THE BRITISH WOODLICE. 43
from moist air, while the outer division (or exopodite) acts as a
protecting cover (fig. 14). In Porcellio, air-tubes (tracheae) may-
be present (see below).
In the male the first
two pairs of abdominal
appendages are specially
modified, the inner divi-
sions (endopodites) being
long and pointed (figs. 15
and 16). The last pair,
or tail appendages, in the
male are often consider-
ably larger than in the
female, and the form of these structures is sometimes of
value in classification.
Alimentary canal.—The
main portion of the alimentary
system is, practically speaking,
a straight tube (fig. 17). Its
first part (not shown in the
figure) is a narrow gullet, which
after passing through the nerve
collar dilates to form a sort of
stomach. Into this the secretion
of four digestive glands is poured by two ducts. These glands
have a somewhat striking appearance, being yellow tubes spirally
coiled, and they end blindly. From the stomach the intestine
runs to the hinder end of the body and passes under the heart.