OBSERVATIONS ON UNICELLULAR EORMS 99 



material in such a position that it is more strongly affected 

 when the ventral surface is illuminated than it is when the 

 dorsal surface is illuminated. The function of the eye-spot 

 will be referred to again later. 



Orientation in negative specimens takes place precisely 

 as it does in positive specimens. The reactions resulting 

 in orientation however are induced by an increase of inten- 

 sity in place of a decrease, as in the case of positive speci- 

 mens; and a change from a position in which the eye-spot 

 faces the light to one in which the ventral surface is exposed 

 induces the avoiding reaction, while in positive specimens 

 it is a change from the latter to the former which causes this 

 reaction. 



After having thus worked out the details in the orienting 

 reactions in Euglenae in the crawling state, I made obser- 

 vations on specimens in the free-swimming state and found 

 the reactions to be essentially the same. A brief account 

 of these observations will be found below (p. 102). 



h. Discussion. — The orientation of Euglena in the 

 crawling state confirms in general the description of the 

 orientation in the free-swimming state given by Jennings 

 (see p. 83). When the organism is not oriented every 

 change from a position in which the light strikes the ventral 

 surface to one in which it strikes the dorsal, and vice versa, 

 due to rotation on the long axis, may be considered a 

 " trial movement." If such a trial movement results in a 

 decrease of light intensity on the sensitive protoplasm in 

 the organism it responds with a definite reaction, after which 

 it repeats the trial movements. Thus it continues until it 

 becomes so directed in its course that rotation on the long 

 axis no longer produces sufficient change of intensity on 

 the sensitive part to induce a reaction. It is evident that 

 this condition is fulfilled when the organism moves toward 

 or away from the general source of light. Orientation can 

 take place in an absolutely constant intensity of light in 

 the field. It is however always induced by reactions which 

 are due to changes 0} intensity on some part of the organism. 



