERATA 



[CH. 



inside out and thus thrust out of the capsule. If the irritation 

 of skin continues the whole capsule will be pressed out by the 

 animal. These thread-capsules are most abundantly developed in 

 the tentacles, and a small amount of observation of the habits 

 of Hydra will show how they are used. If a small Crustacean, 

 or other animal, approaches too near a Hydra, the latter makes 



Fio. 19. Cnidoblast with large nematocyst from the body-wall of Hydra fusca. 

 Highly magnified. From Schneider. 



A. Unexploded. B. Exploded. 1. Cnidoblast. 2. Nucleus of cnido- 

 blast. 3. Cnidocil. 4. Muscular sheath. 5. Wall of 



nematocyst. 6. Thread. 7. Eeflexed processes. 



one swift lash with its tentacles and the luckless water-flea is seized 

 and at the same time paralysed. If we now remove and examine 

 the prey, we shall find it covered with exploded thread- capsules, 

 the threads of which have entered its body, and exerted a poisonous 

 action on it. It is possible to induce a Hydra which is being 

 observed under the microscope to eject its thread-capsules : we 



