CRUSTACEANS 



Gammarus pulex will possibly be surprised to find in animals superficially so 

 unlike a long series of structural resemblances. These however cease to 

 be conspicuous in the pleon. The seven large articulated segments 

 which this part of the animal's body displays alike in the crayfish and 

 the amphipod are in Asellus all consolidated into a comparatively small 

 shield. Beneath this are five pairs of appendages in the male, but only 

 four in the female. The sixth pair, the uropods, project from the caudal 

 shield on either side of its central apex, and since their attachment be- 

 longs to the sixth segment of the pleon the apex may be regarded as the 

 telson or proper terminal segment of the body. 



The terrestrial Isopoda of the county, commonly called wood-lice, 

 are distributed over three families of the Oniscidea. In these the pleon 

 is divided into six segments, of which the first two are always narrower 

 than those of the middle body and usually than those of the pleon itself 

 which immediately follow. On the other hand the sixth segment, 

 though presumably carrying with it the telson in coalescence, is the 

 smallest of all, and like the first two takes no part in forming the outer 

 rim of the body at the sides, though its special point or border com- 

 pletes the circuit at one end. 



In the family Trichoniscidas the county has been fortunate. A 

 gathering of five small specimens obtained by Mr. Webb at Eton Wick 

 has proved to contain representatives of three genera, two of the species 

 being new to the English fauna. The third species may be mentioned 

 first, this being the well-known and widely-distributed Trichoniscus 

 pusillus, Brandt, described by Bate and Westwood under the name 

 Philougria riparia (Koch). 1 Like members of its family in general, it 

 has the pleon attenuated and the inner branch of the uropods as well as 

 the outer conspicuously displayed. The antenna? are geniculate and 

 spiny, with the slender terminal flagellum four-jointed. The back, 

 which is claret-brown with some slight markings, is smooth and shiny, 

 but at the same time under magnification it is seen to be plentifully 

 sprinkled with minute hairs. The pteon has its apex truncate with a 

 slight emargination. It is a character of the genus to have the eyes 

 small but distinct, consisting of only three visual elements embedded in 

 a dark pigment. 2 Mr. Webb reports it from Eton Wick, Langley, 

 Hedsor, and the neighbourhood of Burnham Beeches. Only from the 

 first of these localities comes Trichoniscoides albidus (Budde-Lund), of 

 which the genus and species are alike new to England. The genus was 

 established by Sars in 1898, principally on the character of the eyes, 

 which he found to be ' simple or wholly wanting.' In the present 

 species he describes them as ' distinct, but extremely small, circular, 

 consisting each of a single corneal body, with underlying reddish 

 pigment.' ' This crustacean is, like Trichoniscus pusillus, barely a sixth 

 of an inch long. Preserved in spirit it is quite pallid. Alive, it is 



1 British Sessile-eyed. Crustacea, ii. 456. 



2 Sars, Crustacea of Norway, 'Isopoda,' ii. 160. 

 s Loc. cit. pp. 164-5. 



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