256 ALTERNATE GENERATIONS. 
various phases of this extraordinary series of 
metamorphoses, so different from each other in 
their external aspects, should not have been 
recognized at once, and that this singular Aca- 
leph should have been called Scyphostoma in its 
simple Hydroid condition (see p. 248), Strobila 
after the transverse division of the body had taken 
place (see p. 249), Ephyra in the first stages of 
its free existence (see p. 251), and Aurelia in its 
adult state (see pp. 252 and 254), — being thus 
described as four distinct animals. These vari- 
ous forms are now rightly considered as the suc- 
cessive stages of a development intimately con- 
nected in all its parts, beginning with the 
simple Hydroid attached to the ground, and clos- 
ing in the shape of our common Aurelia, with 
its white transparent disk, its silky fringe of 
tentacles around the margin, its ruffled curtains 
hanging from the mouth, and its four crescent- 
shaped ovaries grouped to form a cross on the 
summit. From these ovaries a new brood of 
little embryos is shed in due time. 
There are other Hydroids giving rise to Me- 
duse buds, from which, however, the Medusz 
do not separate to begin a new life, but wither 
on the Hydroid stock, after having come to ma- 
turity and dropped their eggs. Such is the Ay- 
dractinia polyclina. This curious community 
begins, like the preceding ones, with a single 
