126 LIGHT AND THE BEHAVIOR OF ORGANISMS 



nucleus, ordinarily located near the surface, was visible, 

 and upon others in which opposite sides had a slightly dif- 

 ferent curvature, it could be seen (i) that the same surface 

 continually faces out as they proceed on the spiral course, 

 precisely as in asymmetrical forms; and (2) that a given 

 individual always turns toward the same side in giving the 

 avoiding reaction. The side toward which they turn bears 

 no definite relation to the location of the nucleus or to the 

 curvature of the side. Some turn toward the more convex, 

 others toward the more concave surface. These organisms 

 then, although symmetrical, respond with the avoiding 

 reaction, when mechanically stimulated, just like asym- 

 metrical forms. They stop, usually back quite a distance, 

 turn toward a given side, and then proceed on a new course 

 (Fig. 15). Since there is no known asymmetric structure 

 which bears any definite relation to the direction of turning, 

 and since the organism always turns toward the same side 

 no matter which point on the surface comes in contact with 

 a solid, it is evident that as far as the facts are known they 

 indicate that there is no differential response to a localized 

 stimulation. The same is true in case of the symmetrical 

 form Didinium nasutum. During the turning process it 

 was clearly seen in both Didinium and the swarm-spores 

 that the cilia strike forward on one side and backward on 

 the opposite side, showing a remarkable differentiation in 

 function. 



d. Orientation in light. — The process of orientation was 

 studied in negative forms in direct sunlight. In the quince- 

 seed jelly solution the spores do not orient definitely enough 

 to make it possible to work out their orientation reactions, 

 and positive specimens under normal conditions orient very 

 indefinitely. The method of procedure in this study was in 

 general like that followed in the observations on Euglena 

 and Stentor. 



A small beam of light direct from the sun was allowed to 

 fall on the stage of the microscope, and another beam was 

 reflected at right angles to it with a mirror. The light in 



