INTEGUMENTARY PHOTOSENSITIVITY IN A MARINE FISH 263 



duced here. There were no currents in the tank during the experiments 

 with light, but the sea-water was renewed at frequent intervals. For 

 the purpose of determining the positive or negative character of the 

 hamlet's phototropism, one-half of the water in the tank, which was 

 approximately 15 x 15 x 12 inches in size, was darkened by an opaque 

 black screen; the other half, containing the fish, being fully illuminated 

 by electric lamps of various intensities. Thus were created so-called 

 light and dark fields. If a fish, manifesting the fin and body move- 

 ments characteristic of photic stimulation, 5 remained in the lighted 

 zone, it was regarded as positively phototropic, whereas swimming into 

 the darkened portion of the tank was taken as evidence of negative 

 phototropism. 



In the detection of differences in the degree of sensitivity of various 

 regions of the integument, as well as for securing additional evidence 

 concerning the positive or negative character of the phototropism, the 

 light was localized, rather imperfectly, upon different regions of the 

 body by means of absorbing screens of black pasteboard and mirrors 

 which served as reflectors. It was difficult to procure a delicate local 

 stimulation by these methods and the areas described are large and 

 overlap each other more or less. 



The writer is deeply grateful for the assistance which Dr. E. L. 

 Mark and Dr. W. J. Crozier have rendered in connection with this work, 

 and also for the privilege of study at the Bermuda Biological Station. 

 He also wishes to thank the trustees of the Humboldt Fund for finan- 

 cial aid. 



II. DESCRIPTION OF EXPERIMENTS 



When a normal fish was placed in the testing aquarium, it swam about 

 vigorously for a few moments, bumping the walls with its head or tail. 

 This uneasiness soon disappeared, however, and it settled to the bot- 

 tom, where, unless disturbed, it remained indefinitely. If now its body 

 were sufficiently illuminated, it soon began to swim about, and usually 

 did not resume a resting position until the light was removed or until 

 it reached the dark end of the aquarium. The criterion of stimulation 

 adopted for these experiments was to start the fish in this way from a 

 position of rest and then, by cutting off the light, to cause a resumption 

 of the original attitude. 



5 The fishes when first illuminated often twitched their dorsal fins rapidly and 

 changed position slightly. No increase in the rate of respiration was noted and 

 the fin movements were but little changed in rate. Ultimately illumination led 

 to more or less continuous swimming about. 



