LIGHT REACTIONS AND PHOTOTAXIS 593 



the positive and negative orientation and then finally reverse 

 their reaction, becoming negative with a velocity proportionate 

 to the hght intensity. If the light is very weak, the reversion 

 does not occur at all. As the authors saw positive animals 

 swim from direct sunlight into diffused daylight and negative 

 animals take the opposite course, provided only that they were 

 following the direction of the light rays, they concluded, that, 

 in accordance with Loeb's theory of phototaxis, not light inten- 

 sity, but the direction of the light rays, was the determining 

 factor in orientation. ^ Moreover they did not succeed in finding 

 any influence of sudden changes of intensity on the behavior 

 of the nauplii. They concluded from experiments with red and 

 blue glass, that the short wave-lengths were of greater effect 

 than the long ones. Finally, they tried the effect of changes 

 in temperature and salt-concentration, but without apparent 

 results. Hess, in the course of investigation on the sense of 

 sight in invertebrate animals, arrived at the conclusion, that 

 they all showed the maximum of stimulation in the green and 

 yellow-green parts of the spectrum, not in the blue and violet, 

 as was to be expected by the older experiments with color-screens. 

 This caused Loeb to reinvestigate the question, together with 

 Maxwell. They experimented on the Calif ornian Balanus and 

 Vol vox following the method of Hess. After placing a cuvette 

 containing the animals, in the light of a spectrum, they found 

 the largest gathering in the green. Hess simultaneously obtained 

 similar results with Balanus at Naples. 



I first examined the behavior of the nauplii in white light of 

 a 50 c. p. electric lamp. They were seen to become negative 

 much more rapidly on the first day after hatching, than on the 

 following. From day to day their sensitiveness to light slowly 

 decreased. Brought from the dark into Hght they all show a 

 characteristic reaction, sinking to the bottom more or less quickly, 

 according to the intensity of illumination and collecting there 



' On the evidence offered this conclusion was since shown to have been drawn 

 without cogent reason (Hess, Ewald) and the antithesis of 'intensity' and 

 'direction' not to be justified in this fprm (Ewald, Mast) though the underlying 

 idea was a fruitful one and a step forward. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 13, NO. 4 



