264 HOVEY JORDAN 



A. Reactions of normal fishes 



1. To light and dark fields. Light- and dark-field experiments, in 

 which the half of the aquarium containing the fish was illuminated, 

 were often repeated and almost invariably a decided negative reaction 

 was observed. The fishes with few exceptions swam away from the 

 lighted end of the aquarium when a 60-watt light was used as a stimu- 

 lus. This reaction occurred in approximately 60 seconds, occasionally 

 after a much longer time. Then after a variable period of relative 

 quiescence in the dark, the fish often returned to the lighted zone for 

 an interval, shorter, usually, than the sojourn in the shaded end. Il- 

 lumination usually caused uneasiness, the fish being constantly in mo- 

 tion. In many instances this swimming from light to dark and back 

 to light was repeated several times; but until the photoreceptors became 

 exhausted, successive illuminations seldom failed to cause a retreat to 

 the dark a very definite negative reaction. 



2. To regional stimulation. Negative responses were also obtained in 

 nearly all cases when a portion only of the integument was illuminated 

 by a 100-watt electric light. By testing different areas of the skin 

 separately, the other regions being in comparative dark, it was found 

 that various areas manifested this sensitivity in different degrees. The 

 times of exposure required to produce evidences of stimulation on 

 certain general areas of the body were as follows: Head and eyes, 1-5 

 (average 3) seconds; posterior end of the body, 4-17 (av. 7.6) seconds; 

 and mid-trunk 10-20 (av. 12.5) seconds. In these experiments the whole 

 of the head, the body posterior to the anus and the region between the 

 pectoral fin and anus, were successively, and more or less exclusively, 

 stimulated by the localized light. Each fish was tested several times 

 in rapid succession; though some reactions were inexplicably variable 

 in time, there was on the whole a noticeable uniformity in the duration 

 of illumination which was necessary to produce a negative response. 

 For instance, among the records of the reaction times when the stimu- 

 lus was applied to the head region are many data which are as uniform 

 as the following: 4, 3, 2, 1, 3, 2 seconds. Sometimes the stimulation 

 required was much longer, 60 seconds being the greatest. But as a 

 rule the reactions to localized stimulation were consistent and precise 

 and the failures to respond were few. 



Normal hamlets, then, are negatively phototropic when either the 

 whole, or a considerable part, only, of their integuments are stimulated 

 by electric light. Some areas of the skin, too, usually manifest a 



