44 THE BRITISH WOODLICE.
Circulatory system—The blood being aerated in the
abdominal appendages, we find that the heart is situated towards
the hinder end of the body (fig. 18).
Three main arteries supply the
thorax and head, while the
blood is brought from the gills
to the heart.
Excretory system.—The ex-
cretory organs consist of a (a)
pair of so-called "shell glands,"
which are considered to be the
equivalents of the excretory
tubes or nephridia of annelid
worms. In the Woodlouse
these excretory organs open
on the second pair of maxillae.
They are composed of a
tube (sacculus) closed at one
end and more or less bent upon
itself (5, p. 261) which com-
municates with a labyrinth that
is provided with an excretory
orifice. Matters are eliminated
by the epithelial cells [the his-
tology has been described and
figured in Ligidium hypnorum (66)], which are very large in Ligia
oceanica.