REVERSIBILITY OF HELIOTROPISM OF ARENICOLA LARV/E. 153 



natural sea-water at room temperature, 22 C., it was found that 

 30 minutes after the treatment about 35 per cent, of the larvae 

 became negative to light. Besides sodium and potassium 

 chlorides, m-CaCl 2 , m-MgCl 2 , and 2m-MgSO 4 were tested without 

 success. Temperature experiments also failed in all these 

 mixtures of hypertonic sea-water. 



(6) Effect of Hypotonic Sea-water. The mixture of 30 c.c. of 

 natural sea-water and 20 c.c. of distilled water (which was the 

 optimum dilution) was very remarkable in producing a reversal 

 of heliotropism. The effect was temporary only. The results 

 of several series are summarized in Table II. It is also interesting 

 to note the difference in the time required to produce the maxi- 

 mum results at the three temperatures 12, 22, 32. A Qio value 

 of about 2 is indicated for the reversive process. 



TABLE II. 



EFFECT OF HYPOTONIC SEA-WATER. 



The larvae lived over 15 days and grew well in this hypotonic 



sea-water. 



3. Effects of Isotonic Salt Solutions. 



A question arose whether these reversing effects of hypertonic 

 and hypotonic sea-water were simply osmotic or not. This 

 idea was tested by using isotonic salt solutions. In all of these 

 series the problem of temperature was omitted from considera- 

 tion. The results are given in Table III. 



Since the sodium, lithium, potassium or ammonium chloride 

 and sulphate solutions that were used were all isotonic with 

 sea-water, it is evident that the reversing effect of these salts is 

 not simply an osmotic one. And since the action of these eight 

 salts was different from that of calcium or magnesium chloride 



