Hadley, Behavior of the American Lobster. 241 



the form of reaction displayed by the lobster larva?. This is 

 readily shown, first, by the fact that, at any corresponding time on 

 two successive days (and especially when a moult has intervened), 

 the reactions of the same larvae may be quite dissimilar. Here the 

 reactions are explainable on the grounds of (i) the stage of the 

 larvae and their age in the stage-period; and (2) the intensity of the 

 light and of other stimuli which are brought to bear. This con- 

 clusion is shown furthermore by the fact that larv:i? at correspond- 

 ing times in the different stage-periods usually manifest similar 

 types of reaction. Thus does it appear that, although the diverse 

 forms of reaction are partly due to the differences of intensity in 

 actual illumination, the underlying cause is some physiological 

 change which the larvae undergo as they gradually approach and 

 pass the crises of their moulting-periods. 



^. Fourth Stage. 



We come now^ to a consideration of the reactions to light of the 

 fourth-stage lobsters. It has been observed in previous pages that 

 the most striking change, not only in body-form but also in life- 

 habits, which takes place in the life of the lobster, occurs during 

 the transition from the third to the fourth stage. It is the aim of 

 the present section to analyze the reactions of the fourth-stage 

 larvae and to exhibit the conditions which determine or modify 

 these reactions. 



In the previous pages the lobsters under consideration have been 

 referred to as the "second-day" or the "fifth-day" larvae, etc., as 

 the case might be, and this terminology was of advantage, because 

 the first three stage-periods are so brief that changes may occur 

 even in two consecutive days. The fourth stage-period, however, 

 is much longer (usually eight to twelve days) and the differences 

 in reaction on two consecutive days may be slight or inappreciable. 

 For this reason, then, in the following consideration we shall 

 divide the fourth stage-period into three parts, viz: the early, the 

 mid, and the late fourth stage-period. 



Experiment 2^. Reactions of early fourth-stage lobsters. Case 

 I. Photopathic reactions — July 28, 10 a.m. Ten early fourth- 

 stage larvae were put in the glass-bottomed box B and this was 

 placed over the light-shaft. The arrangement of the colored 

 plates and the resulting orientations were as follows. The results 



