202 BENEST WARREN. 



tion ; also it would seem as if the water becomes so changed 

 in character as to be specifically injurious to Daphnia. 



I. The Reaction of Daphnia magna to Different 

 Strengths of Solutions of Sodium Chloride. 



The Daphnia experimented with were taken from a large 

 tank (3 X 4^ x 1 j ft.), facing a south-east window at University 

 College, London. The tank contained a luxuriant growth of 

 Vallisneria, and a constant sIoav current of water was main- 

 tained. Some two years previously the tank had been 

 stocked from various sources. During the winter the water 

 was kept at a temperature of about 17° C. by a small gas- 

 ring fitted below the tank. 



It was found that a Daphnia placed into a solution contain- 

 ing above 1 percent. NaCl soon becomes uneasy and agitated, 

 and that after a certain time it may turn several somer- 

 saults in the water (sudden exposure to a strong light some- 

 times causes the same phenomenon), and then it sinks to the 

 bottom of the vessel. After this the animal may make 

 several feeble attempts at swimming, but it is soon exhausted. 

 The heart continues to beat, and the antennee and limbs to 

 move for a considerable time, and in the weaker solutions 

 (0"7 per cent., 0'9 per cent., I'O per cent.) this may go on for 

 hours. If after the last trace of movement has ceased the 

 Daphnia be placed in fresh water the animal may recover, 

 and after about ten minutes feeble movements of the heart 

 and limbs may be observed. The Daphnia may recover com- 

 pletely and breed, or it may swim about for a few hours and 

 then die. The condition when movement has ceased may be 

 termed salt-rigor. 



1. The Rate of Killing as a Function of the Strength 

 of the Solution, the Temperature remaining Con- 

 stant. 



The first point to investigate was the time at which salt 

 rigor occurred, using different strengths of solutions but a 



