Reactions of Isopods to Light 285 



ped by some stimulus, it often remains perfectly quiet for a time. 

 But if the stimulation be kept up, the animal usually waves its 

 antennae about in a characteristic manner, lifts its head slightly, 

 turns the anterior end of the body to one side or the other a little, 

 and perhaps turns the entire body more or less and begins to move 

 again. All of these movements may occur in succession, or any 

 one may occur without the others, or only the first part of the 

 series may be gone through with; but the series occurs often 

 enough to be characteristic, particularly with stimuli that are non- 

 directive. This strongly suggests a motor reaction in the sense 

 in which Jennings uses the term. It is not that stereotyped type 

 of reaction, however which characteriszes the typical motor reac- 

 tion. 



With these preliminary statements regarding the actions of 

 Asellus, we are in position to consider the actions of the species 

 at the plane of division between the dark and the light regions. 



If headed toward the illuminated region, the animals sometimes 

 stop abruptly when partly across the plane or immediately after 

 crossing it. This stopping occurred often enough to indicate that 

 it was due to the sudden action of the light on the animal. If the 

 animal in one of its runs, reached the plane before it was well under 

 way, or when it seemed near the end of such an excursion, this 

 abrupt stop was more likely to occur than if the animal crossed the 

 plane while well under way, since in the latter case any stimulus, 

 as has been stated, is likely to be less effective than when the ani- 

 mal IS moving more slowly. Sometimes stopping near the line 

 was apparently due to causes other than that of suddenly coming 

 into the light, e. g., the animal may have reached the end of its 

 run. But in other cases the stopping was so abrupt and the reac- 

 tion so characteristic of Asellus when stimulated, that without 

 question the reaction was due to the influence of the light. 



If the animal stopped at the plane, or so little beyond it that 

 the characteristic movements following stimulation brought its 

 head back partly or entirely over the plane, it almost invariably 

 turned into the dark region at once or followed along the plane 

 a short distance and then entered the dark. Animals which on 

 entering the light region met the plane at a very oblique angle 



