THE CELL 



37 



Protococcus, which covers the north side of trees and stone walls 

 with a green coating of thousands of cells. The one-celled plants, 

 known as diatoms, are very abundant, inhabiting all bodies of 





Fig. 26. — Diatoms. {From A. S. Mann.) 



water, in particular the sea. They are among the most exquisite 

 of Nature's handiwork, being often beautifully patterned (Fig. 

 26). Vying with the diatoms for beauty in the world of micro- 

 scopic organisms are the uni- 

 cellular animals Foraminifera 

 and Radiolaria (Fig. 27), also 

 inhabitants, like the diatoms, 

 of the first few inches of the 

 surface of the sea. The minute 

 plants and animals that live in 

 the surface of the sea are known 

 as the plankton. The Radio- 

 laria and Foraminifera possess 

 a shell, as do the diatoms, and, 

 because of it, have produced 

 large deposits such as the chalk 

 cliffs of Dover. The siliceous 

 shells of the diatoms constitute the deposits which form diato- 

 maceous earth. The shells of diatoms and Radiolaria are of silica; 

 the shell of Foraminifera, of lime. These creatures differ much 



Fig. 27. — Cromyodrymus abietinus . 

 {From " Voyage of the Challenger.") 



