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 7. 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. 



