THE BEHAVIOR OF A PARASITIC COPEPOD 47 



on to a 60 c. p. Mazda bulb, which had previously been arranged 

 in position about a foot from one side of the dish. When first 

 illuminated, all the copepods were found to be at the bottom ; 

 but soon they began to dart about in every direction, ultimately 

 making their way into the lighted area, where they remained. 

 When the light was shifted to the opposite side of the dish, the 

 organisms also changed their position. They moved about 

 actively and in a few moments all had made their way across 

 the dish, to the light side. An object, such as the hand, passed 

 betw r een the light and the dish, produced the same reaction as 

 that brought forth by the shadow in ordinary daylight. 



When an aqueous solution of alum was intercepted between 

 the dish containing the free swimming copepods and the light, 

 in order to absorb the heat from the 'rays, the organisms were 

 found to behave similarly. Of course, the water in the dish 

 served the same purpose to some extent. It is seen then that 

 it is the light which causes the copepods to orient and not the 

 heat produced by the electric bulb. 



In another series of experiments twelve of the animals were 

 used, and the same results were obtained. Here the electric 

 bulb was slowly shifted in a circle around the dish, and the 

 copepods slavishly followed the light. It is remarkable to note 

 with what precision the organisms shift, when the position of 

 the light is gradually changed. Time and time again, the ani- 

 mals made the complete round of the dish. 



The definiteness with which the copepods orient to light is 

 even more easily discerned when an oblong dish is used. Eight 

 copepods were placed into a glass dish whose dimensions were 

 eight inches long, one and one-fourth inches high, one and 

 one-fourth inches wide, and were allowed to remain in the 

 utter dark for about an hour. Then the current was switched 

 on to a 60 c. p. Mazda globe, which was previously arranged in 

 place at one end of the dish. The sudden flash caused the 

 organisms to leave the bottom and to swim actively about in every 

 direction. Shortly, however, they all made their way into the 

 most highly illuminated region and remained there. Now a 

 similar electric lamp, stationed at the opposite end of the dish, 

 was turned on, while at the same time the first globe was ex- 

 tinguished. The copepods soon began to shift their position. 

 One after another, they circled through the water, actively 



