306 W. C. ALLEE 



That is, in the isopods in low oxygen content all their lives, 

 there was less tendency to regulate their response to that normal 

 for stream isopods than was the case with their parents that were 

 kept in the same vessel with the second generation. Another 

 point is significant in this connection. The isopods in the 

 smaller amount of oxygen were seven months old at the close 

 of the trials shown in the table, while those in higher oxygen were 

 only five months old. The exact meaning of this time difference 

 is masked by the fact that the isopods in the low oxygen content 

 had been kept in quiet water which was only infrequently changed, 

 while the others were from running water. The food conditions 

 in the two cases were about the same. 



The second part of the table summarizes the results of more 

 exact tests. The isopods used in both this experiment and the 

 control were reared in the laboratory from the same stock. They 

 were separated when too small to be tested and those that were 

 to be kept in higher oxygen were placed in an aerating device 

 patterned after that of Colton ('08, p. 428). In the form used 

 this consisted of a large wide mouthed glass bottle which could 

 be tightly sealed (fig. 4). 



Two glass tubes extend through the cork. One of these (B) 

 opens above the water in the bottle and is connected at the other 

 end with a filter pump. The other glass tube (A) is drawn to a 

 fine point at the outside end and only a small opening is left for 

 the entrance of air. In this way the bubbles are smaller and so 

 create less current in the water. On the inside this tube extends 

 down well towards the bottom and opens into a larger tube (C). 

 The large tube is about two centimeters in diameter and slopes 

 upward uatil its upper end extends out of the water. Just 

 below the water's surface are three openings (E) blown into the 

 wall of the tube. When the suction is turned on, the air is drawn 

 into the bottle through (A). The bubbles are then drawn into 

 (C) at (D) and escape at (F). 



During the passage through the water some of the gases of the 

 air are absorbed and in tliis way the water is kept at air satura- 

 tion. Whatever current is set up by the passage of the bubbles 

 escapes through the holes (E) on the under side of the large 



