174 INVERTEBRATE ZOOLOGY 



the medusa corresponds, consequently, 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. The 

 medusa being a free-swimming animal, however, 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, while the nervous system is represented only by 

 scattered ganglionic 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 motion of the umbrella is produced which pro- 

 pels the animal through the water. The nervous system consists 

 of a double nerve ring which runs around the margin of the 

 disc and from which delicate fibers pass to the velum and 

 the sense-organs. 



