THE BEHAVIOR OF OTHER INFUSORIA 



125 



tion ("orientation") may be brought about by the selection of varied 

 movements. 



The common hypotrichan Oxytricha fallax, abundant in vegetable 

 infusions, is well fitted for the study of this reaction. A large number of 

 specimens are placed on a slide or trough. When one end of the trough 

 is gradually heated 

 by passing water at 

 a temperature of 

 40 degrees beneath 

 it, the Oxytrichas 

 at this end are 

 seen to become 

 very active, dart- 

 ing about in all di- 

 rections (Fig. 84). 

 As the temperature 

 rises, they give the 

 avoiding reaction, 

 — darting back- 

 ward, and turning 

 to the right. This 

 is alternated with 

 rapid dashes for- 

 ward. Whenever 

 a specimen passes 

 toward the warmer 

 end of the trough, 

 or when it comes 

 in contact with the 

 sides or end, it re- 

 sponds with the 

 avoiding reaction. 

 But a specimen 

 passing away from 

 the heated region, 

 in the direction of 

 the arrow at 14 



(Fig. 84), does not Fig. 84. — Reaction of Oxytricha to heat. The slide is heated at 

 ffive the reaction "*® ^^^ "'• ^" Oxytncha in position I reacts as indicated by the arrows, 

 ^ . . ' repeatedly moving backward, turning to the right, and moving forward, 

 because it is pass- thus occupying successively the positions 1-14. When it finally be- 

 ing from a hot to <^°™^ directed away from the heat, as at 13-14, it ceases to change its 

 ° . , direction of movement, but continues to move straight ahead, thus 

 a cool region. 1 tie reaching a cooler region. 



