SHAPE AND SIZE OF BODY 



21 



sider the aperture which is applied to the surface over whicli the organism 

 is moving as ventral in position, so that a dorsal and a ventral surface 

 can be distinguished (Fig. 8). Many Mastigophora are definitely elongate, 



Fig. 7. — Cercomonas longicauda ( x 2000) : Changes in Shape undergone by 

 A Single Individual during Twenty Minutes' Observation. (Original.) 



and locomotion takes place in the direction of the flagellate end (Fig. 6). 

 In these it is evidently possible to distinguish an anterior from a posterior 

 end. When a mouth aperture or cytostome is present, it is usually near 

 the anterior end, but slightly to one 

 side of the terminal flagella. The 

 surface nearest to which the cytostome 

 lies may be regarded as the ventral 

 surface, in which case it becomes 

 possible definitely to orientate the 

 organism. In the case of such a 

 flagellate as Trichomonas (Fig. 26) it 

 is legitimate to speak of the anterior 

 flagellated extremity of the body, the 

 posterior extremity through which the 

 axostyle protrudes, the ventral surface 

 ^near which the cytostome is placed, 

 and the dorsal surface which is provided with the undulating membrane 

 and its basal fibre. This orientation becomes complicated to a certain 

 extent by the fact that a torsion or twisting of the body towards a spiral 



Pig. 8. — Difflugia constricta : A Shelled 

 Rhizopod from Pond Water 

 ( X 660). (Original.) 



The shell is strengthened by adherent grain 

 of sand. 



