44 



COLLEGE ZOOLOGY 



tions the chloroplasts and pyrenoids degen- 

 erate and disappear. Although the euglenas 

 do not capture and eat other organisms, 

 there are colorless animal-like flagellates 

 which do eat protozoans, algae, and 

 diatoms. 



Locomotion 



In swimming, the flagellum beats back 

 and forth, moving the animal forward. A 

 spiral path is followed, resulting in a straight 

 course through the trackless water, provided 

 no stimulus interferes. Although euglenas 

 possess a definite shape, they are charac- 

 terized by wormlike movements involving 

 waves of contraction to which the term 

 euglenoid movement has been applied 

 (Fig. 20). 



Reactions to light 



Euglenas are easily stimulated by changes 

 in the direction of the light. Most species 

 swim toward an ordinary light such as that 

 from a window; and if a culture is examined, 



most of the animals will be found on the 

 side toward the brightest light. This is of 

 distinct advantage to the animal since light 

 is necessary for the process of photosyn- 

 thesis, just as is true in plants. However, 

 euglenas will swim away from the direct rays 

 of the sun; direct sunlight will kill them if 

 they are exposed to it for a long time. If a 

 drop of water containing euglenas is placed 

 in the direct sunlight with one half shaded, 

 the euglenas will avoid the shady part as 

 well as the direct sunlight, both of which 

 are unfavorable to them. They will remain 

 in a small band between the two, in the 

 light best suited for them, their optimum 

 (Fig. 19). By shading various portions of 

 the body of a euglena, it has been found 

 that the region of the eye spot is especially 

 sensitive. It should be noted that when a 

 euglena is swimming through the water, it 

 is this anterior end which first encounters 

 regions of different light intensity; the ani- 

 mals give the avoiding reaction when they 

 enter less favorable areas. 



Shaded side of vessel 



Euglenas gather in 

 intermediate region 



Direction of light 



Figure 19. Euglena. Reaction to light. The euglenas gather in the intermediate region across 

 the middle, where the light intensity is most favorable for them. (After Jennings.) 



Reproduction 



Reproduction in Euglena takes place by 

 binary longitudinal fission (Fig. 20). The 

 nucleus divides in two by mitosis; then the 

 anterior organelles such as the reservoir are 

 duplicated; and the animal divides longitudi- 

 nally, that is, in an antero-posterior direc- 

 tion, splitting the cell into two equal parts. 

 The old flagellum may be retained by one 

 half, while a new flagellum is developed by 

 the other. Often longitudinal division takes 



place while the animals are in the encysted 

 condition (Fig. 20). These animals are said 

 to be encysted when they have become al- 

 most spherical and are surrounded by a 

 gelatinous wall which they have secreted. In 

 this condition, periods of drought are suc- 

 cessfully passed, the animals becoming active 

 again when water is encountered. Usually 

 in laboratory cultures, cysts are present on 

 the sides of the dish. Before encystment, 

 the flagellum is thrown off, but a new one is 

 produced when activity is again resumed. 



