MOVEMENTS OF DAPHNIA 261 



daphnids under constant conditions maintain themselves 

 at nearly a definite height in the aquarium. They may sink sev- 

 eral centimeters, but usually soon regain the former position 

 by a few rapid strokes of the antennae. Birge (1897) seems to 

 be of the opinion that change of pressure is the controlling factor 

 in this case. It seems that the differences in water pressure on 

 the upper and lower parts of a daphnid must be very small in 

 amount, and that these differences can hardly act as a directive 

 force in the vertical movements. However, -careful experimen- 

 tation is needed to settle this point. 



Although every care was taken to experiment on the animals 

 under as nearly natural conditions as possible, it was not pos- 

 sible to reproduce the conditions of the natural habitat and still 

 be able to control the factors for experimentation. The un- 

 natural conditions in the experiments may have altered the 

 behavior to some extent, but it is believed that the general 

 features of daphnid behavior are as have been indicated. 



Daphnia pulex shows a strong tendency to avoid any solid 

 objects, such as the sides of the dish. This indicates that is 

 is a true plankton animal. 



When a daphnid is approached by a glass rod or other solid 

 object held in the hand there usually ensues a vigorous move- 

 ment away from the source of stimulation. This reaction must 

 be of great value to the animals in escaping enemies. 



When any considerable change is made in the environment, 

 such as changing the animals to water to which they are not 

 accustomed, or by strong heating or cooling, there is a tendency 

 for a great number to get caught on the surface film. Once 

 caught on this film the animals are seldom able to escape but 

 remain there until they are exhausted and die. 



The behavior of Daphnia pulex is regulatory in general. 

 Continued exposure to temperatures much above 25° C. pro- 

 duces feebleness or death in these animals. However, in nature 

 such strong stimulation would produce positive geotaxis and 

 perhaps negative phototaxis which would cause the animals to 

 descend to lower levels in the lake wehre the temperature would 

 be lower. Under ordinary conditions the animals remain near 

 the surface where they find food and the normal conditions of 

 existence. 



One particular case of very peculiar behavior may be men- 



