200 'Journal of Comparative Neurology and Psychology. 

 I. INTRODUCTION AND HISTORICAL SUMMARY. 



Every year is bringing new and valuable additions to our knowl- 

 edge of the behavior of the Crustacea. Most of the investi- 

 gations dealing with this subject are concerned, however, with 

 the Entomostraca, while the behavior of the higher forms has been 

 less studied. It is apparent, moreover, that the experimental 

 work done has been chiefly upon adults, while little attention 

 has been given to the behavior of the larval forms of those Crusta- 

 cea, as the macrurous decapods, which undergo an extensive 

 metamorphosis. It is the aim of the present paper to demon- 

 strate certain phases in. the reactions of larval and early adolescent 

 stages of the American lobster (Homarus americanus) to light, 

 and to analyze these reactions, so far as possible, into their con- 

 stituent factors. 



In the study of reactions to light it is apparent that the lack 

 of a satisfactory terminology has led to considerable confusion. 

 This is manifest when we attempt to apply the definition of positive 

 or negative phototaxis, as given by Loeb, to the types of behavior 

 which we find, for instance, in the lobster and in the shrimp, 

 Palemonetes (Lyon 1907). Loeb (1905, p. 29) states that "posi- 

 tively heliotropic animals are compelled to turn their oral pole 

 toward the source of light and move in the direction of the rays 

 to its source." In the larval lobsters, however, there may be a 

 difference between the signs of body-orientation and what may be 

 called progressive orientation. In body-orientation the animal 

 in question turns with reference to the source of light; in progres- 

 sive orientation it moves tow^ard or from the source of light. Em- 

 ploying these terms, we may say that the body-orientation of the 

 larval lobster under stimulation by light is invariably negative, 

 whereas the progressive orientation may be either positive or 

 negative, as the conditions of the case determine. 



Secondly, what do we mean by intensity and by direction of 

 hght .? Are we justified in assuming that a stimulus such as light 

 can be effjective in causing either kind of orientation through its 

 directive quality } The answer to these questions depends largely 

 upon arbitrary definitions. Yerkes' ( 1903) exposition of what 

 constitutes a phototactic reaction as differentiated from a photo- 

 pathic reaction indicates very nearly the meaning that will be 

 given to these terms in the present paper. Attention may be 



