THE PROTOZOA 



amoeba, such vacuoles are believed to eliminate water from the cell, and only 

 incidentally to serve for the expulsion of the soluble excreta which this water 

 may contain. 



A mass of red pigment at the anterior end of the organism is called the 

 stigma, or eye spot; it seems to be a light-sensitive organelle. The nucleus 

 lies near the center of the cell, surrounded by green chromatophores, the 

 chloroplasts, which fill the cytoplasm. The chloroplasls contain chlorophyll 

 and are responsible for the green color of the cell. This chlorophyll is com- 

 parable with that in the green cells of plants. Between the chloropla.sts the 

 most conspicuous inclusions in the cytoplasm are bodies of characteristic 

 shape, varying between different species, composed of paramylum. This is a 

 complex carbohydrate related to starch, and the paramylum bodies are inter- 

 preted as stored food reserves. There is no flowing of the endoplasm as in the 

 amoeba, although the plasticity of the cytosome is demonstrated when the 

 euglenoid cell changes its shape. 



Movements and Responsiveness. Characteristic expansions and con- 

 tractions of the cell, occurring when the euglena is not in active locomotion, 

 are called euglenoid movements (Fig. 8.10). These are not interpreted as re- 

 lated to progressive locomotion, which is brought about by the action of the 

 flagellum. The flagellum beats in such a way as to propel the organism in a 

 spiral course, rotating upon its long axis. 



By these movements of the cell body, and by spiral swimming, the organism 

 reacts to a variety of stimuli. The behavior with respect to light, a necessary 

 factor in the environment of these plant-like forms, has been studied especially. 

 Observations have shown that a euglena which has been moving toward a 

 source of light gradually changes its direction when the direction of the light 

 is changed and so continues to orient positively toward the light. The adjust- 

 ment involves a complex series of movements, including rotation of the cell 

 upon its long axis; but once the orientation is accomplished, the animal con- 



after A. E. Shipley, 1893, ^oology of the Invertebrata; B, adapted from R. P. Hall and T. L. 

 Jahn, 1929, Transactions of the American Microscopical Society, vol. 48, printed by permission.) 



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