eel 



Fig. 55. — Diagrams illustrating the derivation of the medusa from 

 the hydranth. In the whole series of figures the ectoderm (ec() is dotted, 

 the endoderm (end) striated, and the mesoglcea (msgl) black. 



A, longitudinal section of a simple polype, showing the tubular body 

 with enteric cavity (ent. cav), hypostome {hyp), mouth (mth), and 

 tentacles (/). 



