32 MANUAL OF ANIMAL BIOLOGY 



cell body of Euglena, and other related forms, always divides 

 longitudinally, beginning at the anterior end. Very shortly after- 

 wards if the conditions are favorable, the daughter cells attain the 

 structural organization and size of a typical Euglena. (W. f. 

 22, B.) 



When the environmental conditions of Euglena are unfavorable, 

 due to a lack of moisture or other factors, it has the power of 

 encysting, a process which is quite common among many species 

 of the unicellular organisms. The organism just previous to 

 encystment becomes quiescent and soon assumes a spherical shape. 

 The ectoplasm then secretes a resistant cyst wall which entirely 

 encloses the cell. In this condition the dormant organism is able 

 to withstand unfavorable environmental conditions. When the 

 latter are again favorable the cyst wall is dissolved, and the Euglena 

 once more assumes an active life. In many cases cyst formation 

 is followed by reproduction, for during encystment one or more 

 divisions of the cell body may occur, so that when the cyst wall 

 is later dissolved, two or more individuals escape and begin an 

 active, free-swimming existence. (W. f. 22, C.) 



4. Adaptation 



The stigma in Euglena, as has been stated above, is believed 

 to be sensitive to light rays. Thus the organism is able to attain 

 the particular location in any given environment which is best 

 adapted for carrying on the photosynthetic processes. In a culture 

 of Euglena it will be found, for example, that the individuals, in 

 general, move toward the source of light. However, direct sun- 

 light does not represent the best condition, and may even be fatal. 

 If, therefore, one side of the culture is dark and the other side is 

 in the direct sunlight, it will be found that the organisms tend to 

 congregate in an intermediate zone between the two extremes. 

 This particular region is the one with optimum conditions for pho- 

 tosynthesis. A response to light by an organism is spoken of as 

 phototropism. Euglena, when it moves toward the light, is 

 said to be positively phototropic, and when it moves away from 

 light of high intensity, to be negatively phototropic. 



The phenomenon of encystment, noted above, may be regarded 

 as an adaptive measure which enables Euglena to survive unfavor- 

 able environmental conditions. It, is of common occurrence among 

 unicellular organisms. 



