GENERAL ZOOLOGY 



Mastigamoeba 



Peranema 



Monosiga 



Fig. 8.11. Representative colorless flagellates. Although Mastigamoeba produces pseudopodia, 

 it is considered a flagellate because of the presence of a flagellum. Monosiga, a choanoflagellate, 

 bears a protoplasmic collar surrounding the flagellum. 



tinues its spiral progression in one direction. In general, the euglena responds 

 positively to light of optimum intensity; if the light is very intense, a negative 

 response will be exhibited. In these and other reactions the organism mani- 

 fests the responsiveness characteristic of all cells. 



Nutrition and Metabolism. Possessing chlorophyll, the euglena carries on 

 holophytic nutrition like that of green plants. It is doubtful that ingestion of 

 food ever occurs in Euglena, although such colorless flagellates as Peranema and 

 others do ingest small organisms through the gullet and form food vacuoles. 

 When kept in darkness, Euglena gracilis and other green flagellates lose their 

 green color but continue to live and reproduce for long periods. This is true, 

 however, only if certain organic compounds are present in the culture medium, 

 to satisfy the energy requirements of the cells. Thus it has been established 

 that the same species can maintain itself in the light by holophytic nutrition 

 and in darkness by saprophytic or saprozoic nutrition. In the absence of light, 

 the organism is unable to manufacture its energy-rich compounds by photo- 

 synthesis and must depend on external sources. 



Life Cycle and Reproduction. As in many other protozoans, the life cycle 

 of some species of Euglena includes an active phase, during which the organism 

 moves about, and an encysted phase, during which it is enclosed within a cyst 

 and is non-motile. It is questionable whether Euglena viridis ever undergoes 

 encystment. In this species reproduction occurs by binary fission, which is 

 typically a longitudinal division beginning at the anterior end of the cell 

 (Fig. 8.10). In other euglenas this division may proceed in either the active 

 or the encysted phase. So far as is known, there is no sexual reproduction 



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