12 General Morphology of the Protozoa 



homogeneous core (193, 195). In contrast to the axial filament, the outer 

 cytoplasm is a sol, as indicated by the movement of inclusions. Axial 

 filaments may converge in a central granule (Acanthocystis and related 

 genera) or they may end separately in the cytoplasm (Actinosphaerium). 



FLAGELLA AND ASSOCIATED 

 STRUCTURES 



Flagella 



These organelles are found in Mastigophora and in flagellate stages 

 of Sarcodina and Sporozoa. A typical fiagellum is composed of a sheath, 

 which may be circular, elliptical, or flattened in cross-section, and an 

 inner axoneme. The latter, according to some workers, is the active por- 

 tion of the fiagellum while the sheath is merely protective. Others think 

 that the axoneme is only an elastic support for a contractile sheath. The 

 axoneme arises from a granule, the blepharoplast . and may or may not 

 extend beyond the sheath as a distal end-piece (Fig. 1. 7, F). A terminal 

 knob (Fig. 1. 7, H), instead of a filament, is evident in silver preparations 

 of Trypanosoma rhodesiense (127). The anterior flagella of Hexamitus 

 pulcher (130) also are unusual in that they arise from external rod-like 

 structures (Fig. 1. 7, E) of uncertain significance. 



The finer structure of the fiagellum^ is incompletely known, although 

 investigations with the electron microscope (13, 56, 180, 199) have sup- 

 plemented earlier observations. The axoneme may be composed of one, 

 two (Astasia, Euglena), three {Peraneyna), or perhaps more fibrils, while 

 the sheath apparently contains a spirally coiled filament in certain species. 



The sheath in some flagella shows lateral filaments (Fig. 1. 7, A, C), 

 the mastigonemes (43) or "Flimmer," the nature of which is uncertain. 

 Although observed in living Mallomonas acaroides in dark-field (217), 

 they may be artifacts (173) or may represent fibrils of the sheath which 

 are frayed out laterally under certain conditions (180). At any rate, such 

 filaments appear consistently in some species and not in others. In the 

 stichoneynatic fiagellum (43), a single row of filaments extends along one 

 side of the sheath (Fig. 1. 7, A), as in Astasia and Euglena (13). In the 

 pantojiematic type there are two or more rows of mastigonemes. Only a 

 terminal filament is present in the acronematic, or "lash" fiagellum (174), 

 while the pantacronematic type shows both a terminal filament and one 

 or more rows of mastigonemes. A simple type, found in Cryptomonadida 

 and Dinoflagellida (174), shows neither terminal filament nor masti- 

 gonemes. These characteristics of the fiagellum seem to be constant within 

 various groups and may furnish significant information in studies on 

 taxonomy and phylogeny (174, 217). 



In the majority of flagellates, the flagellum extends forward from its 



^ This subject has been reviewed in several papers (13, 174, 180, 217). 



