Reactions of Isopods 4^9 



eiied the cloth was formed into a rude hanging tube through which 

 the water escaped. While this arrangement certainly did not 

 make the darkened end light-proof, it at any rate cut off nearly 

 all the Hght except that which entered from the light part of the 

 trough. Doubtless a great deal did so enter, but under all cir- 

 cumstances there was a strong contrast between the illuminated 

 part of the trough and the rest as judged by the eye. 



Ten animals were found most satisfactory to experiment with 

 at a time, but occasionally as few as five or as many as twenty 

 were used. In general, currents of two strenghts were used: 435 

 centimeters per minute and 140 centimeters per minute. Currents 

 stronger than 435 cm. per minute swept the animals, especially 

 the Asellus, down the trough and currents weaker than 140 cm. 

 seemed to produce no decided results. 



I. Asellus 



When, after having become quiet, Asellus was subjected to a 

 current of water in the apparatus described, it usually responded 

 very quickly by crawling against the current. Individuals which 

 went down-stream soon turned back from the partition and 

 crawled up-stream. Individuals which reached the upper end 

 did not usually turn back. Hence the rheotaxis was very marked. 



The animals, when first subjected to a current, crept rapidly 

 for a time. In doing so they did not adhere well to the slate bot- 

 tom of the trough, and many were swept off their feet and carried 

 all or part way towards the lower end of the trough. Those swept 

 down ordinarily persisted in crawling up-stream again, and after 

 a time, when they had come to crawl less rapidly, fewer were car- 

 ried back by the current, and they became collected at the upper 

 end of the trough. 



Asellus tends somewhat to remain on the copper gauze parti- 

 tions at the ends of the trough. The great amount of moving 

 back and forth in the trough especially during the early part of 

 an experiment is sufficient warrant that this is not a very disturb- 

 ing factor, however. 



The maximum response as indicated by the number of indi- 



