Some of the species of _Ocypode_ (Plate XV.) dig their burrows between tide-marks, where they are swamped by the advancing tide, and must be excavated afresh when the water retreats. Other species, however, live above high-water mark, and 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 Talitridæ) by springing on them suddenly, "very much as a cat catches mice," but it also fed on dead fish and the like. Of somewhat similar habits are the numerous species of the genus _Gelasimus_ ("Fiddler Crabs"--Plate XV.), which abound on sand and mud flats of tropical shores. These little Crabs are remarkable for the great dissimilarity between the sexes in the form of the chelipeds. In the female both chelipeds are small and feeble, but in the males one of them, either the right or the left, is enormously enlarged, sometimes exceeding in length and breadth the body of the Crab which carries it. What the precise use of this enormous claw may be does not seem to be quite certainly known. It is said to be used as a weapon by the males in fighting with one another, but it seems too clumsy to be very efficient for this purpose. It is often brilliantly coloured, and has been supposed to be a sexual adornment.