34 MANUAL OF ZOOLOGY sect. 



of the specially vegetable green colouring matter, chlorophyll, 

 it is able to utilise for its nutrition the carbon dioxide 

 or " carbonic acid gas " present in the air. The carbon is 

 seized and made use of by the plant cell for the building up 

 of such compounds as starch and sugar, while the oxygen is 

 set free. The animal cell, on the other hand, is continually 

 using up oxygen and giving off carbon dioxide in the process 

 of respiration, while it is unable, in the absence of chloro- 

 phyll, to manufacture such substances as starch and sugar. 

 Thus in this close association or symbiosis between the Zoox- 

 anthella and the Radiolarian, the latter benefits the former 

 by supplying it with carbonic acid and other substances by 

 which it is nourished, while the Alga contributes to the 

 respiration of the Radiolarian by the oxygen which it gives 

 off, and to its nutrition by the sugar and other substances 

 which it forms. 



2. THE MASTIGOPHORA 



We have seen that the spores by which multiplication is 

 effected in some of the Rhizopoda (Heliozoa, Radiolaria) 

 are characterised by the presence of slender whip-like 

 appendages — the flagella. In a great number of Protozoa 

 such a flagellate condition of the cell is not merely a tempo- 

 rary larval one, as in the cases already dealt with, but is the 

 ordinary and permanent condition of the adult animal. 

 These permanently flagellate Protozoa constitute the class 

 Mastigophora — a very numerous group, mostly of very 

 small size. A good example of this class, very abundant in 

 fresh-water pools, in which it may be present in such 

 enormous numbers as to impart to the water a distinct 

 green colour, is Euglena viridis (Fig. 12). Another species 

 or variety of Euglena viridis, is so abundant at times as to 

 colour the water blood-red (Stokes). 



