246 



HYDROID POLYPES 



ectoderm cells are now found to be ciliated, and before long 

 a cavity appears in the previously solid mass of endoderm 

 cells : this is the first appearance of the enteron or digestive 



A 



B 



FIG. 57. Stages in the development of two hydroid polypes, Eao- 

 medea flexuosa (A-H) and Eudendrium ramosum (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 ectodei'm, and endoderm 

 enclosing a narrow enteric cavity. 



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



K, the same after fixation. 



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



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

 (After Allman.) 



