A HISTORY OF WARWICKSHIRE 



wanting in this or that family or genus will make their appearance in 

 another that is nearly related. The same applies to the coalescence of 

 segments. In the tail of a crab, for instance, that of the male will often 

 show only five segments, while that of the female has the normal seven, 

 the explanation being that in the male three are obviously consolidated 

 into one. Frequently lines, grooves, sutures, partial divisions, testify to 

 the intrinsic distinctness of these united portions. In the Entomostraca 

 on the other hand there are always more or fewer than this number of 

 nineteen segments and nineteen pairs of appendages. 



In the Handbook of Birmingham Mr. Thomas Bolton, F.R.M.S., 

 speaking of the Malacostraca says : ' In this class [Crustacea] should be 

 mentioned the freshwater crayfish, Astacus jtuviatilis, not of course a 

 microscopic organism ; but if it were omitted here it could not appear 

 in any of the other reports. This species is fairly distributed in most of 

 the smaller brooks, in the canals and larger reservoirs, but it is not so 

 abundant or so large as it is on the lime formations round Oxford. Two 

 other large microscopic species of this class, the freshwater shrimp, 

 Gammarus pulex, and the water woodlouse, Asellus vu/garis, are always 

 present, the former busy in its office of scavenger in the sandy bottoms 

 of the brooks and ditches, and the latter climbing about, like a monkey, 

 amongst the water weeds, investigating the mass of living and decaying 

 organisms with which the weeds are clothed.' : 



Of the Macrura or long-tailed Malacostraca the only species likely 

 to be found living in Warwickshire was the above-mentioned river 

 crayfish, and this was not likely to be absent. The technical designation 

 of it should rather be Potamobius pallipes (Lereboullet), the name Astacus 

 in strictness belonging to the somewhat similar but really distinct genus 

 of the marine lobster. There is no evidence that we have in England 

 more than one species, or even more than one variety of the river 

 crayfish. A difference in size, however constant as between the speci- 

 mens from two localities, could not be considered of any significance in 

 this respect, since the smaller form might become larger if transferred 

 to a district where there was a better food supply and where the con- 

 stituents of its crustaceous coat were more abundant, while the larger 

 breed might degenerate under the influence of an opposite removal. 

 The two other malacostracan species which Mr. Bolton records are 

 almost certainly present in every one of our English counties. Gammarus 

 pulex (Linn.) has very near relations in the sea and on the seashore, but 

 is itself a widely distributed exclusively freshwater representative of the 

 Amphipoda. The species of this great order are at once distinguished 

 from crabs and crayfishes by being sessile-eyed. They have their eyes 

 firmly seated in the head. They cannot shift them from side to side or 

 up and down as we can ours, nor yet can they lift and lower them or 

 move them to and fro on jointed pedicels after the fashion which gives 

 to many of the stalk-eyed crustaceans a wonderful look of alertness and 



1 Handbook of Birmingham, p. 306. I am indebted to Professor W. W. Watts for calling my 

 attention to this source of information. 



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