OBELI A. 119 



justified, as it first arises as a single polype individual which buds like 

 Hydra, but in this case the bud does not become detached. It remains 

 in continuity with the parent and later buds in its turn. Obelia is 

 therefore a hydroid (or hydra-like) colony produced by asexual budding. 

 The perisarc is secreted by the outer layer or ectoderm and is evidently 

 a necessity to a colonial form to give support. 



Occasionally, at the base of the colony, there may be noticed large 

 ovoid masses completely enveloped in perisarc. These sporosacs contain 

 modified polypes which have no mouth nor tentacles and appear 

 cylindrical in ouuine. Later on the sporosac bursts and the modified 

 polypes are detached from the coenosarc and become free. They 

 are then known as niedusce. 



Fig. 52. ^A Medusa of Obelia. 



Seen from the oral surface, magnified. {Ad nat.') 



* Velum. Sub-umbrellar cavity. 



Ring-canal. 



Sense-organ. -___y<//0^'''''^ / ^^S?\VV ^°"^^- 



Radial Canal. 7, i • 7 v. 1 1 »> tvt .i, 



^ , Mouth. 



A medusa of Obelia is bell-shaped with the opening downwards. 

 The cavity of the bell is known as the stib-timbrellar cavity and in its 

 centre there hangs the maniibriiwi upon which the mouth opens. The 

 mouth leads into a coelenteron which is continued down the wall of 

 the bell by four radial canals. These run to the rim of the bell to fall 

 into a ring-canal, passing completely round the rim. At each of the 

 four corners, at which the radial canals meet the ring-canal, there is 

 a sense-organ usually termed an otocyst. These otocysts are connected 

 by a double nerve-ring. They are probably balancing organs. The 

 opening of the bell is partially reduced by a thin membrane or vehim 

 projecting from the edge of tlie bell. Sections show that the medusa, 

 like the polype from which it is derived, consists of two layers, ectoderm 



