276 



COMPARATIVE ANATOMY 



composed of columnar nutritive cells, 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 

 between the structure of a medusa and that of an ordinary 



rc- 



Fig- 59- 



Development of the medusa in Podocoryne carnea, a Hydroid allied to 

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

 canal ; cc, ring canal ; be, sub-umbrellar cavity ; /;/, 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. 



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 

 centrally with the endoderm of the manubrium and peripherally 



