106 THE ESSEX NATURALIST.
I think, of the entire group, both from the taxonomic and
ecological standpoints.
Woodlice, it may be recalled, are members of the class Crusta-
cea ; a largely aquatic group of Invertebrates possessing
segmented bodies and jointed limbs, and breathing by means
of gills. The particular order of Crustacea to which woodlice
belong—the Order Isopoda—is characterised by possessing
dorso-ventrally flattened bodies and equally proportioned limbs,
whilst the Woodlice as a subsection of the Isopoda are dis-
tinguished by comprising those members which have taken to
a semi- or wholly terrestrial existence, with consequent modifica-
tion of the abdominal limbs for the breathing of moist air—hence
their more scientific designation of "Terrestrial Isopods." Their
aquatic ancestry is however displayed in their close relationships
to the familiar sea-slater (Ligia oceanica), still found on the sea-
shore at low tide.
The species which were selected for the present observations
were as follow :—
Trichoniscus pusillus Br., one of the smallest and most active
species, being only about four millimetres long. The body is
smooth and polished, slender, and of a translucent, reddish-brown
hue. The head bears small side-lobes, and the flagellum (the
terminal part of the antennae) is four-jointed.
Porcellio (Euporcellio) dilatatus Br., about fifteen millimetres
long. The body is conspicuously tuberculated, of a light
brownish-grey colour, and more than half as broad as long. The
head bears a prominent frontal lobe as well as side lobes, and
the flagellum is two-jointed. Air-tubes, or pseudotrachaea, are
present on the first two pairs of abdominal appendages (pleopods),
and these appear as prominent whitish patches on the ventral
side of the abdomen.
Porcellio (Euporcellio) scaber Latr., a very common species,
closely resembling P. dilatatus but somewhat shorter and more
slender, and darker in colour. As in P. dilatatus, air-tubes are
present on the first two pairs of pleopods.
Oniscus asellus Linn., one of the largest species and also one
of the commonest. It reaches a length of sixteen millimetres.
The body is of a slate-grey colour, bearing a mottled appearance
owing to the presence of regularly arranged yellowish markings