21 



ated. During the subsequent life-history of the larva no 

 cavity exists in the body until several other marked chanj^- 

 es have taken place. 



IV. TIIE PLANULA - By the rupture of the egg-merabrane the 

 nearly spherical ciliated larva makes its escape and starts 

 upon the stage in which it is a swimming planula (Fig. 9). 

 Its shape soon changes, becoming longer, and more narrow 

 a+ one pole than the other; this narrower pole is to be the 

 future oral extremity of the larva. The cilia serve to 

 propel the planula in a slow rotating progression through 

 the \'/ater, usually not far from the bottom. The larger end 

 is directed forward in swimining. The time at which the 

 planula appears is in the morning, about twelve hours after 

 fertilization of the egg took place. The length of the 

 larva is now between .1 and .15 mm. ("^ig. 9). This condi- 

 tion persists for a varying time, usually several days. 

 Towards the end of this time the first indications of a coe- 



lenteric cavity appear in the arrangement of the cells at 

 the posterior end of +he swimming larva ("Fig. 10). Their 

 inner margins come to lie in a straight line, following the 

 long axis of the larva (Fig. 10,G). This is better under- 

 stood when we notice that in changing its shape from the 

 spherical morula to the elongated planula the larva also 



