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THE BRITISH WOODLICE.
By WILFRED MARK WEBB, F.L.S., and CHARLES SILLEM.
(With Plates I,—XXV., and numerous other Illustrations).
Introduction.—Having finished a somewhat exhaustive
list of the land and fresh-water molluscs of Essex,1 one of the
present writers felt that if he were to make any further con-
tributions of importance to a knowledge of the fauna of that
interesting county, he must turn his attention to some other
group of animals. It seemed most fitting that some creatures
should be chosen which are commonly met with during the search
for molluscs. Centipedes, millepedes, and woodlice fulfilled
these conditions, and all were collected, but as only seventeen
species of woodlice had at the time been found in England, it
was deemed advisable to study these in detail to begin with.
The present contribution is the result of the undertaking, and we
have thought that a general consideration of the British Woodlice,
with careful drawings from nature of all the species now known
from this country, ought to lead to a more general study of these
interesting creatures and their habits.
Position in the scheme of classification.—The Woodlice
belong to an immense group of invertebrate animals known as
the Arthropoda, the bodies of which are segmented and provided
with jointed appendages for purposes of walking, swimming, and
feeding. Of this group, two large divisions are recognized. The
first contains the forms which breathe by means of air tubes,
such as the Insects; and the second has been constituted for
Crustacea, which breathe by means of gills. The latter are, of
course, adapted more especially for a life in water, but here and
there we come across examples so modified that they can exist
in air. The land-crabs are a case in point, and so are the Wood-
lice. These belong to an order which contains many fresh-water
and marine species, known as the Isopoda.
Geological history.—The known history of the order is a
long one, for remains occur in the Old Red Sandstone (Devonian)
of Herefordshire, and in the Coal Measures. (792). A form
1 "The Non-Marine Molluscs of Essex," by Wilfred Mark Webb; Essex Naturalist,
Vol. x. (1897), pp. 27-48 and 65-81.
2 The numbers in brackets refer to papers mentioned in the Bibliography at the end.