48 THE STORY OF ANIMAL LIFE 



called Radiolarians. The " cyst " consists of a 

 basket-work supporting skeleton of flint; there 

 may be several, one inside the other, and con- 

 nected by radial bars. A living species named 

 Actinomma has three such layers of basket-work, 

 one in the outer layer of protoplasm, one in the 

 inner layer, and a central one. It will perhaps 

 be remembered by the reader that the animals of 

 this group, Radiolaria, are forms described in a 

 previous volume of the series, as so curiously as- 

 sociated in Symbiosis with the algae known as 

 Yellow Cells. 



The famous polishing slate of Bilin in Bo- 

 hemia consists of flinty Protozoan shells ; it is 14 

 feet thick, and a cubic inch has been estimated to 

 contain 41,000,000,000 of the shells. 



While the Radiolarians are marine, the Heli- 

 ozoa, a group in which the skeleton is also pres- 

 ent, but not usually so greatly developed, are 

 predominantly fresh-water forms. Both classes 

 take their name (Ray-animals, Sun-animals) from 

 the stiff radiating rods of the skeleton. 



Strongly to be contrasted with the above 

 groups belonging to the Rhizopoda are the In- 

 fusoria proper, which are characterized by the 

 usual possession of cilia. Cilia (literally " eye- 

 lashes ") are fine hair-like processes of the proto- 

 plasm of the cell, which fringe its exterior; by 

 their constant movement they enable the animal 

 to swim, and at the same time they create a cur- 

 rent in the water, which washes up to the region 

 of the mouth particles which may serve for food ; 

 for these creatures have this very great advantage 

 over Amoeba, and the other forms above referred 

 to, that they possess something which may be 

 called a mouth. That is to say, there is one par- 



