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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.

In Ocypode and Gelasimus the respiratory apparatus is modified for the purpose of breathing air. The gills are similar to those of purely aquatic Crabs, and no doubt serve for respiration when the animal is in the water; but the gill chambers are much more spacious than usual, and the lining membrane is richly supplied with bloodvessels. Air is admitted to the gill chambers by an opening, protected by a brush of hairs, between the second and third pairs of walking legs on each side. It is believed that in this way the gill chamber is fitted to be used as a lung when the animals are out of the water. Similar arrangements in some of the more exclusively terrestrial Crustacea will be mentioned in a later chapter.

There are many Shore Crabs, however, which lead a more or less amphibious existence without showing any marked modifications of structure as compared with their more purely aquatic relatives. On our own coasts, the Common Shore Crab (Carcinus mænas—[Plate IX].) commonly spends several hours each day exposed to the air, and in an aquarium it will voluntarily leave the water if the opportunity be afforded it. On tropical coasts the species of Grapsus and allied genera are often seen clambering with great agility about exposed rocks.

Analogous habits to those of the sand-burrowing, amphibious Crabs described above are shown on a small scale by the Amphipods of the family Talitridæ, known as "Sand-hoppers" or "Beach-fleas." Everyone who has walked over the firm sand near high-water mark on our own shores must have noticed the myriads of actively hopping little creatures disturbed at every step. The commonest species of Sand-hopper on the British coasts is Talitrus saltator ([Fig. 39]), but Orchestia gammarellus is also common. Both species occur together on sandy beaches or among decaying sea-weeds, and are among the most important scavengers of the seashore, picking clean the bones of fish or other animals cast up by the tide. In this country the Sand-hoppers do not, as a rule, venture far above high-water mark; but in warmer climates species of Talitridæ live in the damp forests at great distances from the sea, and deserve to be ranked among the terrestrial Crustacea.

Fig. 39—The Common Sand-hopper (Talitrus saltator), Male, from the Side. × 3. (After Sars.)

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