46 NATHAN FASTEN 



ever, as soon as the disturbing agent ceased acting, the animals 

 again began to sink, until they reached the bottom, and here 

 they moved about in their peculiar way. It becomes evident 

 that this is not exactly a case of geotactic response but merely 

 a result due to the high specific gravity of the copepod. 



VII. REACTIONS TO LIGHT 



i. Behavior in daylight. — Seven copepods were placed into a 

 round dish of water, and allowed to remain near a window for 

 some time. Soon all the organisms gathered near the window. 

 Rarely did one leave the area. If a copepod, through its motion, 

 happened to be brought into the opposite side of the dish, it 

 soon began to adjust itself accordingly, swimming about actively 

 until it was brought into the region of greatest illumination. 



When a hand was passed between the window and the glass 

 dish, the shadow caused the copepods to momentarily dart 

 about. They did not, however, leave the most highly illumi- 

 nated side of the dish, but actively moved about in it. The 

 response in this case is similar to what Whitman (1898) has 

 observed in the leech Clepsine., If a hand is passed over a dish 

 containing a number of Clepsine, the shadow causes the animals 

 to stretch in every direction, as if trying to reach something. 

 Bateson (1887), has found that a passing shadow also causes 

 a similar response in shrimps and prawns. 



Obviously, this mode of behavior is of great value in pro- 

 curing these animals their food. As Mast (1911) says, 'The 

 important point is that the shadow in itself is of no particular 

 importance, but what follows may be." 



In direct sunlight, the copepods were found to behave simi- 

 larly. The reaction was observed many times and the positive 

 phototropism was very striking. 



2. Behavior in artificial light. — All experiments with artificial 

 light were carried on during the evenings. The hatchery was 

 situated a good distance from any of the neighboring dwelling 

 houses, and it was also surrounded by tall trees, so that the 

 laboratory was perfectly dark on the- nights when experiments 

 were conducted. 



Five copepods were placed into a circular dish, eight and 

 one-half inches in diameter, and this was allowed to remain 

 in darkness for about an hour. The current was then switched 



