POLYPS {CUPLIKE ANIMALS) 



29 



eats and digests for the group, another defends by nettling 

 cells, another produces eggs. Each hydra-like part of a 

 hydroid is called a hydranth. Sometimes the buds on the 

 hydra remain attached so long that a bud forms upon the 

 first bud. Thus three generations are represented in one 

 organism. Such growths show us that it is not always 

 easy to tell 

 what consti- 

 tutes an indi- 

 vidual animal. 

 Hydro ids 

 may be con- 

 ceived to have 

 been developed 

 by the failure 

 of budding hy- 



Fig. 42. — The formation of many free swimming jelly- 

 dras to sepa- fishes from one fixed hydra-like form. The saucer-like 



rate from the parts (k) turn over after they separate and become like 



Fig. 43 or 44. Letters show sequence of diagrams. 



parent, and by 



the gradual formation of the habit of living together and 

 assisting one another. When each hydranth of the hydroid 

 devoted itself to a special function of digestion, defence, or 

 reproduction, this group lived longer and prospered ; more 

 eggs were formed, and the habits of the group were trans- 

 mitted to a more numerous progeny than were the habits 

 of a group where members worked more independently of 

 one another. 



As the sponge is a simple example of the devotion 

 of special cells to special purposes, the hydroid is a 

 primitive and simple example of the occurrence of organs, 

 that is of special parts of the body set aside for a special 

 work. 



