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. 172
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