282 PHYLUM ARTHROPODA. 
creep forwards on their “ walking legs.” Their life is toler- 
ably secure, but the frequent moultings during adolescence 
are expensive and hazardous. When hatched the young are 
like miniature adults ; for a time they cling beneath the tail 
of the mother. 
External appearance.—The head and thorax are covered 
by a continuous (cephalothoracic) shield; the abdomen 
shows obviously distinct segments movable upon one 
another. As indicated by the appendages, there are three 
groups of segments or metameres—five in the head, eight 
in the thorax, six in the abdomen, as well as an unpaired 
piece or telson on which the food canal ends. Each of the 
mineteen segments bears a pair of appendages. Among 
other external characters may be noticed the stalked 
movable eyes, the two pairs of feelers, the mouth with six 
pairs of appendages crowded round it, and the gills under 
the side flaps of the thorax. 
(1) The external shell or cuticle, composed of 
various strata of chitin, coloured with pig- 
ments, hardened with lime salts ; 
‘The Bopy WALL } (2) The ectoderm, epidermis, or hypodermis, 
consists of— which makes and remakes the cuticle ; 
(3) An internal connective tissue layer or dermis, 
with pigment, blood vessels, and nerves. 
Internal to this lie the muscles. 
Between the rings and at the joints the cuticle contains 
no lime, and is therefore pliable. It is a layer not in itself 
living or cellular, made by the underlying living skin. As it 
cannot expand, it has to be moulted periodically as long as 
the animal continues to grow. The old husk becomes 
thinner, a new one is formed beneath it, a split occurs 
across the back just behind the shield, the animal with- 
‘draws its cephalothorax and then its abdomen, and an 
empty but complete shell is left behind. The moulting is 
preceded by an accumulation of glycogen in the tissues, and 
this is probably utilised in the rapid growth which intervenes 
‘between the casting of the old and the hardening of the 
new shell. 
How thorough the ecdysis or cuticle-casting is, may be appreciated 
from the fact that the covering of the eyes, the hairs of the ears, the 
lining of the fore-gut and hind-gut, the gastric mill, and the tendinous 
