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 _Coenobita_. 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. [Illustration: FIG. 63--THE SEA-SLATER (_Ligia oceanica_). ABOUT TWICE NATURAL SIZE. (After Sars.)]