XI 



POLYZOA 



389 



the beginning of the enormous stomodaeum which pushes the 

 endodermic mass towards the posterior end of the embryo. 



The embryo now adheres by its oral face, and also by the apical 

 organ, to the vitelline membrane, and at the same time it alters its 

 shape, so that it becomes compressed from side to side. The vitelline 

 membrane seems to be absorbed where it is in contact with the 

 embryo; stiff cilia or sense-hairs appear on the apical organ, and 



prod 



stem 



Fig. 313. — The development of the larva of Membranipora pilosa, (After Prouho.) 



A, young larva just free from the egg-membrane ; ewd, solid mass of cells which will be hollowed 

 out to form the stomach. The stomodaeum is formed, but does not join the stomach as yet. B, 

 young larva in which the stomodaeum has joined the stomach. C, larva a little older than that repre- 

 sented in B, in which the proctodaeal invagination has been formed, ap, apical plate ; co, corona ; 

 proct, proctodaeal invagination ; mes, ectomesoderm. 



powerful locomotor cilia on certain cells of the thickened ridge 

 of ectoderm, termed the mantle, which forms the border of the oral 

 face. The space surrounded by the mantle has become concave, and 

 it is termed the atrium ; into it the stomodaeum opens. The ring 

 of ciliated cells is termed the corona. 



The embryo now becomes a larva and swims about ; the remnants 

 of the vitelline membrane which still envelop it in the middle are 

 brushed off. After the free life has begun the mass of cells forming 

 the endoderm becomes hollowed out and forms the larval stomach. 



