xxin DEVELOPMENT 249 



swims slowly through the water by means of its cilia, the 



A 



B 



FIG. 57. vStages in the development of two hydroid polypes, Lao- 

 uiedeaflexuosa (A-H) and Eitdendrium ramosnm (I-M). 



A, oosperm. 



B, two-celled, and c, four- celled stage. 

 D, E, polyplast. 



F, G, formation of planula by differentiation of ectoderm and 

 endoderm. 



In A-G the embryo is embedded in the maternal tissues. 



H, free swimming planula, showing ciliated ectoderm, and endoderm 

 enclosing a narrow enteric cavity. 



I, planula, after loss of its cilia, about to affix itself. 



K, the same after fixation. 



I,, Hydra-like stage, still enclosed in cuticle. 



M, the same after rupture of the cuticle and liberation of the tentacles. 

 (After Allman.) 



broader end being directed forwards in progression. It then 

 loses its cilia and settles down on a rock, shell, sea-weed, or 



