48 HYDR7E, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 



■^^^ 



tains an ovoid capsule filled with a peculiar, irritating 

 fluid. Within the capsule antl surrounded by the fluid 

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

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

 comes in contact ^vith a tentacle, the latter is stimulated to 



action and cjuickly 

 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 hydraalso 

 protects itself from 

 its enemies by means 

 of the stinging thread 

 "When an ene- 

 my approaches within 

 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 o.xygen into its 



Fig. 17. — Diagrammatic drawing of a hy Ira : CClls. 

 A, stinging thread cell; B, bud; C, body 

 cavity. 



