These Crabs resemble the Cyclometopa in general appearance, but the carapace is very square in outline, and its margins are never so well provided with spines as in the Cyclometopa. The position of the male genital openings is peculiar, since they lie upon the sternum, and are connected with the copulatory appendages upon the abdomen by means of furrows excavated in the sternum. The Catometopa are either littoral or shallow water forms, or else they live entirely on land. The Grapsidae are marine Crabs, _Pachygrapsus marmoratus_ (Fig. 134) at Naples being exceedingly common on rocks at high-water mark, over which it scuttles at a great rate; in the Mediterranean it takes the place of our common _Garcinus maenas_, which is not found there. [Illustration: FIG. 134.—Dorsal view of _Pachygrapsus marmoratus_, × ⅓. (From an original drawing prepared for Professor Weldon.) ] Among the land genera are _Ocypoda_, _Gelasimus_, and _Gecarcinus_ of tropical lagoons and coastal swamps. _Ocypoda_ often occurs in vast crowds in these regions, and digs burrows in the sand. [Illustration: FIG. 135.—_Gelasimus annulipes_, × 1. =A=, Female; =B=, male. (After Alcock.) ] _Gelasimus_ (Fig. 135) is remarkable for the enormous size of one of the chelipedes, generally the right, in the male, which may actually exceed in size the rest of the body. It is not known what purpose this organ serves in the various species. In _Gelasimus_ it is supposed that the male stops up the mouth of the burrow with it when he and the female are safely inside. It is also used as a weapon in sexual combats with other males; but Alcock, from observations made in the Indian Ocean, believes that the males use it for exciting the admiration of the females in courtship, as the huge chela is bright red in colour, and the males brandish it about before the females as if displaying its florid beauty. The species of _Ocypoda_ are exclusively terrestrial, and cannot live for a day in water. The gills have entirely disappeared, and the branchial chambers are converted into air-breathing lungs with highly vascular walls, the entrances into which are situated as round holes between the bases of the third and fourth pairs of walking legs. As their name implies, they can run with astonishing rapidity, and they seem to be always on the alert, directing their eyes, which are placed on exceedingly long stalks, in all directions. Some of the Grapsidae, _e.g._ _Aratus pisonii_, are partially adapted for life on land. Fritz Müller, in his _Facts for Darwin_, alludes to this creature as “a charming lively crab which ascends mangrove bushes and gnaws their leaves.” The carapace can be elevated and depressed posteriorly, apparently by means of a membranous sac, which can be inflated by the body-fluids. This Crab retains its gills and can breathe under water in the ordinary way.