l62 



AN INTRODUCTION TO ZOOLOGY 



cavity, almost as large as the proper one whose walls are lined with 

 an entoderm containing a large number of glandular cells. The body 

 of the hydranth calls for no special notice, but it is suddenly con- 

 stricted at its base, which sits on a kind of perforated shelf at the 

 bottom of the hydrotheca. It is continued through this central 

 hole as a very narrow tubular neck-like portion, the hydrocope, 



FIG. 53. A., part of a colony of Obelia geniculata magnified. 

 From Bourne. 



ht., hydrotheca containing a hydranth ; gt., a gonotheca enclosing a blastostyle with medusa 

 buds ; p., perisarc ; t., terminal growing point. B., a sexually mature female medusa, seen from 

 below ; m., mouth ; r.c., radial canal ; cc., circular or ring canal ; go., gonads. C., diagrammatic 

 longitudinal section through a medusa ; m., mouth ; mb., manubrium ; g.v ., gastro-vascular cavity ; 

 r.c., radial canal ; cc., ring canal ; e.l., 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. 



which, however, soon swells out and becomes continuous with the 

 coenosarc. Thus the gastral cavity of each polyp is continuous with 

 the ccenosarcal canal, and so food eaten by any one individual can 

 be utilised for the benefit of the whole. The function of this zooid 

 is purely nutritive, except in so far as its budding to form another 

 hydranth can be looked upon as a kind of reproduction, which in 



