EHEOTAXIS IN ISOPODA 339 



olism increases with the increase of the oxygen dissolved in 

 water (Lingle '02, p. 83; Martin, '06, p. 303; Loeb, '06, p. 95). 

 Then, too, Pieron ('08, pp. 1020 and 1061) measured the amount 

 of oxygen present at different times in sea water and found that 

 Actinians expand and retract their tentacles as the oxygen ten- 

 sion of the water increases and decreases. These observations 

 were confirmed by experiments in aquaria, but Bohn ('08, p. 1163) 

 questions Pieron's interpretation of his results and attributes 

 these rhythms to much more complex factors. An increase in 

 temperature increases metabolism providing, of course, the in- 

 crease is not too rapid or too great. Caffein is known to have a 

 permanent stimulating effect (Cushny, I.e., p. 248). 



From this evidence, the rheotactic reaction in isopods must 

 depend upon the metabolic state of the animals. All the observed 

 facts concerning their response in nature support this view, except 

 that in young animals the rate of metabolic reaction is higher 

 than in the older ones. Yet the juvenile isopods give either no 

 rheotactic reaction or are very indifferent to the current. But 

 with these young isopods, conditions that favor a high metabolic 

 rate cause positive reactions to appear sooner and to be a great 

 deal stronger than in" those animals kept in conditions that depress 

 metabolism. Then, too, the clinging reaction is strong in these 

 young isopods and even under the most favorable conditions, they 

 do not move about rapidly. Since the positive rheotactic re- 

 sponse seems correlated with the degree of motile activity of the 

 isopods, this tendency to cling in one place would account for 

 the apparent discrepancy in the lack of positiveness in these 

 juvenile Aselli. 



In nature the complex of conditions found in ponds furnishes 

 isopods that give a low positive rheotactic response, while stream 

 conditions produce the opposite result. The amount of oxygen 

 in these two habitats varies greatly, as has already been pointed 

 out, and this variation appears sufficient to account in a large 

 measure for the difference in the reactions. Of course the fact 

 that the pond water usually contains more organic waste products 

 than are found in the streams, may help to cause the difference 

 in the reactions. But this does not seem to be the important 



