CCELENTERATA. 167 



numerous other hydranths, and each of these in turn by 

 budding may produce several medusae. In other words, 

 we have here an animal which reproduces asexuafly. 

 These medusae are the sexual forms, and they produce 

 eggs which grow not into other jelly fishes but into the 

 fixed forms. This phenomenon is known as an alternation 

 of generations, the young resembling not the parent, but 

 rather the grandparent. 



ORDER I. HYDRIDE. 



Here belongs the fresh-water Hydrozoan Hydra in 

 which there is no medusa stage, the animals producing 

 eggs which develop directly into other Hydrse. The 

 fresh-water Hydrae, which are green or brown in color, 

 and about J inch in length, are common in fresh water, 

 attached to plants, stones, etc. 



ORDER II. HYDROMEDUS^E. 



Pennaria is typical of this group. In most of the species 

 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 occasion- 

 ally may be developed into cups (thecce) protecting the 

 hydranths. On the other hand, some of these HydromedusaB 

 exist only as jellyfishes, the eggs which they produce 

 developing directly into other jellyfishes, while in others 

 the medusae do not separate from the colony, but, remain- 

 ing attached, retain more or less clearly the features of 

 the jellyfish, and always produce the eggs. The Hydro- 

 medusae 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. 



