PHOTOTACTIC REACTIONS IN HYALELLA 261 



wide, containing twenty-five cubic centimeters of distilled water. 

 Very slowly and gradually the concentration of the solution was 

 increased by adding constant small amounts of a given chemical 

 at intervals of five to fifteen minutes. A careful record was kept 

 of the number of animals positive and the number negative at 

 certain concentrations throughout the experiments. In every case 

 the light was moved and the reaction of the animals tested from 

 each end of the dish; that is, when the animals had oriented 

 themselves in one end of the dish, the light was then transposed to 

 that end and the reaction tested again. The results were surprising ; 

 in each case the amphipods remained decidedly negatively photo- 

 tactic, even though the concentration was carried far beyond the 

 point at which they became positive when dropped directly into 

 the solution. The concentration in each case was increased to the 

 point where the majority of the animals died; nevertheless they 

 were negative, and decidedly so, throughout the experiment 

 until death occurred. Thus, when the concentration is gradually 

 increased, the Hyalellas were negative in .64 per cent solution 

 of ethyl alcohol, .05 per cent solution of ammonium hydroxide, 

 .022 per cent hydrochloric acid, .05 per cent nitric acid, .43 per 

 cent acetic acid, .029 per cent picric acid, and 0.22 per cent 

 chromic acid. The results will appear more evident by a study 

 of the accompanying table. In this table the first column of fig- 

 ures indicates the per cent solution; following each of these 

 figures, to the right, is indicated the number of animals which were 

 positive and the number negative at each reading of a given 

 chemical at a concentration given in the first column. The line 

 drawn across each column indicates the point at which the animals 

 were positively phototactic when dropped directly into the solu- 

 tion. Where the sum of the positive and negative specimens 

 is not equal to the sum of the positive and negative used at the 

 beginning of the experiment, it indicates that a number of animals 

 died from the effect of the chemical. 



It is evident that it is not chemical change in the tissues which 

 caused the reversal of reaction, for, if it were, it would be impos- 

 sible to increase the concentration and still have the animals 



