Most, if not all, of the Crabs belonging to the tribe Oxystomata are sand-burrowers, and the structure of the mouth parts characteristic of the tribe appears to have been acquired as an adaptation to this habit. As already mentioned, the mouth-frame in these Crabs is triangular instead of square, being produced forwards between the eyes, and the third maxillipeds, which cover it, are also elongated. In this way the exhalent channels carrying the water from the gill chambers open on the front margin of the head, and are exposed even when the Crab is buried. In the different families of this tribe the inhalent openings by which the water enters the gill cavities are protected in various ways, and so arranged that respiration can go on without danger of the gills becoming clogged by sediment. The members of the tribe Hippidea (sometimes called "Mole Crabs"), among the Anomura, have habits somewhat similar to those of the Crabs just described. They are common on sandy beaches in the warmer parts of the globe, and they burrow with great rapidity by means of the curved, flattened end-segments of the legs. The carapace is generally smooth and oval, and the body is compact, the short abdomen being folded up as in the Crabs. In _Albunea_ (Plate XIV.), which belongs to this tribe, a long "antennal tube," which looks very like that of _Corystes_, is believed to have a similar function in connection with respiration when the animal is buried. In this case, however, the tube is formed, not by the antennæ, as in _Corystes_, but by the antennules, so that it affords a striking example of the independent evolution of similar structures from quite different origins. _Hippa emerita_, which is found on the coasts of North and South America, has the mouth parts imperfectly formed, and not adapted for biting; and it is stated by Professor S. I. Smith that the animal feeds in the way that an earthworm does, swallowing the sand through which it burrows, and extracting the nutriment which it may contain. This habit, however, is not followed by other members of the tribe, for Mr. Borradaile found that a species of _Remipes_ in the Maldive Islands could "easily be caught by a bait of Crab at the end of a line, pouncing on it with its sharp maxillipeds, and allowing itself to be flicked out of the sand if the rod be sharply lifted." In the cases mentioned above, the Crustacea do not bury themselves much below the surface of the sand, and do not form definite burrows; but there are many Crustacea which live in open tunnels dug deep into the sand. Some of these belong to the category of amphibious forms, to be mentioned presently; but there are others which live in deeper water, and of which the habits are less open to observation. [Illustration: FIG. 38--_Callianassa stebbingi_ (FEMALE), A SAND-BURROWING THALASSINID FROM THE SOUTH COAST OF ENGLAND. NATURAL SIZE]