140 PHYLUM CCELENTERATA 



brium and called the velum, by means of which the animal 

 swims. In campanularian medusae it is often very narrow and 

 not easily seen; in tubularian medusae it is broad and very 

 noticeable. 



Exercise 7. Make a diagrammatic sketch of a medusa 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 manubrium of the medusa and the hypostome of 

 the polyp do not differ essentially from each other ; the tentacles 

 are also homologous structures. Consequently the exumbrella of 

 the medusa corresponds to the base of the polyp, and just as 

 the latter is attached to the stem at its base, so the medusa is 

 attached to the blastostyle by its exumbrella. The digestive, 

 excretory, respiratory, and circulatory functions are carried on 

 in the medusa as they are in the hydranth. But because the 

 medusa is a free-swimming animal, its muscular and nervous 

 systems are much more highly developed than are the same 

 systems in the hydranth. 



In the latter form 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, 

 and the nervous system is represented only by scattered gan- 

 glionic cells, which are also of ectodermal origin. 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 

 which runs around the margin of the disk and from which delicate 

 fibers pass to the velum and the sense organs. 



