84 TEXTBOOK OF ZOOLOGY 



Behavior 



Euglena usually lives near the surface of the water if the light 

 there is not too intense, and when in the active phase swims about. 

 This animal displays positive phototropism and is easily stimulated 

 by changes in intensity of light. If the light is too intense, there 

 will be a negative response. A medium light is optimum for it. 

 There is naturally an attraction to light in those forms which utilize 

 it in the manufacture of food by photosynthesis. Direct, intense 

 sunlight, however, is injurious to them. When Euglena swims 

 through the water, its anterior end with the flagellum goes foremost 

 and is first to reach any injurious or distasteful environment. 

 When it encounters such a condition in the medium, it stops and 

 turns sharply in another direction and attempts to move out of 

 danger. This is known as the avoiding reaction. In these and other 

 reactions this cell exhibits the irritability that is characteristic of all 

 protoplasm. 



Locomotion and Flag'ellar Movement 



Contractions and expansions take place in Euglenae when they 

 are not actively moving about. These movements resemble waves 

 of contraction (peristaltic contraction) passing over the cell. Some 

 of the larger species move about in a crawling fashion by taking 

 advantage of this movement. This activity is known as euglenoid 

 movement. The chief method of locomotion is swimming by means 

 of the whiplike movements of the flagellum through the water. A 

 spiral path is followed due to the continuous turning of the body. 

 The flagellum is made up of an elastic outer sheath which encloses 

 an axial filament composed of one or more contractile fibrils. 



