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. [Illustration: FIG. 39--THE COMMON SAND-HOPPER (_Talitrus saltator_), MALE, FROM THE SIDE. × 3. (After Sars.)]