34 MANUAL OF ZOOLOGY SECT. 



biosis between the Zooxanthella 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 tem- 

 porary 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). 



The body of Euglena (E, H) is spindle-shaped, and has at 

 the blunt anterior end a depression, the gullet (F, as.), from 

 the inner surface of which springs a single long flagellum (/?.). 

 The organism is propelled through the water by the lashing 

 movements of the flagellum, which is always directed for- 

 wards ; it can also perform slow worm-like movements of 

 contraction and expansion (A D), but anything like the 

 free pseudopodial movements which characterise the Rhizo- 

 poda, is precluded by the presence of a very thin skin or 

 cuticle which invests the body. There is a nucleus (nit.) 

 near the centre of the body, and at the anterior end a 



