LIGHT REACTIONS AND PHOTOTAXIS 601 



reactions of Balanus-larvae. Before describing these experi- 

 ments, it should be mentioned that my results were always 

 controlled by comparison with animals, caught at the same pole 

 of the same vessel as the animals experimented on, but kept in 

 pure seawater of normal temperature. This is important, as 

 the animals change their reaction when exposed to light with- 

 out any other influence being brought to bear on them, and as 

 it is therefore possible only to determine the influence of the 

 agent to be tested by comparison with normal animals under 

 equal conditions of illumination (and temperature). I will 

 mention first the effect of changes in temperature. 



I found without exception, that increase of temperature made 

 positive animals negative and negative animals more negative, 

 and that decrease of temperature made negative animals posi- 

 tive and positive animals more positive. I never noted any 

 uncertainty in this effect of temperature. The amount of the 

 change in temperature necessary for the reversing of the reaction 

 to hght depends on the age and state of the larvae. The rela- 

 tion between the inclination to positive or negative reaction, 



the quotient - indicating the degree of neutralisation of two 



antagonistic processes causing positive or negative reaction, con- 

 stitutes what is called the 'Lichtstimmung' of the organism. 

 The factor of 'Lichtstimmung' must be considered in all obser- 

 vations on the effect of stimulating agents. If, in our case, the 

 nauplii are approaching the negative condition owing to the 

 effect of illumination, a shghter increase in light intensity will 

 effect the change of reaction than if they had just been exposed 

 to light. Young animals are more easily influenced than old 

 ones. A change in temperature of about 5 or 6°C., however, 

 always has the desired effect. At about + 5°C. the nauphi fall 

 into cold rigor; they will stand being treated to more than 30°C. 

 As an increase of about 1°C. will under certain circumstances — 

 especially if the larvae are newly hatched — suffice to make them 

 negative, it is most important to maintain a constant tempera- 

 ture during all experiments on phototaxis. 



