lob 



HYDRA 



AND OBELIA. PHYLUM COELENTERATA 



thickness to form the jelly. The manubrium represents the oral cone 

 and the tentacles stand around it at a greater distance owing to the 

 widening of the body. The arrangement of the organs of a medusa 

 is an excellent example of what is known as radial symmetry. In 

 bilateral symmetry (p. i88) 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 many 

 planes divide the body into like 

 halves. Polyps also are radially 

 symmetrical. 



...5 



Fig. 71. — The medusa of Ohelia, 

 seen from the subumbrella 

 side. — From Shipley and 

 MacBride. 



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

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

 statocyst. 



MOVEMENTS OF THE MEDUSA 



The medusa floats in the sea with 

 the manubrium downwards and the 

 tentacles hanging like the snaky 

 locks of its classical namesake. It 

 swims by contractions of the plentiful 

 musculature of the subumbrella side, 

 which drive water out of the um- 

 brella and send the animal in the 

 opposite direction. The contractions 

 are started by impulses which originate in the nerve net at the 

 umbrella margin. There nervous transmission is facilitated by the 

 nerve-rings — two specially well-developed circular tracts of the 

 net — and there is provision for keeping balance by means of 

 eight sense organs, known as statocysts, situated each at the base 

 of one of the tentacles. These are small hollow vesicles each 

 containing a calcareous body which hangs in a single cell that 

 secreted it. The swaying of the calcareous bodies against fine 

 processes on sense-cells which line the outer side of the vesicle 

 gives rise to impulses by which the movements of the animal are 

 regulated through the nervous system, stronger contractions 

 being caused on the side which is at the lower level. 



REPRODUCTION 



Each medusa is of one sex only. The generative organs, which 

 are not fully developed till after the animal is set free, are four in 



