22 



underwent a slight rearrangement of its cells. The endo- 

 derm was when first formed in the shape of a spherical . 

 iriass, a-'id its cells were all conical, rauiatinf^ from the 

 centre to the surface. Eiit as elongation took place in the 

 fonnation of the planula, the cells were stretched out into 

 a cylinder, and +heir inner ends overlapped irregularly, as 

 is sho'^m in the anterior end (A) of figure 10. v/hen the 

 coelenteron begins to be developed, the inner ends of these 

 upper endoderinal cells change their position somewhat, and, 

 as stated, meet along a continuous line. At the same time 

 a change is to be noticed in the ectodermal cells at the 

 exterior of the future oral pole (Fig. 10, 0). The cell- 

 walls at +his point become less distinct, and finally a 

 disintegration of the boundaries leaves the tissue an un- 

 differentiated layer of protoplasm. Before separation of 

 the tissue to form the definitive coelenteric cavity, the 

 larva stops swira^ning, loses its cilia, and settles do'.vn up- 

 on the bottom. The larger end, which was directed forward 

 in swinrriing, is dovmv/ard. Between the free-swimming stage 

 and the sessile hydra stage there freciuently, though not 

 always, intervenes a condition which reminds one of a mi- 

 nute planarian in its shape and movements. The planula 

 settles down upon the bottom, and slowly glides along by a 



