A HISTORY OF BUCKINGHAMSHIRE 



indebted to the researches of Wilfred Mark Webb, Esq., F.L.S., F.Z.S., 

 and to those of Miss Johnson and her sister. Some of the species, it is 

 true, are so uniformly distributed over the whole neighbourhood that 

 their existence in this county could not have been reasonably disputed. 

 But the examples which my very obliging friends have sent from definite 

 localities dispense with any necessity of depending on speculative infer- 

 ence, and, as will be presently seen, the results are not confined to those 

 customary forms the occurrence of which could have been predicted. 

 Here, as in all other counties, the Amphipoda are represented by Gam- 

 marus pulex (Linn.). It has the advantage of being everywhere obtain- 

 able from ponds and brooks. But it is of small size, so that for the proper 

 understanding of its separate details some optical enlargement is almost 

 essential. Otherwise it is a convenient object of study for the beginner, 

 not only by reason of its great abundance, but also because in many re- 

 spects it seems to offer a standard of comparison, a simple pattern from 

 which the very numerous genera and species of Amphipoda diverge and 

 radiate. Its insignificant size might readily put an observer off the 

 thought of comparing it with a crayfish, but the organization is essen- 

 tially the same. Here however the shield or carapace, instead of being 

 produced backward as far as the pleon, stops short over the first maxilli- 

 peds. Hence there is left uncovered the middle body, consisting of 

 seven segments, which as a rule are all articulated. To each of these is 

 attached a pair of appendages. The first two pairs, which are generally 

 used for grasping, have been called gnathopods, and these are homologous 

 with the second and third maxillipeds of the crayfish. Maxillipeds and 

 gnathopods alike mean jaw-feet, the words being intended to teach that 

 the appendages in question are foot-like in form or in origin, but con- 

 cerned with the food in point of function. All but the first of the seven 

 pairs of legs may carry branchial vesicles, comparable with the more 

 complicated gills which are hidden beneath the cheeks of the carapace 

 in the higher crustaceans. 



Along with G. pulex at Eton and at Iver Mr. Webb has taken 

 the isopod Asellus aquaticus (Linn.). The two species are almost con- 

 stant companions and perhaps equally abundant, but at some times and 

 some places one or the other may be found to predominate in numbers. 

 The Isopoda can scarcely be considered so united a group as the Amphi- 

 poda, and if any genus could be selected as a central representative it 

 would scarcely be Asellus. Nevertheless, as the only aquatic isopod of 

 our inland counties the species of it above mentioned deserves attention. 

 It is as curious as it is common. Like other genuine isopods it agrees 

 with the amphipods in having sessile eyes and seven uncovered articu- 

 lated segments of the middle body, and differs from them in having its 

 breathing organs in the pleon instead of attached to gnathopods and 

 walking legs. But an extreme ventral flattening gives it a highly pecu- 

 liar appearance. By this shape it is enabled to adhere very closely to the 

 leaves and stems of the water weeds about which it climbs. Those who 

 are at the pains to compare its appendages pair by pair with those of 



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