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. 

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