44 COLLEGE ZOOLOGY 



swims toward an ordinary light such as that from a window, and 

 if a culture containing Euglena is examined, most of the ani- 

 mals will be found on the brightest side. This is of distinct 

 advantage to the animal, since light is necessary for the assimi- 

 lation of carbon dioxide by means of its chlorophyll. If a drop 

 of water containing Euglence is placed in the direct sunlight and 

 then one half of it is shaded, the animals will avoid the shady 

 part and also the direct sunlight, both of which are injurious to 



them, and will remain in a 

 small band between the two 

 "* ' in the light best suited for 



them, that is, their optimum 

 (Fig. 23). By shading various 



^. portions of the body of a 



Euglena it has been found 

 that the region in front of 



FIG. 23. Phototropism of Euglena. ,1 , ... 



Diagram showing the reaction of Euglena the e ye-SpOt IS more Sensitive 



to light. The light comes from the direc- than any other part. It 



tions indicated by the arrows, while the u u i~ j ^i i 



opposite side of the vessel is shaded, as should be noted that when 



indicated by the dots. The Euglena Euglence are Swimming 



gather in the intermediate region across ,v i ,1 , ,1 



the middle. (From Jennings.) through the Water it IS this 



anterior end which first 



reaches an injurious environment; the animals give the avoiding 

 reaction at once, and are thus carried out of danger. 



Reproduction. - - Reproduction in Euglena takes place by 

 binary longitudinal division (Fig. 22, E). The nucleus divides 

 by a primitive sort of mitosis. The body begins to divide at 

 the anterior end. The old flagellum is retained by one half, while 

 a new flagellum is developed by the other. Frequently Euglena 

 become spherical and secrete a gelatinous covering, called a 

 cyst. Periods of drought are successfully passed while in the 

 encysted condition, the animals becoming active when water is 

 again encountered. Sometimes division takes place during 

 encystment (Fig. 22, F, G). One cyst usually produces two 

 Euglence, although these may divide while still within the old 



