314 OBELIA CHAP. 



style, produced at the edge into sixteen very short tentacles, 

 and having a blunt process, the manubrium, projecting from 

 the centre of the concave surface. They are ultimately set 

 free through the aperture in the gonotheca as little medusa 

 or jelly-fish (B D). 



The structure of a medusa must now be described in 

 some detail. The saucer-shaped " bell " or umbrella (Figs. 

 78 B D, 79 and 80) is formed of a gelatinous substance 



end.fam 



FIG. 79. Dissection of a medusa with rather more than one quarter of the umbrella 

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

 striated, and the mesogloea black, circ. c. circular canal ; end. lam. endoderm- 

 lamella ; gon. gonad ; /. lithocyst ; mnb. manubrium ; mth. mouth ; rad. c, radial 

 canal ; vL velum. 



(Fig. 80, D, msgt) covered both on its inner surface, or sub- 

 umbrella, and on its outer surface, or ex-umbrella^ by a thin 

 layer of delicate cells (ect). The clapper-like manubrium 

 (mnb) is formed of two layers of cells, precisely resembling 

 the ectoderm and endoderm of Hydra, and separated by a 

 thin mesogloea ; it is hollow, its cavity (ent. cav) opening 

 below, i.e. at its distal or free end, by a four-sided aperture, 



