ALTERNATE GENERATIONS. 247 
bratile cilia that cover the surface. At this stage 
of its existence such an embryo is perfectly free, 
but presently its wandering life comes to.an end: 
it shows a disposition to become fixed, and pro- 
ceeds to choose a suitable resting-place. I use 
the word “‘ choose”’ advisedly ; for though at this 
time the little embryo seems to have no developed 
organs, it yet exercises a certain discrimination 
in its selection of a home. Slightly pear-shaped 
in form, it settles down upon its narrower end. 
It wavers and sways to and fro, as if trying to get 
a firm foot-hold, and force itself down upon the 
surface to which it adheres; but presently, as if 
dissatisfied with the spot it has chosen, it sud- 
denly breaks loose and swims away to another 
locality, where the same examination is repeated, 
not more to its own satisfaction apparently, for 
the creature will renew the experiment half a 
dozen times, perhaps, before making a final selec- 
tion, and becoming permanently attached to the 
soil. In the course of this process the lower end 
becomes flattened, and moulds itself to the shape 
of the body on which it rests. Once settled, this 
animal, thus far hardly more than a transparent 
oblong body, without any distinct organs, begins 
to develop rapidly. It elongates, forming a kind 
of cup-like base or stem; the upper end spreads 
somewhat; the depression at its centre deepens ; 
a mouth is formed that gapes widely, and opens 
