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. CHAPTER VII REMARKS ON THE DISTRIBUTION OF MARINE AND FRESH-WATER CRUSTACEA