36 THE BEHAVIOR OF LOWER ORGANISMS. 



the two sides, the animals on crossing the line should become oriented 

 by turning directly away from the source of light, as shown in the 

 diagram (Fig. 12) at a. The animal would then be expected to swim 

 in the direction x-v as shown by the specimen a, i, 2, 3, 4. 



It cannot be held that the real source of light for the Stentors is that 

 reflected from the bottom or sides of the dish in the lighted region, and 

 hence coming on the whole from a direction perpendicular to the line 

 xy, for the behavior of the Stentors shows that this is not the case, 

 A Stentor in the shaded region, close to the line x-y, as at c, Fig. 12, 

 receives whatever light there may be thus reflected exactly as it does 

 after it has crossed the line, yet it shows no reaction and does not 

 orient itself in any way. On the other hand, as soon as it crosses the 

 line x-y, so as to receive the light coming from the window, it reacts 

 strongly, as we shall see. It is thus clearly the light from the window, 

 coming in the direction shown by the large arrows, that causes the re- 

 action ; hence the Stentor ought, according to the direct orientation 

 theory, to orient itself in the line of these rays. 



When a Stentor, swimming at random, reaches the line x-y, it 

 reacts by stopping suddenly, then turning toward its aboral side, 

 then swimming forward. It thus swims about until its anterior end is 

 again within the shadow, where it continues to swim forward (Fig. 12, 

 <5, i, 2, 3, 4). Often the first reaction is not sufficient to direct it into 

 the shadow ; in this case the reaction is repeated ; one to three reactions 

 almost invariably bring the Stentor back into the shadow. It has no 

 particular orientation in the shadow, but swims in whatever direction 

 it happens to be headed. 



Very frequently the animals react when the anterior end alone has 

 crossed the line, so that less than the anterior half of the body is lighted. 

 In other cases the animal swims completely across the line, sometimes 

 for a distance greater than its own length, into the light, before it reacts. 

 In any case the reaction is that above described. 



Does the Stentor, when it turns on entering the light, always turn 

 away from the source of light, as the theory of direct orientation 

 requires ? 



At the moment of crossing the line into the light the Stentor may 

 occupy various positions. It will be well to note specifically the re- 

 action in certain of these positions, as we obtain here the observations 

 which furnish an exact and crucial test of the direct orientation theory. 



I. The Stentor may reach the line with the aboral side directed 

 toward the source of light (Fig 12, o). It therefore turns (as usual) 

 toward its aboral side. It thus swings its anterior end toward the 

 source of light, in the direction opposite that required by the direct 



