Habits, Reactions, and Associations in Ocypoda arenaria. 27 



Usually at low temperatures, not above 35° C, a dark coloration occurs, 

 provided the eye is stimulated by light. 



At high temperatures, above 35° C, a light coloration is the rule, and it 

 occurs independently of the intensity of light. 



The temperature limits given above are those obtained from a study 

 during the hot summer months, but it may be that in winter, when the 

 average temperature is much lower, the limits would be diflferent. 



These results may be stated in another way. Dark coloration occurs at 

 comparatively low temperature in diffuse or direct sunlight. 



Light coloration occurs at comparatively high temperatures when light 

 is absent. 



Light coloration occurs at comparatively low temperatures and at medium 

 temperatures when light is absent. 



Light coloration occurs at comparatively high temperatures in diffuse or 

 direct sunlight. 



The blanching of individuals on the sands of Loggerhead Key is prob- 

 ably due to the high temperature alone. (See plate i.) The results of 

 Experiment XI indicate that it is not a reversed light reaction. On the other 

 hand, the blanching in the photographic dark-room may be due to the absence 

 of light alone. 



The dark coloration of individuals occurs only when the eye is stimulated 

 by light, and then only when the temperature is comparatively low. The 

 indications are that it is not the result of the direct action of light on the 

 pigment-cells or on the nervous system. 



EQUILIBRATING ORGANS-" AUDITORY ORGANS." 



For many years an auditory function was ascribed to certain organs 

 found in the basal joint of the antennules in decapods, but since the work of 

 Bethe (1895, 1897), Beer (1898, 1899) and Prentiss (1901) we have strong 

 evidence for the assumption that the so-called " auditory organs " do not 

 have the auditory function, but that they are organs which are important 

 in the maintenance of equilibrium. So far as I know we have not as yet 

 any good evidence that the Crustacea hear, and in those cases which have 

 been recorded the supposed reaction to sound was probably due to tactile 

 stimuli. 



Although Prentiss (1901) stated in his paper on " The Otocyst of Deca- 

 pod Crustacea " that Gelasimus pugilator, a brachyuran decapod, living much 

 of the time on land, did not react to sound-waves, I was much interested to 

 see if such an active and highly developed land-crab as Ocypoda was able 

 to hear. 



The " auditory organs," or otocysts, as I shall call them, are situated in 

 the basal joint of the antennules, as in nearly all the decapods (plate 4), and 

 they are partly protected by the rather broad but short rostrum. 



