CRUSTACEA OF THE SEASHORE 105 



are practically terrestrial animals, only entering the 

 water occasionally, and, indeed, unable to survive 

 prolonged immersion. The work of excavating the 

 burrows has been watched in several species. The 

 Crab comes out of the burrow sideways, carrying a 

 load of sand between two of the walking legs on the 

 rear side. By a sudden movement the sand is jerked 

 away to some distance, where it accumulates in a 

 little heap, and the Crab dives into the burrow for 

 another load. Most of the Crabs belonging to this 

 genus possess a curious " stridulating organ " on one 

 of the large claws, by means of which they can 

 produce a buzzing or hissing sound. On the inner 

 surface of the "hand" there is a raised patch, 

 which, when examined with a lens, is seen to be 

 made up of a series of fine ridges, like the teeth of 

 a file. When the limb is bent in towards the body, 

 this patch can be rubbed up and down against a 

 sharp-edged ridge or scraper on the third segment of 

 the limb, and in this way the sound is produced. 

 What the use of the sound may be is not quite clear, 

 but there is probability in Dr. Alcock's suggestion 

 that it serves to warn intruders that the burrow is 

 already occupied. These Crabs run very swiftly, and 

 one species was seen by Professor S. I. Smith to 

 catch Sand-hoppers (Amphipods of the family Tali- 

 tridse) by springing on them suddenly, " very much 

 as a cat catches mice," but it also fed on dead fish 

 and the like. 



