HYDROZOA. 307 



the sexual forms, and they produce eggs which grow not 

 into other jellyfishes but into the fixed forms. This phe- 

 nomenon is known as an alternation of generations, the 

 young resembling not the parent, but, rather, the grand- 

 parent. 



ORDER I, HYDRIDE. 



Here belongs the fresh- water Hydrozoan the Hydra 

 in which there is no medusa stage, the animals producing 

 eggs which develop directly into other Hydra?. The fresh- 

 water Hydrae, which are green or brown in color, and about 

 a quarter of an inch in length, abound in fresh water 



ORDER II. HYDROMEDUSJE. 



The Pennaria is typical of this group. In most there is 

 that alternation of fixed and free-swimming forms which has 

 already been described. In the fixed stage the colony is 

 usually protected by a perisarc which occasionally may be 

 developed into cups protecting the hydranths. On the 

 other hand some of these Hydromedusae exist only as jelly- 

 fishes, the eggs which they produce developing directly into 

 other jellyfishes. The Hydromedusae are abundant in all 

 seas, and are among the most beautiful and interesting 

 of all the animals with which the naturalist has to deal. 

 Only two or three species occur in fresh water. 



ORDER III. SIPHONOPHORA. 



These may be defined as colonies of jellyfishes, arising 

 by budding. In these colonies the medusae become special- 

 ized in different directions. This specialization in some 

 forms may go so far that we have the jellyfishes modified 

 into (1) a float supporting the colony; (2) swimming- 

 bells by means of which it moves; others (3) for feeding, 

 still others (4) for digestion, and again others (5) for re- 



