CRUSTACEANS raptorial claws and ambulatory legs, swimming-feet and tail-feet not only by function but by form are in many particulars so unlike one another that comparison often seems out of the question. But here again extended inquiry brings the most unexpected agreements to light. The normally whip-like character of the antennas, for example, is found displaying itself in each of the other sets of appendages, and in turn disappearing from the antenna? themselves, which forego their pliant lashes to become stiffly pediform or broad and shovel-like. The crab has six pairs of mouth-organs and five pairs of trunk-legs. The wood- louse has four pairs of mouth-organs and seven pairs of trunk-legs. The difference would be startling but for the simple explanation that the last two pairs of jaws in the crab are equivalents of the first two pairs of trunk-legs in the woodlouse. The latter animal is probably of older lineage than the crab, possibly as well savoured as the shrimp, and is certainly represented in Surrey by some of our rarer English species. Within the last few years several species have been added to the number of terrestrial Isopoda known in our islands. The total is still a modest one, for the moment reaching twenty-one in England and not quite so many in Ireland, though two continental species occur there which have not yet been found between the Irish and the English Channels. These woodlice of Great Britain and Ireland are distributed among four families. One of these, the Ligiidas, takes its name from the genus Ligia, which has been distinguished as maritime rather than marine, because it loves the coast though it does not go into the sea. In the same family stands an inland genus, Ligidium, Brandt. Of this the oldest species was called Oniscus bypnorum by the celebrated Baron Cuvier in 1792, its name meaning woodlouse of the mosses. It loves the shelter of the woodlands, where the mould is crumbling, where the leaves lie thickly, where there is abundance of moss and moisture. The very next year after it had been named in France by Cuvier it was independently observed in Germany by Persoon, who styled it O. agi/is, the nimble woodlouse, with appropriate allusion to the agility of its movements. Presently afterwards it was removed by Bosc from the too vaguely comprehensive genus Oniscus into one of narrower limits, being named by him and by several subsequent French writers Ligia bypnorum. Bosc declares that it is found * on the shores of the sea under mosses.' About this statement the suspicion may arise that Bosc was not speaking from personal observation, but that having assigned the species to Ligia he thought himself bound to make it live, as species of Ligia usually live, by the lip of the sea. It is certainly not confined to such a situa- tion, since it occurs in northern and central Europe everywhere, often gregariously. 1 Its distinctness from Ligia was in due course noted by Brandt, who in 1833 established for it the genus Ligidium and called the species L. persoonii, in which he was followed by numerous writers. Koch however in his Crustacea of Germany called it Zia agi/is, so far 1 Budde-Lund, Crustacea Isopoda Ttrreitria, p. 256 (1885). 189
