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AN INTRODUCTION TO ZOOLOGY 



is said to be encysted (Fig. 39 F and G) . In this condition periods 

 of drought are successfully passed, the animals becoming active 

 when water is again encountered. Usually in cultures brought 

 into the laboratory many cysts are found on the sides of the dish. 



FIG. 40. Diagram of the reaction of Euglena when the light is decreased. 

 The organism is swimming forward at i ; when it reaches 2 it is shaded. 

 It thereupon swerves toward the dorsal side, at the same time continu- 

 ing to revolve on the long axis, so that its anterior end describes a circle, 

 the Euglena occupying successively the positions 2-6. From any of 

 these it may start forward in the directions indicated by the arrows. 

 (From Jennings.) 



Encystment frequently takes place without any apparent cause, 

 the animal resting in this condition for a time and then emerg- 

 ing again to its free swimming habit. Before encystment the 

 flagellum is thrown off, a new one being produced when activity 

 is again resumed. 



