PHYLUM CCELENTERA I'A 



125 



structed on the same general plan as the latter. Figure 76 illus- 

 trates in a diagramatic fashion the resemblance between the 

 polyp (A) and the medusa (B) by means of longitudinal sections. 

 If the medusa were grasped at the center of the aboral surface 

 and elongated, a hydra-like form would result. Both have sim- 

 ilar parts, the most noticeable difference being the enormous 

 quantity of mesoglea (tnsgl) present in the medusa. 



FIG. 77. Physdlia or Por- 

 tuguese man-of-war, a colonial 

 HYDROZOON. (After Agassiz.) 



FIG. 78. Diagram showing 

 possible modifications of medu- 

 soids and hydroids of a hydro- 

 zoan colony of the order SIPHO- 

 NOPHORA. e, gastrozooid with 

 branched, grappling tentacle, /; 

 , dactylozooid with attached 

 tentacle, //; /, generative mcdu- 

 soid; k, nectophores (swimming 

 bells); /, hydrophyllium (cover- 

 ing piece) ; m, stem or corm; 

 n. pneumatophore. The thick 

 black line represents entoderm, 

 the thinner line ectoderm. (From 

 Lankester's Treatise, after All- 

 man.) 



