ARTICULATA. 103 
twenty-one, of which seven belong to the head, trunk, and tail, respectively, 
and when a smaller number appears, it is generally to be attributed to the 
fusion of several into one. In fig. 22, pl. 78, the head is observed to be 
distinct, whilst in jg. 1 it forms a single piece with the trunk, the tail being 
articulated and turned beneath, although partly visible in figs. 3 and 8. In 
Limulus ( fig. 34), the segments of the tail are fused into a single shelly 
plate, as well as those of the head and trunk or cephalo-thorax, the shield or 
plate of which is called the carapace. 
The moulting of the larger crustacea takes place about once a year, but 
Mr. Lyell mentions a species of Cancer covered with oysters six years old, 
so that the shell could not have been renewed during this period. Young 
and growing animals moult frequently, as in Daphnia, which may change 
every two days. Previous to moulting, a crustacean becomes sickly and 
refuses food, the carapace becomes loosened, and the corium beneath 
secretes a new, soft, and membranous shell, which finally becomes calcareous. 
When the old shell has become loosened, the animal, after various exertions, 
extricates itself from it in about half an hour, withdrawing the antenne, 
feet, and even the gills, from an external film. The shell of the limbs splits 
to allow the large extremity to pass, and it sometimes happens that a limb 
is left in the old shell. Some of this class have the power of throwing off a 
limb. 
The Crustacea feed mostly upon animal food, which is taken sometimes 
solid and sometimes in a liquid state, and the oral organs are modified 
accordingly. Some small species are very destructive to the timber of ships 
and docks. ‘ 
In forms like the lobster, swimming is effected by striking downwards 
and inwards with the tail, which forces the body in a backward direction. 
In the branchiopoda the feet are used in swimming, not being adapted to 
walking; and in the entomostraca (figs. 27, 29), the body is impelled 
through the water by the feet, in a succession of jerks. 
The walking feet of the Crustacea generally amount to ten or fourteen. 
In some (fig. 1, &c.), the anterior pair are robust, the extremity provided 
with a movable thumb forming pincers. Sometimes the end of a foot 
ean bend, so as to form a hook; and in other cases the last joint closes upon 
an extension or enlargement of the preceding joint, forming an organ of 
prehension. 
There are two mandibles, four lower jaws, succeeded by six auxiliary or 
foot-jaws, followed by the feet; and when there are fourteen feet, the four 
anterior ones are transformed trom the four posterior foot-jaws. 
Most of the Crustacea see well, and the eyes are either sessile or placed 
upon pedicles. Some of the parasitic species are without eyes in their 
perfect and affixed state, although provided with them when young, and 
capable of swimming about. As in the insects, both compound eyes and 
stemmata are found in this class, but the former are the most common. In 
a few cases both are found together. 
There are sufficient reasons to believe that the sense of smelling is present 
in the Crustacea, although its special organ has not been discovered. 
307 
