124 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



Class Euglenea 



The euglenoid flagellates constitute a well-defined group 

 resembling the phytomonads in the nature of their photosynthetic 

 pigments, but differing from them in several morphological 

 features and in the production of paramylum, a starch-like reserve 

 material that is not stained by iodine. A whole spectrum of 

 metabolic patterns ranging from obligate photoautotrophy to 

 heterotrophy is illustrated by series of species found in nature, 

 and apochlorotic strains also varying in the complexity of their 

 organic needs are readily produced experimentally from species 

 of Euglena. Thanks to such seductive biochemical qualities, 

 combined with an interesting degree of morphological diversifi- 

 cation and an ubiquitous distribution, they are among the most 

 familiar protozoa. The two best-known genera, Euglena and 

 Peranema, are the only ones to have been studied thoroughly in 

 the electron microscope; information on flagellar structure of 

 several more is available. 



A flagellum of Euglena gracilis ; prepared for electron microscopy 

 by permitting unfixed or osmium- vapor-fixed cells to dry, is seen 

 in Fig. 42, PL XII. The unilateral array of long (up to 4 ^), slender 

 mastigonemes, and the striated bands bordering the flagellar axis, 

 have been seen in species of Euglena, Astasia, Phacus, Rhabdomonas, 

 Entosiphon, and Peranema (Brown, 1945; Pitelka, 1949; Chen, 

 1950a, 1950b; Houwink, 1951; Manton, 1952; Brown and Cox, 

 1954; Pitelka and Schooley, 1955). Like the pantoneme flagellum 

 of the chrysomonad-xanthomonad-brown-alga group, the 

 stichoneme flagellum of the euglenoids is a distinctive and 

 characteristic type. However, Pitelka and Schooley (1955) found 

 that under certain conditions — specifically when cells were fixed 

 in osmium tetroxide buffered to a pH at or somewhat more 

 alkaline than that of the culture fluid in which they were growing — 

 the euglenoid mastigonemes remained smoothly and compactly 

 wrapped around the flagellum (Fig. 43, PL XII). It has seemed 

 unlikely that the thickset fringe of delicate, flexuous filaments 

 could be anything but a hindrance to rapid movement of the 

 euglenoid flagellum; but if these normally adhere to the flagellar 

 surface they would, like the short, stout, presumably rigid masti- 

 gonemes of chrysomonads, effectively increase its diameter. The 



