ALTERNATE GENERATIONS. 239 
lower end to some surface, while the inversion of 
the upper end has formed the mouth and diges- 
tive cavity, and the expansion of its margin has 
made the tentacles, the very simple story of the 
fresh-water Hydra is told. But the last page in 
the development of these lower Radiates is but 
the opening chapter in that of the higher ones, 
and I will give some account of their trans- 
formations as they have been observed in the 
Acalephs. 
On shells and stones, on sea-weeds or on 
floating logs, there may often be observed a 
esrowth of exquisitely delicate branches, look- 
ing at first sight more like a small bunch of 
moss than anything else. But gather such a 
Coryne mirabilis, natural size. 
mossy tuft and place it in a glass bowl filled 
with sea-water, and you will presently find that 
it is full of life and activity. Every branch of 
this miniature shrub terminates in a little club- 
shaped head, upon which are scattered a num- 
ber of tentacles. They are in constant motion, 
