GENERAL CHARACTERS 153 



two important food-stuffs starch and proteids. The Radio- 

 larian may therefore be said to keep the Zooxanthellae con- 

 stantly manured, while the Zooxanthellae in return supply 

 the Radiolarian with abundance of oxygen and of ready- 

 digested food. It is as if a Haematococcus ingested by an 

 Amoeba retained its vitality instead of being digested : it 

 would under these circumstances make use of the carbon 

 dioxide and nitrogenous waste formed as products of kata- 

 bolism by the Amoeba, at the same time giving off oxygen 

 and forming starch and proteids. The oxygen evolved would 

 give an additional supply of this necessary gas to the Amoeba, 

 and the starch after conversion into sugar and the proteids 

 after being rendered diffusible would in part diffuse through 

 the cell-wall of the Haematococcus into the surrounding 

 protoplasm of the Amoeba, to which they would be a valuable 

 food. 



Thus, as it has been said, the relation between a Radio- 

 larian and its associated yellow-cells are precisely those 

 which obtain between the animal and vegetable kingdoms 

 generally. 



The Diatomacece, or as they are often called for the sake 

 of brevity Diatoms, are a group of minute organisms, in- 

 cluded under a very large number of genera and species, so 

 common that there is hardly a pond or stream in which they 

 do not occur in millions. 



The general form of Diatoms is very various : they may be 

 rod-shaped, triangular, circular, and so on. Their essential 

 structure is, however, very uniform : the cell-body contains a 

 nucleus (Fig. 36, A, mi) and vacuoles (vac), as well as two 

 large chromatophores (chr) of a brown or yellow colour; 

 these are found to contain chlorophyll, the characteristic 

 green tint of which is veiled, as in Zooxanthella, by diatomin. 



