RHEOTAXIS IN ISOPODA 329 



parts were painted dead black. Two centimeters from each end 

 a screen of brass wire netting was placed in order to keep the 

 isopods from the current introducers. These introducers were 

 of brass tubing in the form of a capital T. An even distribution 

 of the current across the pan was assured by having the cross 

 bar of the T punctured by eight equidistant holes, each 3 mm. in 

 diameter. The water was withdrawn by a brass tube 2 cm. in 

 diameter placed at the middle of the box with its lower side 4 cm. 

 from the waxed bottom. This was also drilled with equidistant 

 holes, which were guarded by wire screening to prevent the isopods 

 from escaping. 



Tap water was introduced at one end and the boiled or the high 

 carbon dioxide water, as the case might be, was allowed to 

 flow in at the other. The most striking results were obtained 

 with a flow of 200 to 400 cc. per minute at each end of the pan. 

 This current spread over the 10 cm. width of the pan, gave a flow 

 of 5 to 10 cc. to each square centimeter of cross section, for each 

 minute, and isopods do not react definitely to a flow of this 

 strength. 



With this device, a gradient of from 1.82 cc. of oxygen at one 

 end of the pan to 8.14 cc. at the other, was obtained. For 

 changes in salts see table 9 and discussion on p. 296. The half 

 bound carbon dioxide was decreased Tec. per liter and the nitro- 

 gen content was lowered from 18 to 3 cc. per liter. In all cases 

 a control was run in a box, the exact duplicate in every way 

 of the experimental one. The only difference between the two 

 sets of conditions was that in the control the same kind of 

 water was introduced at both ends, the rate of flow being the same 

 as in the ej^perimental box. From five to ten isopods were used 

 at a time and the experiments ran from one to six hours. Read- 

 ings were taken at five, ten or fifteen minute intervals. In all 

 cases the trials were made in very diffuse light or in total darkness. 

 The readings were taken with a one candle power white light. 

 The temperature was kept near enough that of the water from 

 which the isopods were taken so that there was no temperature 

 interference. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 13, NO. 2 



