34 INTRODUCTION TO ZOOLOGY CHAP. 



mouth-tube, and extended in the form of four radial canals, 



which run into a circular canal round the margin of the bell 



(Fig. 15). 



The concave under surface of the bell is partially closed 



by a skin-like ledge, or velum, which grows inwards from its 



margin. 



From this margin also, just below each of the four radial 



canals, hangs down a pair of solid tentacles. At the base of 

 each of these is a little mass of pig- 

 ment, to which perhaps is due a certain 

 sensitiveness to light possessed by these 

 medusae ; hence these pigment masses 

 are termed "eye-spots." 



Each medusa swims like a jelly-fish 

 by alternately contracting and expand- 

 ing its bell, and so forcing water out 



Fm. 15. Diagrammatic re- and in - This movement is brought 



presentation of the Canal about by muscle processes of the skin 



system of the Medusa of ce U S) suc h as those in Hydra, and also 



by special muscle fibres long nucleated 



anal. skin cells in fact in these medusae 



there is, in some parts of the body, a 



layer of these muscle fibre cells, between the skin and 

 digestive layers, as well as the much thicker layer of non- 

 cellular jelly or mesogloea. 



The nervous system is also more highly developed, for 

 there is a double ring of nerve cells with fibre-like processes 

 round the margin of the bell, as well as an irregular network 

 of the same just below the skin cells on the inner surface of 

 the bell. 



This greater differentiation of the muscular and nervous 

 systems is doubtless due to the more varied needs of the freer, 

 more independent life of the medusa, compared with that of the 

 more or less stationary Hydroids. The medusa swims away to 

 fresh regions and must be more active and "alive," to face 

 successfully the new, varying conditions of life. 



The reproductive cells are produced usually in the skin cells 

 of the mouth-tube, along four radial lines corresponding to 

 the radial canals. The sperm cells are formed on one medusa 

 and the egg cells on another. 



