HYDROZOA. 



109 



power of alternately raising and ' depressing its 

 hydrophyllia, so as to render them agents of pro- 

 pulsion. 



Fig. 22. 



Morphology of PHYSOPHOiiiD^ : — a, Athoryhia rosacea; h, 

 Thyso^hora Philippii; a, pneumatophore ; y, nectocalyx ; y', rudi- 

 mentary nectocalyx ; (p, hydrocysts ; ir, polypite ; t, tentacles ; 

 fi, hydrophyllia. (About the natural size.) 



