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). 

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