56 



THE PROTOZOA 



tion with a flagellum, and is to be regarded as a web of the ecto- 

 plasm (periplast) connecting the flagellum to the surface of the 

 body. Such a condition may arise either by attachment of a back- 

 wardly-directed trailing flagellum to the side of the body, as in 

 Trichomonas (Fig. 5) and Trypanoplasma (Fig. 36), or by the 

 shifting backwards of the point of origin of an anterior flagellum, 

 as is well seen in the transition from crithidial to trypanif orm phases 

 in the development of trypanosomes (Fig. 131). As a rule, only the 

 proximal portion of the flagellum is involved in the formation of 



Sb. 



FIG. 27. Pleuronema chrysalis. M, The undulating membrane ; o, mouth ; 

 N, macronucleus ; n, micronucleus ; c.v., contractile vacuole ; f.v., food 

 vacuole ; a., anal pore. After Schewiakoff, magnified 660 diameters. 



the undulating membrane, and the distal portion projects freely 

 beyond it ; but in some cases a distal free portion of the flagellum 

 may be quite absent, and then flagellum and undulating membrane 

 are co-extensive (Fig. 12, J). Undulating membranes in Flagellata 

 appear to be specially related to the endoparasitic mode of life, and 

 in free-living species they are found rarely, if ever ; they may be 

 regarded as an adaptation to life in a broth-like medium, such as the 

 intestinal contents, or the blood of a vertebrate, containing many 

 suspended particles or corpuscles. In such cases the membrane 

 may assist the organism to force its way between the solid bodies 

 suspended in the fluid medium. Undulating membranes may, how- 



