MOVEMENTS OF HERRING AND OTHER MARINE FISHES. 323 



of the presence of so many green plants. Carbon dioxide is 

 probably more important in connection with movements of the 

 fishes than in the matter of restricting their breeding places. 



IV. Reactions of Fishes to Chemical Conditions in 

 Sea-Water. 



I. Conditions and Methods of Study. 



• The experiments were performed in a gradient tank. Water 

 of two kinds was used in the experiments. One kind was 

 allowed to flow into one end at a definite rate and another kind 

 into the other end at the same rate. It flowed out at the middle 

 at the top and at the bottom so that the two kinds of water met 

 at the center. The outflow at the center did not of course 

 prevent the mixing of the two kinds of water and thus the middle 

 section, equal to one half or one third of the tank was a gradient 

 between the two kinds of water. The tank used in these experi- 

 ments was 122.3 cm. by 15 cm. by 13 cm. deep. The front wall 

 was of plate glass and a plate glass top was used at times. Water 

 was allowed to flow in at both ends at the same rate (usually 

 600 c.c. per minute) through tees the cross bars of which con- 

 tained a number of small holes. The cross bars of the tees were 

 at the center of the ends of the tank behind screens. The drain 

 openings were located at the center near the top and in the 

 bottom. The outer openings of the drain tubes were at the 

 level of the water in the tank. The water flowed in at the ends 

 and drifted toward the center and flowed out through the drains. 

 We found no evidence that fishes react to the slight current thus 

 produced. Since each half of the tank held about 9 liters, it 

 required 15 minutes to fill it or to replace all the water in 

 one of the halves. The tank was enclosed under a black hood. 

 Two candles (electric lights being wanting) were fixed in the 

 rear and above the center of the two halves, i. e., above a point 

 midway between the screen partition and the center drain. 

 The light was 15-20 cm. above the surface of the water which was 

 13 cm. deep. The room was darkened during the experiments 

 which were observed through openings in the hood above the 

 fights or through the glass side late at night. Fishes do not 

 usually note objects separated from them by a light. 



