HYDROID COLONIES 



103 



is Ohelia geniculata, which is found growing upon seaweeds near 

 low watermark on the British coast (Fig. 67). 



ANATOMY OF THE POLYP 



Certain comparatively unimportant differences distinguish the 

 polyps of Ohelia from those of Hydra. The tentacles are more 

 numerous and, instead of being hollow, have a solid core of large 

 endoderm cells, with very stout walls of intercellular substance 



-end.t. 



Fig. 67.— 

 Obelia 

 lens. 



-Part of a colony of 

 seen under a hand 



Fig. 68. — A longitudinal section of a 

 hydranth of Obelia, highly magnified. 



ect.. Ectoderm ; end., endoderm ; end.t., endoderm of 

 the tentacles ; hyth., hydrotheca ; or.c, oral cone ; 

 St. I., structureless lamella. 



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 (Figs. 68, 69). This 

 is continuous at its base with a root-Hke arrangement of tubes 

 known as the hydrorhiza, on the surface of the seaweed. The tubes 

 of the whole structure are known as the coenosarc, and the 



