Habits, Reactions, and Associations in Ocypoda arenaria. 17 



The so-called compensatory movements of the eye-stalks, such as have 

 been described for other Crustacea, occur in Ocypoda when it is tilted either 

 from right to left or anteriorly and posteriorly. The angle in the latter case, 

 however, is quite small. It was noticed that when an individual was tilted for- 

 ward or backward until the limit of movement was reached the eye-stalks 

 were immediately lowered into their sockets. Another interesting observation 

 was the following: When an ocypoda was picked up, the normal position 

 with reference to the ground being maintained, it almost invariably dropped 

 the eyes into the grooves, but when the crab was lowered again the eyes 

 were raised as soon as the legs were allowed to touch the ground. In these 

 experiments I kept hold of the crab even after it had been placed on the 

 floor. Occasionally the e3'-es were raised in mid-air, not while moving up 

 or down, however. The eyes are usually drawn down into the sockets when 

 they are touched by the experimenter, and in the case of some individuals 

 it was found that the same behavior was brought about when the crab was 

 scratched with a pointed instrument along the middle of the carapace. There 

 is considerable variation among difl:"erent individuals with respect to the reac- 

 tion from scratching, but in the same individual the result is quite constant. 



Ocypoda lives along wind-swept shores and the wind seems to have no 

 ill effect on the eyes. Even in a strong gale the eyes are seen standing erect, 

 apparently unaffected. An attempt was made to cause individuals to lower 

 the eye-stalks by blowing suddenly and sharply upon them, but it was not 

 successful. In the case of the land-crab (Gecarcinus) the eyes were dropped 

 immediately when the same experiment was tried upon them. It was 

 found, however, that when an ocypoda was brought close up to the mouth 

 of the experimenter and warm air was gently breathed on the eyes they 

 were immediately drawn into the sockets. They remained erect when the 

 crab was brought close up to the mouth without breathing on them. 



There is no doubt that Ocypoda can distinguish a large object, such as a 

 person, many yards off (at least 50 yards), as any one can testify who has 

 seen these crabs rim away on the approach of man. By this statement it is 

 not meant that this crab has vision, that it sees things as human beings do, 

 although I would not deny the possibility of imperfect pictures being 

 formed on their eyes, but that its eyes are sensitive to changes in intensity 

 of light and that it is able to see the outlines of objects where the contrast 

 in the amount of light reflected by the object and that reflected by the sur- 

 roundings is great enough. 



While Ocypoda can see objects which are not moving, as the experi- 

 ments D and E prove and as frequent observations of individuals ap- 

 proaching objects indicate, its behavior shows that the eyes are much more 

 sensitive to objects in motion. 



The following interesting performance, which was witnessed from the 

 window of a laboratory built partly on the beach and partly out in the 

 water, shows how well Ocypoda reacts to moving objects. One of the most 



