A CAMPANULARIAN HYDROMEDUSAN 173 
If they are alive, observe the swimming motions. Note the 
radiate type of structure. The body resembles an umbrella in 
shape, having a convex and a concave side, and is bordered 
by a fringe of tentacles. The former is called the exumbrella or 
aboral side, the latter, the subumbrella or oral side. In the center 
of the latter side is the proboscis-like projection called the 
manubrium, at the distal end of which is the mouth. This opens 
into the gastro-vascular space, which comprises the space within 
the manubrium and also a system of canals in the disc-like 
body. These canals consist of four or more radial tubes, which 
extend from the base of the manubrium to the periphery of the 
disc, and are there united by a circular tube which runs parallel 
with the margin of the disc and close to it. 
Count the marginal tentacles. At the base of certain of the 
tentacles are minute sense-organs, called lithocysts, which are 
probably organs of equilibrium. Find them. 
Near the middle of each radial tube notice a prominent 
swelling on the subumbrella. These are the sexual glands and 
are specialized portions of the ectoderm. The sexes are sepa- 
rate in meduse; the sexual glands have the same appearance 
in the two sexes. 
Around the inner margin of the subumbrella, at the base of 
the tentacles, is a muscular membrane extending towards the 
manubrium called the velum. In campanularian medusz it is 
often very narrow and not easily seen ; in tubularian meduse it 
is broad and very noticeable. 
Exercise 8. Make a diagrammatic sketch of a medusa and label 
all of its parts. 
The medusa is a more highly specialized form than the polyp, 
although they are homologous forms and are essentially alike in 
structure. The manubrium of the medusa and the hypostome 
of the polyp do not differ essentially from each other; the 
tentacles are also homologous structures. The exumbrella of 
