594 WOLFGANG F, EWALD 



on the side facing the lamp. If the lamp is attached at a level 

 with the animals, outside the glass containing them, they mostly 

 remain near the bottom and but a small column of nauplii is 

 seen along the side up to the surface. Not before the lamp is 

 lifted considerably above the surface level does the aggregation 

 of naupUi near the surface increase. If one now inserts a smoked 

 glass between lamp and glass vessel, all the animals instanta- 

 neously rise towards the surface, but some of them will sink 

 again after some time. By inserting a second smoked glass the 

 same effect may be attained a second time, causing practically 

 all animals to collect near the surface. If one now takes away 

 one of the smoked glass panes, all nauplii will begin rising again 

 for just a moment, but soon commence sinking. One observes 

 a column of animals moving down fairly rapidly and finally 

 stopping; gradually they begin reascending. Obviously the nau- 

 plii of Balanus show a typical reaction to changes of intensity of 

 illumination, just as I was able to demonstrate it for Cladocera 

 and Copepods. Increase of illumination (within certain limits) 

 causes first slight acceleration, then inhibition of locomotion, 

 making the animal sink. Decrease of illumination causes accel- 

 eration of locomotion, making the animal move toward the source 

 of light. The same absolute hght intensity will cause the 'nega- 

 tive reflex' when following a weaker illumination and the 'positive 

 reflex' when following a stronger one. This shows the nauplii of 

 Balanus to adapt themselves to different light intensities, pro- 

 vided these intensities remain unchanged for a sufficient time. 

 If one brings the source of Hght vertically above the glass vessel, 

 one does not generally succeed in observing the sinking move- 

 ment after increase of illumination. The animals collect near the 

 surface and their locomotion, directed vertically upwards, is suffi- 

 cient to keep them there even after a decrease in the energy of 

 locomotion. With the hght coming from the side the locomotion 

 is directed horizontally, and even small changes in the rate of 

 locomotion will result in a conspicuous change of position. 



I now tried to find out which wave lengths of the spectrum had 

 the strongest influence on the reactions to changes of intensity 

 of illumination described above. Positively phototactic animals 



