236 ALTERNATE GENERATIONS. 
alternate generations, by which they give birth to 
Medusz, while the Medusz, in their turn, repro- 
duce. the Hydroids, from which they spring. But 
first, a few words upon the growth of Radiates in 
general. : 
There is no more interesting series of trans- 
formations than that of the development of Ra- 
diates. They are all born as little transparent 
globular bodies, covered with vibratile cilia, 
swimming about in this condition for a longer or 
shorter time; then, tapering somewhat at one 
end and broadening at the other, they may be- 
come attached by the narrower extremity, while 
at the opposite one a depression takes place, 
deepening in the centre till it becomes an aper- 
ture, and extending its margin to form the ten- 
tacles. All Radiates pass through this Polyp-like 
condition at some period of their lives, either 
before or after they are hatched from the eggs, 
though they do not all attach themselves per- 
manently. In some it forms a marked period of 
their existence, while in others it passes very 
rapidly, and is undergone within the egg; but, 
at whatever time and under whatever conditions 
it occurs, it forms a necessary part of their devel- 
opment, and shows that all these animals have 
one and the same pattern of growth. 
This difference in the relative importance and 
duration of certain phases of growth is by no 
