REVERSIBILITY OF HELIOTROPISM OF ARENICOLA LARVAE. 155 



NH4C1 solutions, the most notable exit of pigment from the 

 larvae the sign of increased permeability according to Lillie 

 took place at about ten or twenty minutes after the treatment. 

 In the case of Na 2 SO 4 solution about 5 per cent, of the larvae 

 became negative at about ten minutes, while in NH4C1 solution 

 the larvae were evenly distributed at about 20 minutes after the 

 treatment. The maximum reversal in the former was produced 

 at about two hours and in the latter at about one hour after the 

 treatment. The reversing effect of these two salts, therefore, 

 seems to be too slow to be regarded as the direct action of ions 

 which have diffused into the larvae. At the same time, the 

 effect is not on the boundary membrane alone, because these 

 salts must have modified and penetrated the membrane long 

 before the change of reaction in the larvae occurred. The larvae 

 treated with Na2SC>4 and NH,,C1 stick tightly to the window side 

 of a beaker 7 or 8 minutes after the treatment. This fact may 

 explain the difficulty. That is to say, if they had been free in 

 swimming, they might have reversed their heliotropic reaction 

 more promptly than they actually did. 



As to the way in which the behavior of organisms is modified 

 by electrolytes such as salts, we are as yet in the dark. At any 

 rate, the reversal of positive heliotropism of the larvae by these 

 electrolytes is an insufficient index by which to gain an under- 

 standing of the mechanism of their physical or chemical influence 

 on the behavior of organisms. 



4. Effects of Artificial Sea-water. 



The artificial sea-water that the writer used was a van't Hoff 

 mixture of the following composition : 



0.52 m. Nacl 100 c.c. 



0.52 m. KC1 2.2 c.c. 



0.35 m. CaCl 2 1.5 c.c. 



0.35 m. MgCl 2 7-8 c.c. 



0.90 m. MgSO4 3-8 c.c. 



To this a trace of NaHCOs was added as Loeb advised. This 

 artificial sea-water with or without the alkali affected little the 

 positive heliotropism of the larvae. About five per cent, of them 

 showed negative heliotropism. Besides these, a few larvae in 



