194 J A ^" B - DE.MBOWSKI. 



surface of the legs II., III., IV., the crab forces its own body 

 below the pellet, grasping it with the legs 2, 3, 4. The sand is 

 carried then to the mouth of the burrow and the legs 2, 3, 4, 

 helped effectively by the small claw adjust the pellet at the edge 

 of the hole. The second or the third pellet close the hole entirely, 

 while the crab still continues the same work, grasping the sand 

 at the bottom and adjusting it at the roof. As the work pro- 

 gresses the chamber containing the fiddler comes to lie still 

 deeper under the surface and the sand-cork becomes still thicker. 

 When the bottom of the jar is reached Uca spends a considerable 

 time in working at the end-chamber and finally it quiets down, 

 sometimes for many days. During the whole work the crab 

 has no chance of measuring the length of the burrow as it is 

 working itself through the sand. I think therefore that this 

 factor may be also excluded. 



If the importance of both mentioned factors is somewhat 

 doubtful, it is sure that the degree of moisture plays a role in 

 estimating the depth of the burrow. Even during the low tide 

 I always found water at about one foot below the surface of the 

 fiddler-ground. The burrow cannot go so far, for the walls of 

 the end-chamber would collapse under the water. Testing the 

 natural burrows with a long pipette I never found water there. 

 The question may be solved experimentally. Usually I per- 

 formed my experiments in the following way. I put sand into 

 the jar and poured enough water to cover it completely. By 

 stirring, droplets of air were removed. Then a long pipette was 

 forced through the sand to the very bottom of the jar and the 

 water was sucked out as far as possible. Under such conditions 

 the crabs dug always to the bottom. But if we put the pipette 

 only to half of the thickness of sand sucking water out, the 

 animal will stop working when the level of water is reached. 

 The degree of moisture influences then the depth of the burrow. 



Unfortunately it is also sure that this factor is not the sole one. 

 The natural burrows measured on an area of about one square 

 meter on a perfectly flat ground may reach a very different level 

 below the surface of the sand in spite of the exactly equal rising 

 of the degree of moisture for all of them. Thus I am unable to 

 answer exactly even such a simple question as why the burrow 



