48 HYDE^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 



tains an ovoid capsule filled with a peculiar, irritating 

 fluid. Within the capsule and surrounded by the fluid 

 is a long, slender, hollow thread coiled into a spiral. If, by 

 chance, a foreign body — for instance, a tiny crustacean — 

 comes in contact with a tentacle, the latter is stimulated to 



action and quickly 

 winds about the for- 

 eign body. Then from 

 each stinging cell the 

 coiled.thread, which is 

 hollow^ quickly rolls 

 out and pierces the 

 skin of the captive. 

 The poisonous fluid 

 is then discharged di- 

 rectly into the wound 

 through the hollow 

 in the thread. 



Means of protec- 

 tion. — The hydra also 

 protects itself from 

 its enemies by means 

 A of the stinging thread 



Fig. 17. — Diagrammatic drawing of a hydra : Cells. When an Cne- 

 ^.stinging thread ceU; B, bud; C. body my approaches within 

 cavity. •' '^^ 



reach, the stinging 



threads are shot out from many different cells, and the 

 antagonist, if not too large, is overcome by the poisonous 

 fluid. Even if not overcome, the enemy is often frightened 

 into retreat. 



How the hydra breathes. — This primitive animal has 

 no special organs of respiration. It takes oxygen into its 



