POLYPS {CUPLIKE ANIMALS) 



2 9 



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 i d s 

 may be con- 

 ceived to Jiave 

 been developed 

 by the failure 

 of budding hy- 

 dras to sepa- 

 rate from the 

 parent, and by 

 the gradual formation of the habit of living together and 

 assisting each other. When each hydranth of the hydroid 

 devoted itself to a special function of digestion, defense, 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 

 each other. 



As the sponge is the first, lowest, and simplest ex- 

 ample of the devotion of special cells to special pur- 

 poses, the hydroid is the first, lowest, and simplest 

 example of the occurrence of organs, that is of special 

 parts of the body (groups of cells) set aside for a 

 special work. 



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

 fishes from one fixed hydra-like form. The saucer-like 

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

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



