[CHAPTER IX]

CRUSTACEA OF THE LAND

There is every reason to believe that the Arthropoda, like the other great groups of the animal kingdom, had their origin in the sea; but they must have invaded the dry land at a very early period, and most of the classes into which the group is divided—the Arachnids, Myriopods, and Insects—are now predominantly terrestrial in their habits. The Crustacea alone have remained for the most part aquatic animals, and only in a comparatively few cases have they succeeded in adapting themselves completely to an air-breathing existence. As already mentioned, a considerable number, both of marine and of fresh-water species, are more or less amphibious in their habits. Thus, the common Shore Crab of our own coasts and the Grapsoid Shore Crabs of warmer seas voluntarily leave the water and scramble about among the rocks between, and even above, tide-marks. Some Crabs, like Ocypode and Gelasimus (see [Plate XV].), have gone farther towards becoming land-dwellers, since their gill chambers are adapted to serve as lungs for breathing air, and some species may even be drowned by keeping them in water. The marsh-dwelling or fresh-water Crabs of the genus Sesarma (see [Plate XXIII].) and allied genera are also apparently to some extent air-breathers, and one species, Aratus pisonii, is stated by Fritz Müller to climb mangrove bushes and to feed on their leaves. Some Crayfishes, like the Engæus of Tasmania (see [Plate XX].), already mentioned, are practically land animals. Finally, some Amphipoda, closely allied to the Sand-hoppers of British coasts, live in damp places on land, although they do not show any conspicuous modifications of structure to adapt them to this mode of life. Of one of these Amphipoda, Talitrus sylvaticus, Mr. G. Smith writes: "This species of land-hopper is widely distributed in the highlands of Tasmania, being found under logs and leaves in the forests on Mount Wellington, and in very great abundance in the beech-forests on the mountains of the west coast."

It will thus be seen that it is impossible to draw any sharp distinction between aquatic and terrestrial Crustacea, and it is chiefly from motives of convenience that we have left to be dealt with in this chapter three groups of land-dwelling Crustacea—the Land Crabs of the family Gecarcinidæ, the Land Hermits (Cœnobitidæ), and the Land Isopods, or Woodlice (Oniscoidea).

PLATE XXVI

A WEST INDIAN LAND-CRAB, Gecarcinus ruricola. (REDUCED)

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