THE PROTOZOA 



A variety of other amoebas inhabit the intestine of man and lower animals, 

 probably more species than can now be recoa^nized. Most of these appear to 

 be relatively harmless "messmates," or commensals, living within the larger 

 animal but not markedly disadvantageous to the host. 



The Flagellata 



In the class Flagellata are included the protozoans possessing one or more 

 flagella (singular, fiagellum), or whip-like extensions of the cytoplasm, during 

 the more representative phases of the life cycle. Flagella are primarily 

 organelles of locomotion; in some species they also assist in feeding. Flagella 

 are also found in many species of Sarcodina but usually only during a limited 

 part of the life cycle. In a similar manner, amoeboid stages occur in the life 

 cycles of many flagellates. The existence of both flagellate and amoeboid 

 stages in a single species suggests a close relationship between Sarcodina and 

 Flagellata. The Flagellata also exhibit a close relationship with plants, since 

 many of the flagellate Protozoa possess chlorophyll and are sometimes indis- 

 tinguishable from unicellular plants. Among such plant-like flagellates are 

 species of the genus Euolerta. 



The Euglena: General Structure. A typical euglena (Fig. 8.9) is covered 

 by a thin pellicle, comparable with the cell wall in plant cells and often 

 marked externally in a spiral pattern. The pellicle is stifl" enough to pre- 

 serve the contours of the organism as it swims through the water but flexible 

 enough to allow the changes of shape called euglenoid movement. The anterior 

 end of the organism bears a mouth-like notch, from which a flask-shaped 

 cavity extends a short distance into the cell. The single fiagellum protruding 

 from this cavity arises from two branches, each of which originates in a 

 granule, or blepharoplast. From one blepharoplast a fiber extends to the 

 nuclear membrane. The fiagellum itself consists of a central axial filament, 

 formed by the union of the two branches, and a surrounding, spirally wound 



Fig. 8.8. Entamoeba histolytica. 

 A, trophozoite or vegetative 

 form; B, a cyst. (.Adapted 

 from W. Balamuth in F. A. 

 Brown, Jr., el al.. Selected 

 Invertebrate Types, copyright 

 1950 by John Wiley and 

 Sons, Inc., printed by per- 

 mission.) 



239 



