172 



ANA&THBOPODA 



FIG. 200. Several hydroid colonies (Sertu- 

 larid) each containing from 20 to 30 animals. 

 Photograph enlarged three diameters. 



zoophytes or polyps and 

 each has the same 

 structure as the Hydra. 

 There are two kinds of 

 polyps One is known 

 as the feeding polyp, 

 and has for its func- 

 tion the securing and 

 digesting of food to be 

 distributed throughout 

 the colony; the other is 



the reproductive polyp or Uastostyle, which produces the 



young. The food polyps are recognized by the tentacles 



absent in the blastostyles which 



contain biscuit-shaped bodies, the 



medusae buds. These one by one 



emerge as minute umbrellalike 



forms called medusce. , . 

 They swim about freely, never 



attaining a sufficient size to be 



seen clearly with the naked eye, 



and the female produces eggs, 



while the male gives forth sperm 



cells each of which uniting with 



an egg fertilizes it, i.e., renders it 



capable of developing into an ani- 

 mal. The young one soon fixes 



itself to a weed or stick, and in a 



few weeks by producing budding 



young like the Hydra forms a 



colony. The hydroids are there- 



FIG. 201. A hydroid colony of 

 six polyps ; /, feeding polyp ; 

 r, reproductive polyp ; m, a 

 medusa. 



