CONTROL OF RHEOTAXIS IN ASELLUS 183 



The results of experiments with 11 isopods are shown in 

 figure 8. The charting is the same as in the preceding figure 

 except that the abscissae spaces represent two hours in place of 

 one. As usual, responses obtained from isopods with an initial 

 reaction of over 70 per cent positive are disregarded. Of the 

 isopods tested 55 per cent were stimulated at some time during 

 their treatment. The 17 of the 35 tests which showed some 

 increase in the positive response averaged 49 per cent more 

 positive than before treatment. Six showed no change in the 

 positive reaction and the 12 trials that were less positive gave an 

 average decrease of 24 per cent. Altogether there was an average 

 increase in positiveness of 16 per cent for the 35 trials. 



Peters ('04) found that Very pure' distilled water causes 

 Stentor to lose salts but that the animals do not swell, i.e., 

 there is no intake of water. Garrey (vide Mathews '04) reports 

 that treating Chilomonas with distilled water increased their 

 irritability and Mathews suggests this is because the protoplasm 

 is brought more nearly to the neutrality point. However it 

 is possible that there is an intake of water and there is some 

 evidence (Riddle '14) that such an increase would result in an 

 increased metabolic rate which could account for the increase 

 in positive rheotaxis. This subject will be considered further 

 on p. 186. 



III. NON-ELECTROLYTES 



Cane sugar 



Cane sugar (rock crystal sugar) in M/2 solution exerts an 

 osmotic pressure of about 12 atmospheres (see references to 

 work of Morse and his students in literature list)^ which is near 

 three times that exerted by N/10 potassium or calcium chlorides 

 and if the depression or stimulation results given by these salts 

 were due to osmosis cane sugar should give greatly increased 

 effects. That the sugar at this strength causes depression is 

 shown in table 4, which exhibits results obtained by exposing 

 24 isopods to M/2 cane sugar for 52-75 minutes. As usual 



^ Garrey '15 found that 1/2 G. M. cane sugar solution gave —1.15 to — 1.155 

 which would give an osmotic pressure of about 14 atmospheres. 



