Page:VCH Derbyshire 1.djvu/156

 A HISTORY OF DERBYSHIRE thirteen pairs of appendages all appear to be attached to or covered by the great shield called the carapace. The theory of this shield is that it is composed of two or three, or perhaps more, of the original seg- ments grown together and extended backwards and forwards over several other segments, which have in consequence become to a great extent dorsally indistinguishable. Although on the ventral surface there are marks from which it is not difficult to infer a primitive separation, it will be found that the leg-bearing segments of the lobster are all firmly coherent. But one of the differences which separates the river crayfish from its marine counterpart consists in this, that it has the segment carrying the hindermost pair of legs freely movable. 1 A more easily appreciable distinction is afforded by the second antennas, these having near the base a scale or plate which in the crayfish is large but very small in the lobster. In the sessile-eyed Malacostraca there is further evidence of that original independence that is claimed for each appendage-bearing seg- ment. Here the carapace with rare exceptions is limited to a connection with the two pairs of antenna? and four pairs of mouth-organs, while the following seven pairs of appendages are more or less conspicuously leg-like, and each pair is attached as a rule to a movable trunk-segment. Thus a flexibility of body is secured of which crabs are altogether devoid, and which in lobster and crayfish is transferred to the abdomen. The first sessile-eyed group that concerns us has received the general name of Isopoda, meaning equal-footed or like-footed. This was con- ferred upon it because the species earliest taken into account in compara- tively modern classification were seen to have all the fourteen legs of the trunk or middle body nearly alike. In the order as now more fully known such a character is far from being constant. There are many exceptions, and in some of them differences of size and shape in the series of legs are carried to an extreme. Nevertheless in a good number of marine forms and undoubtedly in our terrestrial isopods, which go by the humble vernacular name of woodlice, there is sufficient similarity and equality in the feet to justify the title which was given to the order by the French naturalist Latreille about a hundred years ago. In regard to the woodlice or Oniscidea I may affirm from personal observation that five species are found in Derbyshire. It is probable that there are at the least double that number. But at all events five were seen in a little coppice on a hill near Matlock Bridge. These represent four out of the eleven genera at present known to occur in Great Britain. They are divided between two families, the Oniscida? and the Armadillidiidas, which are separated by some well marked dis- tinctions and by others not unimportant but less obvious. Those who have never studied woodlice scientifically must yet be conscious of the habit which some of them share with the hedgehog. Every one indeed is familiar from childhood with the little creatures that on the slightest hint of danger or touch of intrusion roll themselves up into the like- 1 See Huxley, The Crayfish (1881), pp. 152, 237. 104