258 "Journal of Comparative Neurology and Psychology. 



entering the end window. The resulting reaction was invariably 

 and definitely negative; and this with light of all the intensities 

 used in the previous cases. 



Conclusions from experiments on contact-irritahility versus reac- 

 tion to light — Although these experiments can hardly be called 

 critical, they demonstrate that the presence of the sanded area in 

 the box did modify the reactions of the fifth-stage lobster. That 

 there was manifested a tendency to remain in contact with the sand, 

 to burrow in it, and not to be dislodged by such intensities of light 

 as would normally rout the entire group of lobsters and send them 

 to the end of the box farthest from the light. These facts, more- 

 over, cannot be said to hold true for the fourth-stage lobsters that 

 were used in the foregoing experiments, and which showed no well 

 defined preference for the sanded area, at least in the early part 

 of the stage-period. 



VI. MECHANICS OF ORIENTATION. 



The aim of the present section is to report the results of a series 

 of observations w^hich were made in order to answer the following 

 question: By what movements of the lobster larvae are the reac- 

 tions to light accomplished ? In our effort to answer this question 

 we shall, for the present, attempt to avoid so far as possible con- 

 siderations which deal directly with the ultimate causes of orienta- 

 tation; in other words, we shall limit ourselves to the observation 

 of the actual movement of the body, or of certain parts of the body, 

 of individual larvae; and attempt to show what relation exists 

 between these movements and the external factors which appear 

 to determine them. First, however, it is necessary to establish 

 some points regarding the natural behavior of the larvae when the 

 influence of external stimuli is at the minimum. 



I. The normal behavior of the larvce — In view of the fact that 

 swimming constitutes the chief activity of the larval lobsters, our 

 question resolves itself into the following: What is the nature of 

 the normal swimming ^. When one first observes the behavior of 

 individual larvae amidst the thousands contained in the large hatch- 

 ing bags no difference is evident in the swimming of the first three 

 stages. In all instances the back of the larva is, for the most 

 part, uppermost, the abdomen bent under and downward at an 

 angle of about 60° from the longitudinal axis of the cephalo- 



