HYDROMEDUSAE 



side of each bend what looks like a tiny conical sherry-glass mounted on 

 a stalk (Fig. 36, h). In this conical receptacle there resides an individual 

 of the colony a polyp or hydroid individual, so called because in the main 

 features of its organization it agrees with Hydra. The body wall is com- 

 posed of the same layers as in Hydra there is a large oral cone with a 

 wide mouth (Fig. 36, o.c), and a ring of tentacles like those of Hydra 

 except that they have a solid core of much vacuolated endoderm cells, the 

 coelenteron not extending into them. The lx)dy of the polyp is continued 

 downwards as a tube composed of the same layers and this in turn joins 

 the thread-like stolon 

 similar in structure 

 which meanders over 

 the surface of the stone 

 or seaweed. 



The cuticle is greatly 

 developed in Obelia 

 forming a thick horny 

 protective layer the 



9 



bl 



o.c. 



perisarc (Fig. 36, ps). 



This covers the whole 



surface of the colony 



and its branches. At 



the base of the polyp 



the perisarc loses its 



intimate contact with 



the cells of the ecto- 

 derm and expands to 



form a wide cup the 

 fdrotheea which sur- 

 mnds the polyp (Fig. 

 h). Here and 



icre in the angle between a hydrotheca and the stem of the 

 )lony there is present a vase-shaped gonotheca (Fig. 36, g). This, like 



the ordinary hydrotheca, contains an individual of the colony but one 



rery different from the ordinary hydroid polyp. This individual the 



lastostyle (Fig. 36, bl} is somewhat piston-shaped, its end being in the 

 >rm of a flattened disc without trace of tentacles or mouth opening. 



tts special function is that of reproducing by budding, and numerous 

 ids may commonly be seen projecting from its surface. In their young 

 ige, as may be seen towards the lower end of the blastostyle, the 

 ids are just like those of Hydra but the older buds (Fig. 36, M), visible 



FIG. 36. 



Obelia. Portion of hydroid colony, x 25. I>1, Blastostyle ; 

 g, gonotheca ; h, hydrotheca ; M, medusa bud ; o.c, oral cone ; 

 ps, perisarc. 



