l8O JAN H. DEMBOWSKI. 



particulars. Nevertheless they seem to indicate that the 

 existing literature on the fiddler crabs has not exhausted all 

 possibilities. 



THE BURROWING INSTINCT. 



All my observations were done on Uca pugilator, the sand- 

 liddler. In the environs of Woods Hole this species lives on 

 sand banks communicating with the ocean by means of a com- 

 plicated system of ponds and channels. On account of this 

 circumstance the rising tide is very quiet and the animals are 

 never exposed to waves. In fact so slow is the movement of the 

 water that at the low tide one may see clearly sand-pellets 

 removed by the animal from its burrow during the previous low 

 tide. This particular is rather important to note, as many 

 habits of Uca are correlated even with slow tidal changes. 



XVatching the animals in the field furnishes only data which 

 are already known and there is no need to relate them once more. 

 But one particular concerning the shape of the natural burrows 

 deserves some attention. I studied it, pouring a solution of 

 white plaster into the burrow and digging the mould out after 

 it became hard. According to Pearse (12, 13) the burrows of 

 Uca pugilator rarely exceed 75 cm., which I found to be correct. 

 The burrows are nearly always oblique to the surface of the 

 ground and they show a general tendency to become somewhat 

 horizontal at the inner end. But there are very many modifi- 

 cations and it would be rather difficult to say which is the type. 

 Sometimes the beginning of the burrow is vertical and then it 

 bends sharply, as in Fig. i, a; sometimes the inner end of an 

 oblique burrow suddenly becomes vertical as in Fig. i, c. They 

 show, however, some common features, as they are always some 

 what bent, never perfectly straight. Every burrow ends with a 

 marked swelling the end-chamber where the animal often 

 remains. In some cases the end-chamber is so large that its 

 inhabitant may turn in it in every direction, in others it allows 

 only turning around the transversal axis of the body. Lfailed 

 to notice any difference connected with the sex of the crab. 

 Also I have never seen a branching passage, as described by 

 Cowles for Ocypoda arenaria (5). I found such a passage in 

 some burrows of Uca pugnax but the burrow of Uca pngilator is 



