ANATOMY OF MEDUSA 



247 



endoderm lining the cavity of the manubrium is thick, and 

 composed of columnar nutritive, and numerous gland-cells, 

 but it is reduced to a layer of simple cubical cells in the 

 radial and ring canals. The diagram shows a feature which 

 is of importance in enabling a comparison to be established 



Fig. 53. 



Development of the medusa in Podoeoryne carnea, a hydroid allied to 

 Obelia ; ec, ectoderm ; en, endoderm ; bn, bell nucleus ; re, radial 

 canal ; cc, ring canal ; be, sub-umbrellar cavity ; m, manubrium ; el, 

 endoderm lamella ; t, tentacles ; v, velum, a circular curtain running 

 round the mouth of the bell, not developed in the medusa of Obelia ; 

 ov, ova developing on the wall of the manubrium. For further 

 description of the figures see text. 



between the structure of a medusa and that of an ordinary 

 Hydroid polype. The section is represented as being slightly 

 oblique, so that on the left side of the picture it passes through 

 a radial canal, but on the right side it passes through the solid 

 substance of the umbrella. It can be seen that the mesoglcea 

 is traversed by a thin sheet of solid endoderm continuous 



