168 INVERTEBRATE ZOOLOGY 
Observe the arrangement of the marginal tentacles; also of 
the oral tentacles. At the base of the groups of marginal 
tentacles are minute sense-organs, the ocelli; they are charac- 
terized by the presence of pigment and are sensitive to light. 
Note the four swellings on the side of the manubrium; these 
are the sexual organs and are specialized portions of the ecto- 
derm. The sexes are separate; 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 broad 
muscular membrane extending inward called the velum. 
Exercise V. Make a diagrammatic sketch of the 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 different vegetative functions are carried on 
in the medusa as they are in the hydranth. The medusa being 
a free-swimming animal, however, its muscular and nervous 
systems are much more highly developed than the same systems 
are in the hydranth. In the latter the only muscles present 
are delicate fibers, elongated projections of the inner ends of 
ectodermal cells, which cause movement in the tentacles and 
the body of the hydranth, while the nervous system is repre- 
sented only by scattered ganglion cells, which lie among the 
ectoderm cells. In the medusa the velum is the principal organ 
of locomotion. It contains bands of ectodermal muscle fibers, 
by the contraction of which the motion of the umbrella is pro- 
duced which propels the animal through the water. The nervous 
system consists of a double nerve ring of ganglion cells and fibers 
‘running around the margin of the disc, from which delicate 
fibers run to the velum and to the sense-organs. 
