The statement that the Robber Crab climbs lofty trees was first made by the Dutch naturalist Rumphius, in the beginning of the eighteenth century. Its accuracy has been often doubted or denied since then, and only finally put beyond dispute by a photograph taken on Christmas Island by Dr. Andrews, which shows one of these Crabs in the act of descending the trunk of a sago-palm. It seems not impossible that the habits of the animal may vary to some extent in different localities, and that where food is abundant on the ground the tree-climbing habit may be in abeyance. If this were so, it would explain the very definite statements made by some observers, that Birgus does not climb trees.

In localities where coconut palms abound, Birgus feeds largely on the nuts, tearing off the fibrous outer husk and breaking open the shell by hammering with its powerful claws at one of the "eye-holes." According to Darwin in his "Naturalist's Voyage," the pincers of the penultimate pair of legs are used for extracting the contents of the nut, but this observation does not seem to have been confirmed. In spite of its name of "Coconut Crab," however, Birgus by no means feeds exclusively on coconuts. On Christmas Island, where until recently there were no coconut palms, the Crabs are exceedingly abundant, and, according to Dr. Andrews, they "eat fruits, the pith of the sago-palm and the screw-pines, dead rats and other carrion, and any of their fellows that may have been injured.... They are excellent scavengers, and have a curious habit of often dragging their food long distances before attempting to eat it. I have seen a Crab laboriously pulling a bird's wing up the first inland cliff, half a mile or more from the camp whence it had stolen it."

Large specimens of the Robber Crab may be at least a foot in length of body when the abdomen is straightened out. Their great strength is illustrated by the fact, related by Darwin, that specimens placed in a strong biscuit-tin, of which the lid was secured by wire, escaped by turning down the edges with their claws, and in doing so actually punched holes quite through the tin.

The breeding habits and mode of development of the Robber Crab have often formed the subject of inquiry by naturalists, but it is only recently that Dr. Willey has been able to prove definitely that the female visits the sea for the purpose of hatching off the eggs, and that the young are hatched in the zoëa stage. The larvæ obtained by Dr. Willey have been described by Mr. Borradaile, who finds that, as was to be expected, they closely resemble those of Cœnobita. There appears, however, to be no such simultaneous migration of the Crabs towards the sea as has been described in the case of the Gecarcinidæ. The statement, quoted by Darwin, that Birgus visits the sea every night for the purpose of moistening its branchiæ, cannot be universally applicable, since the Crabs are often found, as on Christmas Island, at distances from the sea which put a nightly journey to it out of the question.

Of all Crustacea, the most completely adapted to terrestrial life are the Land Isopods, or Woodlice, which may be found in every garden. It is true that most species are found in damp places, although some that inhabit the sandy deserts of Asia and Africa must be content with a very slight degree of humidity; and in no case is their dependence on moisture greater than, for instance, that of many Insects and Arachnids which are regarded as typically terrestrial animals. Since there is reason to believe that the Woodlice have been derived from marine ancestors—they show no special affinities to the fresh-water Isopoda, like Asellus—it is interesting to find that the most primitive forms, which have departed least from the general Isopod type, are commonly found on or near the seashore. The "Sea-slater," Ligia oceanica ([Fig. 63,]), which is abundant in rocky places on our own coast, is one of the most primitive forms. It has a broad, flattened, greenish-brown body, about an inch long, and it runs quickly, creeping into narrow crevices of the rocks, so that it is not easy to catch. The antennules, as in the other land Isopods, are very minute, but the antennæ are long, and have, besides the five segments which form the "peduncle," a "flagellum" of about twelve short segments. The uropods or tail appendages are long, each with two slender, pointed branches. On the under-side of the abdomen can be seen the five pairs of pleopods, each with two plate-like branches attached to a very short peduncle. As in most aquatic Isopods, the plates of the pleopods are soft and thin, and appear adapted to act as gills, although the outer plate of each pair is somewhat stiffer than the inner. The Sea-slater is generally found just above high-water mark, probably always within reach of the salt spray, and it is said sometimes to enter the water of rock-pools.

Fig. 63—The Sea-slater (Ligia oceanica). About Twice Natural Size. (After Sars.)

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In almost every garden there may be found, under flower-pots and the like, a Woodlouse, about two-thirds of an inch long, of a brown colour, with yellowish blotches arranged in a row on each side of the back. This is Oniscus asellus, a species widely distributed in Europe and North America. It has the antennæ shorter than in Ligia, and the flagellum is composed of only three segments. The uropods are quite short. The endopodites of the pleopods are membranous gill-plates, which serve for respiration in the moist air in which these animals generally live. The exopodites are stiff plates which cover and protect the delicate endopodites; it is probable that they also aid in respiration, for they contain a system of minute channels, filled with air, where the cuticle is separated from the underlying cells. As these channels are nowhere open to the outside, the air must find its way in by diffusion through the cuticle.