ALTERNATE GENERATIONS. Zoo 
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, keeping 
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- 
tount of the Hydroids, including the story of the 
