Habits, Reactions, and Associations in Ocypoda arenaria. 7 



believe that it is used for escape by the ocypodas when they are hard pressed. 

 It seems most remarkable that these crabs should often dig the escape passage 

 out almost to the surface without breaking through, thus leaving the place 

 of exit concealed. 



While the above description of the burrows applies to the large majority, 

 there is considerable individual variation to be seen both in the construction 

 of the burrow and the manner of doing the work. Some have arched roofs 

 with an almost level floor, and others are almost round in cross-section. 

 While, as has been described, the burrow usually points directly away from 

 the water, there are some that extend obliquely, and in one or two cases bur- 

 rows have been found which lead toward the water. Some individuals, 

 especially the older ones, finish these burrows carefully, while others do not ; 

 some work quickly and others slowly ; some start many burrows, but finish 

 none; and many drive individuals out of the burrows they have made and 

 take possession. 



As would be expected, Ocypoda usually digs its burrow in a place where 

 the sand is firm and damp below the surface. It scratches out a shallow 

 hole with its walking appendages, making it deeper and deeper, until it 

 is unable to simply scratch the sand away, when it begins to carry it out in 

 the following manner (plate 2, fig. a) : Using the first and second walking 

 appendages and chelae of one side, it rakes up quite a good-sized load, hold- 

 ing it firmly between the appendages and the body proper. It then crawls 

 out of the hole and deposits the sand, sometimes throwing it by a quick 

 movement of the chelae and walking appendages for a distance of almost a 

 foot, sometimes carrying it away 2 feet or more and depositing it quietly, 

 and occasionally dropping it immediately in front of the mouth of the burrow. 



As is well known, Ocypoda has one chela decidedly larger than the other, 

 and in nearly every case in which an individual was observed making its 

 burrow it went down into the tunnel sideways with the side having the 

 small chela foremost, using that side in the digging and carrying of the 

 sand (fig. 2). Out of 17 cases in which ocypodas were observed going down 

 into their holes, after the burrow was constructed, the side with the small 

 claw led the way 16 times. The small-chela side is undoubtedly better 

 adapted for digging, which may account for this phenomenon. Further- 

 more, under these conditions the crab is better protected from any enemy 

 which may come down after it, since its large fighting chela is presented 

 to the intruder. 



On some occasions I have noticed individuals lying on their backs in 

 the tunnels digging awa>' at the roof, and this is probably the manner in 

 w^hich the vertical passage is started. 



Many ocypodas exhibit considerable care in the construction of their 

 burrows, especially the entrance to them. Sometimes, when the approach to 

 the opening is quite steep, the crab will pile the sand that it brings from the 



