i 7 8 



MANUAL. OF ELEMENTARY ZOOLOGY 



the organs of a medusa is an excellent example of what is 

 known as radial symmetry. In bilateral symmetry (p. 24) 

 the parts of the body are arranged on each side (right and 

 left) of a plane, in such a way that no other plane will 

 divide the body into two halves which are alike. In radial 

 symmetry the parts of the body are arranged about a point 

 in such a way that innumerable planes divide the body into 

 like halves. 



The medusa floats in the sea with the manubrium down- 

 wards and the tentacles hanging like the 

 the V Medu8a° f sna -ky locks of its classical namesake. It swims 

 by contractions of the plentiful musculature of 

 the subumbrella side, which drive water out of the umbrella 

 and send the animal forwards in the opposite direction. 



These movements are co- 

 ordinated by the nerve rings 

 — two specially well - de- 

 veloped circular tracts of the 

 nerve plexus situated at the 

 edge of the umbrella. We 

 have here the rudiment of a 

 central nervous system, and 

 it is interesting to find that 

 the animal is paralysed if the 

 nerve rings be cut away. 

 Balance is kept by means of 

 eight sense organs, known 

 as statocysts, situated each 

 at the base of one of the 

 tentacles. These are small 

 hollow vesicles containing 

 calcareous bodies and lined 

 by cells from which fine sense hairs project into the vesicle. 

 The swaying of the calcareous bodies against the hairs 

 gives rise to impulses by which the movements of the 

 animal are directed through the nerve ring. 



The medusae are of opposite sexes. The generative 

 organs are not developed till after the animal 

 is set free. They are four in number and lie 

 on the subumbrella below the radial canals. Each con- 

 sists of a knob of ectoderm, into which passes a short branch 



Fig. 113. — -The medusa of Obelia, 

 seen from the subumbrella side. 

 — From Shipley and MacBride. 



s, Mouth, at end of manubrium ; 2, ten- 

 tacle ; 3, gonad ; 4, radial canal ; 5, 

 statocyst. 



Reproduction. 



