OBELIA, 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 sforosacs contain 
modified polypes which have’ no mouth nor tentacles and appear 
cylindrical in outline. Later on the sporosac bursts and the modified 
polypes are detached from the ccenosarc and become free. They 
are then known as medusa. 
Fig. 52,—A MEDUSA OF OBELIA. 
Seen from the oral surface, magnified. (Ad nat.) 
Velum. Sub-umbrellar cavity. 
Ring-canal. 
Sense-organ. 
Radial Canal. 
A medusa of Obeléa is bell-shaped with the opening downwards. 
The cavity of the bell is known as the szb-umbrellar cavity and in its 
centre there hangs the #zazubrzumz upon which the mouth opens. The 
mouth leads into a ccelenteron 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 velum 
projecting from the edge of the bell. Sections show that the medusa, 
like the polype from which it is derived, consists of two layers, ectoderm 
