52 



THE PROTOZOA 



every direction. A flagellum consists of an elastic axial core 

 enclosed in a contractile sheath or envelope (Fig. 24), from the 

 extremity of which the core protrudes freely in some cases, forming 

 a so-called "end-piece." The flagellum takes origin from a more 

 or less deeply-seated granule, the blepharoplast, or basal granule, 

 which will be described in dealing with the nuclear apparatus 

 (p 82, infra). The elastic axis, arising from the blepharoplast, 

 can be regarded as a form-determining element of encloplasmic 

 origin, the sheath as an ectoplasmic motor substance. A flagellum 



is usually cylindrical in form, with the axial 

 filament central in cross-section, but may be 

 band-like, with the axial filament at or near 

 one edge ; it is usually of even thickness 

 throughout its whole length, but when the 

 axial filament is exposed to form a terminal 

 end - piece the flagellum tapers to a fine 

 point. 



Like pseudopodia, flagella serve primarily for 

 locomotion, and secondarily for food-capture, 

 which is effected by causing food-particles to 

 impinge on some point or aperture at the surface 

 of the body, where they are ingested. In their 

 relation to locomotion two types of flagella can 

 be distinguished, termed by Lankester pulsella 

 and tractella respectively. A pulsellum is 

 situated at the end of the body Avhich is 

 posterior in movement that is to say, it is a 

 flagellum which by its activity propels the body 

 forwards. Flagella of this type occur in Oxyrrhis 

 (p. 278) and in the Choanoflagellata (p. 271), 

 but are comparatively rare in the Protozoa. In 

 the majority of cases the flagella are tractella 

 that is to say, their action is such as to drag the 

 body after them hence they are situated at 

 the end which is anterior in progression. Con- 

 sidered generally, the movements performed by 

 tractella are of two types. In some cases the entire flagellum is 

 thrown, into even, sinuous undulations, and the body of the 

 flagellate progresses with a smooth, gliding movement, which may 

 be extremely rapid, and is then well expressed by the French 

 phrase " mouvement en fleche ": this type of movement is well 

 seen in the trypanosomes and allied genera, such as Leptomonas, 

 etc. In most free-living flagellates, however, the flagellum is held 

 out stiff and straight for the proximal two-thirds or so of its 

 length, while the distal third performs peculiar whirling or pulsating 



FIG. 24. Structure 

 of the flagellum of 

 Euglena. ax., Axial 

 filament ; c.p., con- 

 tractile protoplasm 

 enveloping the 

 axial filament ; e.p., 

 end - piece of the 

 flagellum, consist- 

 ing of the axial fila- 

 ment exposed ; r, 

 root of the flagel- 

 lum passing into 

 the body (compare 

 Fig. 84). After 

 Eiitschli (3). 



