CONTROL OF RHEOTAXIS IN ASELLUS 185 



just as a decrease in positiveness does when isopods are highly 

 positive at the start. Nine isopods or 38 per cent of those tested 

 were stimulated by this treatment for this length of time. The 

 isopods reacted to water currents after fourteen hours exposure 

 to sugar solutions and fully recovered from the depression when 

 placed in tap water. Needless to say the longer treatment 

 caused a greater depression. 



Although cane sugar depresses positive rheotaxis it does not 

 do so to the extent that would be expected if the effects of the 

 calcium ions reported above were due to osmosis. No experi- 

 ments were run testing whether or not the effects of sugar could 

 be offset by different ions as in muscle or nerve preparations 

 but three attempts at recovery using distilled water were some- 

 what successful. The best case follows: The rheotactic reaction 

 of isopod 290 was changed by three hours treatment with M/2 

 cane sugar from 80 per cent positive, 20 per cent indefinite to 

 20 per cent negative, 60 per cent indefinite, 20 per cent zero. 

 After 3 hours 30 minutes in once distilled water the response 

 was 60 per cent positive, 40 per cent indefinite. This is not the 

 expected result if distilled water acts by a differential removal of 

 salts for cane sugar should act in the same manner. Loeb ('03) 

 found with a marine Gammarus that distilled water and cane 

 sugar solutions had approximately the same toxic effect and 

 ascribed this to the loss of electrolytes or ions into each solution. 

 He suggested that the exit of antagonistic salts takes place with 

 unequal rapidity or in unequal relations. If the effect on rheo- 

 taxis were explained on this basis one would have to assume 

 that calcium or strontium salts escape into distilled water and 

 potassium or sodium salts into cane sugar and finally that by 

 withdrawing first enough of one set and then enough of the other 

 the rheotactic reaction would be restored to its original condition. 

 This may be what happens but the observed results can be amply 

 and more simply explained on the basis of a change in water 

 content, with the distilled water allowing an increase in water 

 which should increase the metabolic rate as suggested above and 

 the cane sugar removing water which should decrease the rate 

 of metabolism, and this is what actually happens under treat- 

 ment with cane sugar (p. 193). 



