9 8 



MANUAL OF ZOOLOGY 



SECT. 



margin. By means of this system of canals the food, taken 

 in at the mouth and digested in the manubrium, is 

 distributed to the entire medusa. 



The edge of the umbrella is produced into a very narrow 

 fold or shelf, the velum (Fig. 42, vl.\ and gives off the 

 tentacles (/.), which are sixteen in number in the newly-born 

 medusa (Fig. 40, D.\ very numerous in the adult. At the 

 bases of eight of the tentacles two in each quadrant are 



end fa m 



WTT-I 



ft* 



FIG. 42. Dissection of a medusa with rather more than one-quarter of the umbrella 

 and manubrium cut away (diagrammatic). The ectoderm is dotted, the endo- 

 derm striated, and the mesoglcea black. circ. c. circular canal ; end. lam. 

 endoderm lamella ; gon. gonad ; /. lithocyst ; mnb. manubrium ; mth. mouth ; 

 rod. c. radial canal ; vl. velum. 



minute globular sacs (/.), each containing a calcareous particle 

 or lithite. These are the marginal sense-organs or lithocysts : 

 they were formerly considered to be organs of hearing, and 

 are hence frequently called otocysts : in all probability their 

 function is to guide the medusa by enabling it to judge of 

 the direction in which it is swimming. The marginal 

 organs, in this case, may therefore be looked upon as organs 

 of the sense of direction. 



