CONTROL OF RHEOTAXIS IN ASELLUS 181 



the average rheotactic reaction was depressed by 25-37 minutes 

 treatment from 40, 32, 28 to 23, 25, 52 per cent positive, negative 

 and indefinite. The isopods lost correlation in this solution in 

 about two hours making the toxicity-depression ratio approxi- 

 mately 31 per cent which is the lowest ratio shown by any 

 potassium salt. None of the six isopods tested with sodium 

 hydroxide were made more positive and the average rheotactic 

 reaction was also markedly' depressed. 



These results with alkalies obviously are based on too little 

 work to conclude that hydroxyl ions wdll not stimulate positive 

 rheotaxis but since there was no evidence of such stimulation 

 the tests were discontinued. 



Distilled water 



To what extent are the results just recorded due to osmosis? 

 It will be remembered that Asellus is a fresh water isopod inhabit- 

 ing in this case pond water that must exert some osmotic pressure, 

 especially during the summer season when the solutes present 

 are more concentrated. Obviously however the effects recorded 

 cannot all be due to osmosis because as has been shown equi- 

 molecular sohitions of certain of the salts had opposite effects. 

 For example N/10 potassium chloride with an osmotic pressure 

 of about four atmospheres is very efficient in causing isopods 

 to give a more positive response. The same strength solution 

 of calcium chloride with an osmotic pressure only sUghtly greater 

 has the opposite effect. In all probability these were more 

 concentrated than the pond water to which the isopod- were 

 accustomed. 



By the use of distilled water it is possible to obtain a condition 

 where the osmotic pressure is less than that of the pond water 

 though the action of distilled water may be due to some other 

 factor than osmosis. Water redistilled in glass is quite toxic 

 for these isopods but the water furnished by automatic laboratory 

 stills will allow the isopods to live as long as five days. Bullot 

 ('04) obtained similar results with the fresh water Gammarus. 



