A CAMPAXULARIAX HYDROMEDUSAX 159 



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 eight 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 medusae ; 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 medusas it is 

 often very narrow and not easily seen ; in tubularian medusae 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 



