^2 LIGHT AND TEE BEHAVIOR OF ORGANISMS 



represented in Fig. ii. Under certain conditions some 

 forms cast off the flagellum, sink to the bottom and crawl 



about in a manner to be described in 

 detail later. In the encysted state, as 

 is well known, they are inactive. 



h. Historical account. — It has long 

 been known that these organisms in 

 their free-swimming state orient and 

 -c.v. swim toward a source of light, and 

 Stahl (1880, p. 410) found that if the 

 light intensity is high they become 

 negative, i.e., they swim away from 

 the source of light. Engelmann (1882, 

 Fig. II. Side view of p. 396) observed that if Euglenae are 



anterior end of Euglena ,1 1 • j ^ • ' ±. 



viridis, after Wager;., eye- Hiountcd on a slide contammg a spot 

 spot; /, flagellum; e./., en- of relatively Strong light they collect 



largement in flagellum; c.r., • j • . i • . • , 



contractile vacuole. ^"^ dense masses m^ this spot just as 



Paramecia collect in regions contain- 

 ing a little CO2. They swim into it without any ap- 

 parent reaction, but when they reach the boundary on 

 the way out they stop suddenly, turn around, and thus 

 remain in the illuminated area. Engelmann called this 

 reaction Schreckhewegung, shock-movement, and Jennings, 

 avoiding reaction. Engelmann also proved that the ante- 

 rior end of E. viridis is more sensitive than the posterior. 

 Jennings (1904) however was the first to demonstrate 

 the connection between the shock-movement, the sudden 

 turning when subjected to a decrease in illumination, 

 and orientation, although the idea expressed in the fol- 

 lowing words shows that Engelmann (1882, p. 395) was 

 also very near the truth in this matter: " Falls sie, was 

 bei schnellem Vorwartsschwimmen wohl einmal geschieht, 

 gans ins Dunkel hineingekommen sind, sistiren sie doch 

 so fort die weitere Vorwartsbewegung, drehen um eine 

 ihrer kurzen Axen, probiren — oft unter bedeutenden 

 Gestaltsanderungen — in verschiedenen Richtungen fortzu- 

 kommen bis sie endlich wieder ins Licht gerathen." 



