A TUBULARIAN HYDROMEDUSAN 135 



brium to the periphery of the body, and are there united by a 

 circular tube, which runs parallel with the margin and close to it. 

 Observe the arrangement of the marginal tentacles, and also of 

 the oral tentacles, if these are present. At the base of the groups 

 of marginal tentacles are minute sense organs, the ocelli ; they 

 are characterized 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 

 ectoderm. The sexes are separate. Around the inner margin of 

 the subumbrella, at the base of the tentacles, is a broad, muscular 

 membrane extending inward and called the velum, by means of 

 which the animal swims. 



Exercise 6, Make a diagrammatic sketch of the medusa as far as 

 observed and label all 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 hy- 

 dranth, and the nervous system is represented 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 mo- 

 tion of the umbrella is produced 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 

 disk, from which delicate fibers run to the velum and to the 

 sense organs. 



