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 iise 

 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 



