xiv STRUCTURE OF CELL-WALL 155 



digested food. It is as if a Hsematococcus 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 Amceba, 

 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 Amceba, 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 DiatomacecR^ or Diatoms, as they are often called for 

 the sake of brevity, are a group of minute organisms, in- 

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

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

 they do not occur in millions. 



Diatoms vary almost indefinitely in form : they may be rod- 

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

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

 nucleus (Fig. 35, 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. 

 The cell is motile, executing curious, slow, jerky or gliding 

 movements, the cause of which is still obscure. 



The most interesting feature in the organization of diatoms 

 is however the structure of the cell-wall : it consists of two 



