REVERSIBILITY OF HELIOTROPISM OF ARENICOLA LARVAE. 165 



IV. SUMMARY. 



1. At room temperature nearly all Arenicola larvae in the early 

 swarming stage in normal sea-water are positively heliotropic. 

 At a temperature 10 C. higher (3O-34) about half of the larvae 

 become negative to light after about 5 minutes' exposure. At 

 10 C. lower than room temperature (ca. 12), about 20 per cent. 

 of the larvae become negative after about 25 minutes exposure. 



2. In sea-water made hypertonic by addition of NaCl or KC1 

 some larvae (20-35 P er cent.) become negative. 



3. In sea-water made hypertonic with CaCl 2 , MgCl 2 or MgSO 4 , 

 no reversal was observed. 



4. In hypotonic sea-water, about 90 per cent, of the larvae 

 reversed their heliotropism; reversal takes place more rapidly 

 the higher the temperature (between 12 and 32). 



5. Isotonic solutions of ammonium, potassium, lithium, sodium 

 chlorides and sulphates made in sea-water reverse the positive 

 heliotropism of the larvae. But isotonic solutions of magnesium 

 and calcium chlorides and sulphates in sea-water do not. The 

 reversing effect of these salts seems, therefore, due to the cations. 

 This suggests an action on the plasma membranes (cf. the mem- 

 brane theory of Ostwald-Bernstein). 1 The effective order of 

 these cations is 



Na+<Li*<K+<NH 4 + 



6. In artificial sea-water, the positive heliotropism of the 

 larvae is not noticeably different from that in natural sea-water. 

 A trace of alkali makes apparently no difference. 



7. Hypertonic sodium and potassium chloride solutions added 

 to the artificial sea-water reversed the positive heliotropism of 

 the majority of the larvae. 



8. In sodium-free artificial sea-water, the larvae became motion- 

 less immediately after the treatment (effect of hypo tony). 



9. In potassium-free artificial sea-water, many became negative 

 to light, and remained at the bottom of the negative side of the 

 beaker. 



10. In calcium-free artificial sea-water, the larvae became 

 motionless about 15 minutes after the exposure. Magnesium 



1 Bernstein, J., Pfliiger's Arch., Bd. 122, S. 129. 



