72 



ELEMENTS OF BIOLOGY 



gaseous wastes are also excreted as in amcebae. In fact, this direct 

 transfer of materials through the cell wall is common to all cells 

 of all animals. In complex multicellular organisms additional ma- 

 chinery is provided whereby these substances may be conveyed to 

 and from the cells, but the essential facts concerning the processes 

 as regards each cell are identical in amoeba, paramoecium, or Man. 





ZOOCHLORELLA 



Fig. 



22. — ParamcEciitm btirsaria containing symbiotic plant cells, Zoochlorella. 



(Re-drawn after Conn.) 



Symbiosis. Certain species of Paramoecium, for example, 

 Paramoocium bursaria, are frequently infected with a green plant 

 cell growth (Fig. 22). This small unicellular green plant, Zoochlo- 

 rella, is found in sufficient numbers to give the animal a green 

 color. This relation between the host paramoecium and the invader 

 plant cell is a curious partnership, for not only does neither of the 

 associates harm the other, but they are mutually helpful. Such a 



