Coelenterata. 



315 



thread capsules, stinging cells, or nematocysts. These 

 are elliptical, and before being disturbed have a fine 

 thread coiled within. When the hydra is irritated, the 

 nematocysts are discharged and the threads are suddenly 

 darted out. At the base of each thread, close to the 

 capsule, are a few fine barbs, and it is supposed that a 



Cell containing 

 thread capsule 



Thread capsule 

 partly discharged 



Thread capsule 

 fully discharged 



FIG. 181. STINGING CELLS OF HYDRA, HIGHLY MAGNIFIED. 



poisonous substance is contained within the capsule. At 

 any rate, the result of coming in contact with this sort 

 of apparatus is a stinging sensation, like that caused by 

 nettles, but, of course, the hydra is so small that it does 

 not produce much effect except on very small animals. 

 But let a small crustacean, such as a water flea, swim 

 against a hydra's tentacle, and it is usually paralyzed at 

 once. Then the tentacles draw the victim to the mouth 

 and it is swallowed. 



