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. 
