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.
A great many other Catometopa are land-crabs; but we may specially mention the genus Gecarcinus, related to the marine Grapsidae, which has representatives in the West Indies and West Africa. The Crabs of this genus may live in sheltered situations several miles from the sea, but in spring the whole adult population rushes down in immense troops to the shore, where breeding and spawning take place; and when this is completed they migrate back again to the land. The young pass through the normal larval stages in the sea and then migrate inland.[[157]]
Fam. 1. Carcinoplacidae.—The carapace is rounded and broader than long, usually with toothed front margin. The orbits and eyes are normal, and not much enlarged. Geryon, in the deep littoral of the northern hemisphere. Euryplax, Panoplax, etc., in the American coastal waters. Typhlocarcinus, etc., in the Indo-Pacific.
Fam. 2. Gonoplacidae.—The carapace is square, with the antero-lateral corners produced into spines. The orbits are transversely widened, and the eye-stalks long. Gonoplax, widely distributed in the littoral zone. G. rhomboides in British and European seas.
Fam. 3. Pinnotheridae.—Carapace round, with indistinct frontal margin. Orbits and eyes very small, often rudimentary. The members of this family live symbiotically or parasitically in the shells of living Bivalve Molluscs, corals, and wormtubes in all seas except the Arctic. Pinnotheres pisum is fairly commonly met with off the English coasts in the mantle-cavity of Cardium norwegicum.
Fam. 4. Grapsidae.[[158]]—Carapace square, the lateral margins either strictly parallel or slightly arched. The orbits and eyes are moderately large, but the eye-stalks are not much lengthened. Littoral, fresh-water, and land. Pachygrapsus marmoratus (Fig. [134]), the common shore-crab of the Mediterranean. Sesarma, with fresh-water and land representatives in the tropics of both hemispheres. Cyclograpsus, marine in the tropical littoral.
Fam. 5. Gecarcinidae.—Carapace square, but much swollen in the branchial region. Orbits and eyes moderately large. Typically land forms, which only occasionally visit the sea or fresh water. Cardisoma is a completely circumtropical genus, with species in tropical America, West and East Africa, and throughout the Indo-Pacific. Gecarcinus in West Indies and West Africa.
Fam. 6. Ocypodidae.—Carapace square or rounded, generally without teeth on the lateral margins. The orbits transversely lengthened, eye-stalks usually very long. The members of this family generally inhabit the mud-flats and sands of tropical coasts; in the southern hemisphere they extend far into the temperate regions. Macrophthalmus, with numerous species, in Indo-Pacific. Gelasimus (Fig. [135]), in the tropics of both hemispheres. Ocypoda, with similar distribution.