Habits, Reactions, and Associations in Ocypoda arenaria. 



31 



that Carcinus does not see in the way we do is undoubtedly true also of 

 Ocypoda, but I believe that the latter has, through the use of the eyes, a 

 sense of position and distance; else how could it run so quickly and often 

 so accurately after objects thrown on the sand several feet away? 



Not only is Ocypoda quite accurate in returning to its burrow after 

 moving away from it in a straight line, but it also returns accurately after 

 running around in different directions on the beach. However, individuals 

 differ much in this respect. The following observation showed very nicely 

 how an ocypoda could return directly to its burrow even after feeding 

 along the beach and showed also that it could return when the dark opening 

 of the burrow was hidden. This crab had made its home so that the open- 

 ing was in a hollow formed by the heel of a shoe and on this account the 

 entrance was concealed (fig. 9). Starting from A, the specimen made its 

 way leisurely to B (12 feet), feeding on the way; from B, it slowly went 

 to C (10 feet) continuing its feeding; then the approach of a man caused 

 the crab to run for its 

 burrow (about 13 feet 

 away), and it did so 

 directly and accurate- 

 ly even though, as 

 stated above, the en- 

 trance was hidden. 

 Similar exhibitions of 

 accuracy, when, how- 

 ever, the opening of 

 the burrow was not 

 hidden from view, 

 were frequently ob- 

 served. The young 

 individuals are much 

 less accurate in this 

 respect, and while an 

 observer is walking 

 along the beach many 

 of the small ones will be seen running about aimlessly as though lost, while 

 the adults nearly always reach their burrows easily. 



While Ocypoda undoubtedly depends to a considerable extent on its 

 eyes during movement toward a definite object or place, as is indicated by 

 the inactivity of individuals with painted eyes, there are other factors which 

 are almost equally important. The care which many of the older indi- 

 viduals take in preparing the surface of the sand immediately in front of the 

 entrance to their burrows leads one to believe that they are sensitive to the 

 contour of surfaces. Such a sense of the contour of surfaces has already 



WATtR LINt 



Fig. 9. — IlUistrating accmacy of return to burrow when 

 entrance was hidden from view. 



