314 



OBELIA 



CHAP. 



attached by the middle of the convex surface to the 

 blastostyle, 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 gono- 

 theca 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 



entf.fam 



yen. 



FIG. 79. Dissection of a Medusa ('' 25) 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 ; 

 rod. c. radial canal ; vl. velum. 



substance (Fig. 80, D, msgl) covered both on its inner 

 surface, or sub-umbrella, and on its outer surface, or 

 ex-umbrella, by a thin layer of delicate cells (ecf). 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 mesoglo2a ; it is 

 hollow, its cavity (ent. cav) opening below, i.e., at its 

 distal or free end, by a four-sided aperture, the mouth 



