236 ALTERNATE GENERATIONS. 



alternate generations, by which they give birth to 

 Medusae, while the Medusae, 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 



