174 



MANUAL OF ELEMENTARY ZOOLOGY 



A common example is Obelia geniculates, which is found 

 growing upon seaweeds near low watermark on the British 

 coast. 



Certain comparatively unimportant differences distinguish 

 the polyps of Obelia from those of Hydra. 

 obeiia: The tentacles are more numerous and, instead 



the Polyp. of being hollow, have a solid core of large 

 endoderm cells, with very stout walls of inter- 

 cellular substance and highly vacuolated contents. In the 

 ectoderm the muscular fibres are independent cells with 



nuclei of their own, lying 

 below the epithelium. The 

 oral cone is very large and 

 forms a chamber above the 

 rest of the enteron. From 

 the middle of the basal disc 

 of each polyp the body-wall 

 is continued as a narrow tube, 

 which joins the tubes from 

 other polyps so as to form a 

 branching structure like the 

 body of a flowering plant. 

 This is continuous at its base 

 with a root-like arrangement 

 of tubes on the surface of the 

 seaweed, known as the hydro- 

 rhiza. The tubes of the 

 whole structure are known as 

 the ccenosarc and the polyp 

 heads as hydranths. The 

 whole colony is enclosed in 

 a horny case or perisarc, 

 which is secreted by the 

 ectoderm and follows closely 

 the outline of the body, but is separated from it by a small 

 space, bridged by processes from some of the ectoderm 

 cells. At the base of each hydranth the perisarc expands 

 into a cup or hydrotheca, into which the hydranth can be 

 withdrawn. 



The generative organs are not borne by the polyps, but 

 by special bodies, which originate as members of the colony, 



Fig. 108. — Part of a colony of 

 Obelia seen under a hand 

 lens. 



