THE BRITISH WOODLICE. 45
(b) Masses of cellules in the head, very greatly developed in
Ligia oceanica (but numbering scarcely more than ten in Oniscus
asellus), which have no external
opening. They also function as
excretory organs (5, p. 263), and
have been called "cephalic nephro-
cytes."
(c) Other "branchial nephro-
cytes" are situated on the dorsal
surface between the last thoracic and
the first abdominal segments, as well
as between those that follow, with
the exception of the last two ; they
are in distinct patches, one on each
of the middle line in Ligia, but more
or less continuous in
Oniscus (5, p. 265).
(d) The digestive
glands have also been
shown to be excretory
(5, p. 270).
Nervous system.—
The nervous system con-
sists of paired ganglia in
the head, above the
alimentary canal which
send off nerves (commis-
sures) that meet below, to
form a double nerve cord
with ganglia at intervals
(see fig. 19).
Reproductive organs.
—In the female there are
a pair of ovaries in the positions shewn in fig. 20 ; and ducts run
to the underside of the fifth thoracic segment.