REVERSIBILITY OF HELIOTROPISM OF ARENICOLA LARV.E. 157 



naturally positive to light. Such was not the case in Arenicola 

 larvae, as just stated. 



Elimination of KC1 had a different effect. After transferring 

 the larvae into the artificial KCl-free sea-water at 23 C., prac- 

 tically all showed positive reaction immediately after the treat- 

 ment, though a few soon became negative. About 20 minutes 

 after the treatment, about 20 per cent, of all larvae were at the 

 top of the positive side of the beaker, about 30 per cent, at the 

 bottom of the same side, and about 50 per cent, at the bottom of 

 the negative side. If the larvae collected at the bottom of the 

 beaker farthest from the source of light could be called negative, 

 one sees a puzzle hardly capable of solution. For, as already 

 seen, addition of 2 c.c. of isotonic KC1 solution to 50 c.c. of the 

 of the natural sea-water also reversed about 70 per cent, of the 

 larvae. In other words, either addition or elimination of potas- 

 sium salt produced a negativating effect on the larvae. Besides 

 potassium, however, there was another negativating component 

 in the artificial sea-water, namely sodium. In the unbalanced 

 artificial sea-water, sodium ions might exert a negativating effect 

 on the larvae. 



In the calcium-free artificial sea-water at 23 C. all larvae swam 

 about at the bottom of the beaker. Their activity was much 

 diminished; and about 15 minutes after the treatment a majority 

 of them became almost motionless. CaCl 2 in the artificia-1 sea- 

 water as well as in the natural one is therefore essential to the 

 life and activity of the larvae, although of the five salts it is 

 present in the smallest proportion. It is well known that calcium 

 and magnesium antagonize the toxic action of sodium and potas- 

 sium. Magnesium alone seemed, however, not to be sufficient 

 in this case. 



In the magnesium chloride-free or sulphate-free artificial sea- 

 water at 23 C., immediately after the treatment practically 

 all the larvae were positive. About 10 minutes after the treat- 

 ment about 60 per cent, or more of them were positive, about 

 20 per cent, negative, mostly at the bottom of the negative side 

 of the beaker, and 20 per cent, or less were scattered at the bot- 

 tom. During I to 5 hours after the treatment about 70 per cent, or 

 less were positive, and the rest were at the bottom of the negative 



