242 COMPARATIVE ANATOMY 



below low-water mark, generally growing on the broad fronds 

 of the oar-weed (Laminaria). A portion of a colony is shown 



Fig. 51- 

 A , part of a colony of Obelia geniculata magnified ; hi, hydrotheca contain- 

 ing a hydranth ; gt, a gonotheca enclosing a blastostyle with medusa buds ; 

 /, perisarc ; t, terminal growing point. B^ a sexually mature female 

 medusa, seen from below ; ?«, mouth ; re, radial canal ; cc, circular or 

 ring canal ; go^ gonads. C, diagrammatic longitudinal section through a 

 medusa ; ?«, mouth ; 7«^, manubrium ; gv^ gastrovascular cavity ; rc^ 

 radial canal ; cc, ring canal ; e/, endoderm lamella ; oc, ocellus ; ot^ otocyst. 

 The section is supposed to pass through a radial canal on the left side and 

 an adradial tentacle on the right. Endoderm black ; mesoglo^a shaded ; 

 ectoderm represented by a broken line. Z>, the bases of two tentacles 

 magnified, showing oc, ocelli; ot^ an otocyst on an adradial tentacle; cc, 

 ring canal. 



in fig. 51, A. The colony consists of a root-like branching 

 stem, the hydrorhiza, which adheres to the oar-weed and 



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