Fig. 77. — Diagi-ams illustrating the derivation of the medusa from the hydranth. 

 In the whole series of figures the ectoderm {ect) is dotted, the endoderm (jend) 

 striated, and the mesogloea (msg-l) black. 



A, longitudinal section of a simple polj^p, showing the tubular body with enteric 

 cavity {ent. cav\ hypostome {kvP\ mouth (mth.\ and tentacles {t). 



V, transverse section of the same through the plane a 6. 



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 



