1Q The Philippine Journal of Science 1915 



the sand is sharply stamped with the foot the disappearance 

 begins. Almost simultaneously each individual ceases feeding 

 and begins to dig with the legs of one side at the same time 

 rotating so that the digging follows a spiral. The result is that 

 a circular mound covering the crab is soon thrown up which 

 usually has a small hole in the center of it, but this hole is 

 quickly closed by wet sand pushed up from below (Plate III, 

 figs. 1 to 3). As all of this is done within two or three seconds, 

 it is evident that they work very rapidly. The individuals of 

 a group of feeding crabs are usually packed together so that 

 they touch one another, and since they dig down where they are 

 when they stop feeding the circular mounds interfere with one 

 another, producing a large patch of disturbed sand in which the 

 individual mounds are almost indistinguishable (Plate II, fig. 

 2). These patches of turned up sand are often very conspic- 

 uous when surrounded by the smooth surface of sand left by 

 the receding tide. If the observer waits for a few minutes with- 

 out moving, a few individuals of the sunken group make their 

 appearance by digging themselves out along the line of a spiral 

 and moving away to form a little group of their own or joining 

 large groups some distance away. More little groups from the 

 buried army appear at intervals and do the same or join with 

 one another. In Plate II, fig. 3, is seen a vertical section made 

 through the mound of a single individual and it shows the little 

 spherical cavity which the myctiris makes after it gets below 

 the surface. I was unable to determine whether or not this 

 cavity filled with air really remains in the natural habitat after 

 the tide has risen and covered the sand flat, but judging from 

 the results obtained in attempting to reproduce such a condition 

 in the laboratory it seems possible. Several specimens of Myc- 

 tiris were placed in a receptacle half -filled with wet sand taken 

 from the sand flats. After they had dug down and had been 

 allowed enough time to make cavities, the receptacle was filled 

 with water. Later in the day I dug down in the sand at a place 

 where a crab was supposed to be and in so doing released the 

 crab and a large bubble of air. The latter undoubtedly filled 

 the cavity occupied by the myctiris and was held in place by 

 the firmly packed sand surrounded on all sides by water- 



