THE PROTOZOA OF THE SEA SHORE 



111 



in order to preserve the continuity of the deeper with the surround- 

 ing protoplasm. They have often a central contractile cavity, and 

 further show their claim to a higher position in the animal scale 

 than the preceding classes by the possession of little masses of cells 

 and a certain amount of fatty and colouring matter. 



Some of the radiolarians live at or near the surface of the 

 ocean, while others thrive only at the bottom. The former, in 

 some cases, appear to avoid the light, rising to the surface after 



FIG. 62. A GROUP OF RADIOLAHIAN SHELLS, MAGNIFIED 



sunset ; and it is supposed that the phosphorescence of the sea is 

 due in part to the presence of these animals. The latter may be 

 obtained from all depths, down to several thousand fathoms. 



The beauty of the radiolarians as a class lies in the wonderful 

 shells that protect the great majority of them. These shells are 

 composed not of carbonate of lime, as is the case with foraminifers> 

 but of silex or silica, a substance that is not acted on by the 

 strongest mineral acids. They are of the most exquisite shapes, 

 and exhibit a great variety of forms. Some resemble beautifully 



