254 'Journal of Comparative Neurology and Psychology. 



tage only when another stimulus of known effect is present and 

 operative. For instance, if the two conditions of stimulation which 

 respectively bring about a photopathic and a phototactic reaction 

 are so arranged as to oppose one another (i. e., by determining 

 opposite reactions in the larvae), and if the constant effect of one 

 set of conditions is known, then it is possible to form an estimate 

 of the persistency of the reaction determined by the opposed set of 

 conditions. For example, if light rays of low intensity coming 

 through the end of box B, resulted in driving the enclosed larvae, 

 which had just previously given a negative photopathic reaction, to 

 the opposite end of the box, and at the same time forced them from 

 a region of low into a region of high intensity, we should say that 

 the negative photopathic reaction of these larvae was of slight 

 importance as compared with the phototatic. If, on the other 

 hand, it was learned by experiment that the rays entering the end 

 window of box B would not force the negatively photopathic larvae 

 from the dark into the brightly illuminated end of the box, but 

 resulted in their gathering in the middle of the box (for instance, 

 in the green or orange area) then it might be inferred that the neg- 

 ative photopathic reaction had a greater influence in determining 

 the final reaction of the larvae, although it was in this case directly 

 and strongly opposed by the tendency to manifest a phototactic 

 reaction. In the following experiments, made to discover the 

 value of contact-irritability in determining the reaction of the 

 larvae, the principle mentioned above was made use of, and in this 

 instance a combination was made between experimental conditions 

 w^hich would allow the demonstration of contact-irritability, and 

 those which would insure the manifestation of negative phototaxis 

 if no other modifying conditions (such as contact-irritability) were 

 present. But before going farther with the description of the 

 technique of the experiments, a few observations on the behav- 

 ior of the lobster larvae under natural circumstances may be con- 

 sidered. This may form a better basis for the consideration of 

 experiments dealing with contact-irritability versus reaction to 

 light under the especially devised conditions to be described. 



It might reasonably be imagined that the loss of the swimming 

 branches (exopodites) of the thoracic appendages, which takes 

 place with the entrance to the fourth stage, would at once deter- 

 mine a very radical change in the habits of lobster larvae. We 

 should surmise that the larvae would immediately abandon their 



