146 THE ESSEX NATURALIST.
those species producing the biggest families having the smallest
larvae, as already mentioned. At first these newly-hatched
larvae are incapable of movement within the pouch, their tiny
limbs being tightly doubled up against their bodies, but after about
three days they begin to wriggle their legs and antennae, and
these movements can be followed beneath the semi-transparent
brood plates of the mother.
Meanwhile the mother animal seems quite unconcerned with
her large family which she carries about in this manner, although
as the larvae increase in size they become crowded more and
more upon each other, forcing the brood plates further and
further apart, until they almost touch the ground with their
load. Finally, the plates are forced open and the larvae, now
able to take care of themselves, are liberated into the outside
world to commence a completely free existence. They are not
always liberated all at once, however ; sometimes one or two
days intervene between the liberation of the first and last larvae.
During the first day of their free lives these tiny woodlice
undergo their first moult ; the soft, delicate integument which
clothed them throughout their larval lives being replaced by
one of firmer, stouter texture, whilst the old skin is usually
broken up and all or part of it eaten. It is common to find large
numbers of these glistening, transparent, first-cast exuvia
amongst a family of newly liberated young.
After this first moult they are of a pretty pearly white colour,
with a pair of conspicuous brown eye-spots and brown chitinous
jaws, which are now employed for feeding. Consequently the
gut becomes increasingly opaque with the solid food material
swallowed, and shows up as a prominent dark band running
from mouth to anus. Their bodies are by this time completely
segmented, and rudiments of cephalic lobes have appeared on
the head, but the seventh thoracic segment,—the last segment
to appear,—is still only in a rudimentary condition situated
beneath the sixth. The absence of the seventh thoracic segment,
as well as the seventh pair of legs, at this early free-living stage
is a feature common to all Isopods.
In the species under consideration the seventh thoracic
segment does not appear in the normal adult position—i.e.
behind the sixth—until the second moult occurs, whilst the