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. Si- 



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 ; m, mouth ; re, radial canal ; cc, circular or 

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

 medusa ; m, mouth ; mb, manubrium ; gv, gastrovascular cavity ; re, 

 radial canal ; cc, ring canal ; el, 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 ; mesoglcea shaded ; 

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

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

 ring canal. 



in 



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

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



