FIG. 80. Diagrams illustrating the derivation of the medusa from the hydranth. 

 In the whole series of figures the ectoderm (ect) is dotted, the ciuloclerm (end) 

 striated, and the mesoglcea (msgl) black. 



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

 cavity (ent. cav), hypostome (hyp), mouth (mth), and tentacles (/). 



A', transverse section of the same through the plane a b. 



B, the tentacular region is extended into a hollow disc. 



C, the tentacular region has been further extended and bent into a bell-like form, 



316 



