POLYPS {CUPLIKE ANIMALS) 



31 



does not produce a hydroid, but a jellyfish ; the egg of the 

 jellyfish does not produce a jellyfish, but a hydroid. This is 

 called by zoologists, alternation of generations. A complete 

 individual is the life from the germination of one egg to 

 the production of another. So that an "individual" con- 

 sists of a hydroid colony fixed in one place together with 

 all the jellyfish produced from its buds, and which may 

 now be floating miles away from it in the ocean. Bathers 

 in the surf are sometimes touched and stung by the long, 

 streamer-like tentacles of the jellyfish. These, like the 

 tentacles of the hydra, have 

 nettling cells (Fig. 41). 



The umbrella-shaped free 

 swimming jellyfish is called a 

 medusa (Fig. 44). 



Coral Polyps. — Some of the 

 salt water relatives of the hydra 

 produce buds which remain 

 attached to the parent without, 

 however, becoming different 

 from the parent in any way. 

 The coral polyps and corallines are examples of colonies of 

 this kind, possessing a common stalk which is formed as 

 the process of multiplication goes on. In the case of coral 

 polyps, the separate animals and the flesh connecting them 

 secrete within themselves a hard, limy, supporting structure 

 known as coral. In some species, the coral, or stony part, 

 is so developed that the polyp seems to be inserted in the 

 coral, into which it withdraws itself for partial protection 



(Fig. 45). 



The corallines secrete a smooth stalk which affords 

 no protection, but they also secrete a coating or sheath 

 which incloses both themselves and the stalk. The 



Fig. 45. — Coral Polyps (tenta- 

 cles, a multiple of six). Notice 

 hypostome. 



