NOTES ON THE HEHAVIOR OF THE FIDDLER CRAH. 



factors. They bear undoubtedly only on the time of digging. 

 In some experiments I put a glass ring at the periphery of the 

 jar covering it with a layer of sand about 3 cm. high. The crab 

 digging close by the wall soon reached the obstacle and for 

 several minutes it tried to remove it. It abandoned then the 

 burrow and began to dig another one in the neighborhood which 

 it abandoned in turn. The animal made 5 successive holes 

 until it succeeded in bending the canal towards the middle of the 

 jar and in avoiding the obstacle. The total amount of expended 

 energy was certainly much greater than in the usual case and 

 yet the animal dug its hole to the very bottom of the jar. The 

 whole process lasted about 6 hours, during which Uca was 

 constantly working, while an ordinary burrow is made easily in 

 half an hour. In another series of experiments I mixed the sand 

 with at least the same quantity of small stones (about 6 mm. in 

 diameter). The resistance of the ground was very much in- 

 creased, in fact about 3 times as measured roughly by determining 

 the weight which was necessary for driving a nail into the sand 

 and into the mixture. Nevertheless the crab succeeded in 

 making a regular hole reaching to the bottom of the jar, which 

 lasted however for about 20 hours. The amount of energy is 

 different depending on the degree of inclination of the canal 

 towards the horizon, as carrying the sand out from a steep 

 burrow requires a greater strain. And again this does not bear 

 on the length of the burrows. 



Another possibility is that the crab may estimate the total 

 length of the burrow while carrying the sand pellets out. But 

 one observation speaks against the validity of this factor. The 

 mode of burrowing described above is a type which does not 

 mean however that it is to be observed the most frequently. 

 In very many cases the crab works in a slightly different manner. 

 When the burrow is just deep enough to hide the animal, Uca 

 comes out of it, turns its big claw towards the burrow and 

 enters again. The legs II., III. and IV. grasp the sand from 

 the bottom, but the pellet is not carried out of the hole. The 

 legs push it towards the opening, while the body remains at the 

 same spot. The pellet comes to lie between the sternum of the 

 crab and the wall of the canal. Still pushing it with the external 



