it is also convex. The surface of the sub umbrella, on the o- 

 ther hand, is convex except in this same thinner marginal area. 

 This is evidently an adaptation to the animal's habit of rest- 

 ing on the bottom with its oral side uppermost. 



The gelatinous layer of the exumbrella is fii'm and elas- 

 tic and when the animal comes to rest on a flat surface and 

 the subumbrellar muscles art relaxed, this jelly tends to as- 

 sume its normal shape and the slight suction which is thus 

 produced gives the animal somethirg of a hold on the bottom, 

 and makes it less liable to be disturbed by the action of waves 

 and currents. The marginal zone of jelly is much thinner and 

 is capable of motion independently of the central concave disk, 

 and it is by its frequent movements that currents of water are 

 kept up bringing food and oxygen to the animal . 

 Structure of the Jelly . 



Keller's description (1883) of the stn.cture of the jelly 

 in C. polypoides would apply equally well to our species. Be- 

 sides connective tissue fibres the jelly contains sevei-al kinds 

 of elements. First there are scattered through it many star- 

 shaped cells that remind one of osteoblasts and are probably 

 analogous to them. Ham.ann (1881) speaks of them as Collo- 

 b lasts. Then there are great numbers of the so-called green 

 cells, especially in th'i subumbrella. These may be isolated 



