ALTERNATE GENERATIONS. 235 



parent form, and thus, by alternate generations, 

 maintain two distinct patterns of animal life 

 within one cycle of growth. 



Perhaps, of all the three Classes of Radiates, 

 Acalephs are the least known. The general 

 interest in Corals has called attention to the 

 Polyps, and the accessible haunts of the Sea- 

 Urchins and Star-Fishes have made the Echino- 

 derms almost as familiar to the ordinary observer 

 as the common sea-shells, while the Acalephs are 

 usually to be found at a greater distance from 

 the shore, and are not easily kept in confinement. 

 It is true that the Hydroids live along the shore, 

 and may be reared in tanks without difficulty ; 

 but they are small, and would be often taken 

 for sea-weeds by those ignorant of their true 

 structure. Thus this group of animals, with 

 all their beauty of form, color, and movement, 

 and peculiarly interesting from their singular 

 modes of growth, remains comparatively un- 

 known except to the professional naturalist. 



It may, therefore, be not uninteresting or use 

 less to my readers, if I give' some account of the 

 appearance and habits of these animals, "teeping 

 in view, at the same time, my ultimate object, 

 namely, to show that they are all founded on 

 the same structural elements and have the same 

 ideal significance. I will begin with some ac- 

 count of the Hydroids, including the story of the 



