JAN H. DHMHOWSKI. 



chamber. The principal instrument is now the small claw which 

 is repeatedly pressed with its external surface against the wall 

 flattening the sand and hardening it. During the work the crab 

 changes its position and it is often to be seen lying on its back 

 and working at the roof. When the burrow is finished the 

 animal will remain motionless, sitting in the end-chamber for a 

 long time. 



The female works in a closely similar way. But as both 

 chelae are equal there is no preference to either side of the body. 

 In pushing the sand the claw of the side external to the burrow 

 works as the other legs. From time to time the female will 

 change the working side. On the average the work is accom- 

 plished a little sooner than in the male which is possibly due to 

 the changing of the side digging which gives to it a chance for 

 resting. One observation points in fact that the legs may soon 

 become tired. When a crab runs along the sand for some paces 

 it will stop from time to time quickly changing the side directed 

 forward. 



If the crab meets any obstacle while digging, its behavior 

 depends very much on the stage of the work at which this 

 occurred. Small stones are simply carried out like a sand pellet. 

 Bigger stones which are too heavy for the animal cause an 

 abandoning of the hole if they lay near the surface. In such 

 cases the animal will start digging another burrow at some spot 

 near to the first, changing the spot many times until it finds a 

 suitable one. But if the obstacle lies deeper the efforts of the 

 crab may last for a long time and often it rather prefers to change 

 the direction of the burrow than to abandon a work which is 

 half done. This has been observed already by Schwartz and 

 Salir. The peculiar form of the burrow on the Fig. I, e, is due 

 to changing the direction of burrowing on account of some 

 obstacle. 



The described typical mode of working is connected with 

 many difficult problems. 



i. There is the characteristic shape of the burrow to be 

 explained. If we make a perpendicular hole in the sand about 

 5 cm. deep and drive the crab into it the animal soon starts 

 working. But it never goes in the direction ot the hole. The 



