780 CRUSTACEA. 
number of five or seven pairs; a head, sometimes not distinct from the 
trunk, with two or four antenne, and two moveable, compound, and ofter 
pedunculated eyes. The organs of generation are at the base of the feet, 
or at the extremity of the body. The Crustacea are, in general, to be re- 
cognised from their solid envelope, which is sometimes extremely hard, as 
when the calcareous matter of the covering predominates over the mem- 
branous portion; but, according to the families and genera, the calcareous 
portion diminishes in quantity, and the corneous material becomes predo- 
minant, ull at last the covering seems simply membranous. 
The antennz in this class are jointed, setaceous, and generally four in 
number. In some, the head is intimately united to the thorax, or is indis- 
tinguishable as a separate organ. The shield in this case forms a large 
covering over the thorax, which is called the shell. In others, where the 
head is distinct, the body is divided into seven segments, to which the feet 
are attached below.. The body is often terminated posteriorly by a tail com- 
posed of many segments. The feet, in general, are from ten to fourteen, 
and with six articulations, the two natural feet, and sometimes the two or 
four following ones, being terminated in a kind of forceps; at other times, in 
simple hooks; and in some, the termination of the feet seems adapted only 
for swimming. 
The Crustacea have two eyes, in some species elevated on moveable 
peduncles, in others sessile. These eyes are, in general, compound or reti- 
culated ; but in some, both eyes are united into one. The mouth is provided, 
in general, with two mandibles, a labiwm below, and from three to five 
pairs of jaws. To the first pair, or the first three pairs of these, the name 
of the feet-jaws has been applied, as being formed by the two or six anterior 
feet of the animal, modified by their position near the mouth, and not proper 
for locomotion. The branchie are exterior in the Crustacea, although often 
concealed, and placed at the sides, feet, or under ‘the tail. They are, how- 
ever, generally at the base of one part of the feet, and are composed of 
pyramidal lamine, or tufted filaments. 
The nervous system in this class is very similar to that of the arachnides 
and insects. It consists principally of a ganglion or brain, placed before and 
above the intestinal canal, and of an elongated double chord, with ganglions 
or knots, placed on the lower surface of the body, sometimes extending its 
whole length, at others forming towards the middle, a medullary circle, with 
radiated elongations. The Crustacea enjoy the faculty of sight; many of 
them, that of hearing; and they possess the senses of smell and taste; but 
their sense of touch is obscure, from their calcareous or horny covering. 
The class Crustacea is divided by Latreille into five orders, founded upon 
the difference in structure and form of the branchie, the manner in which 
the head is connected with the trunk and the organs of mastication. The 
first three were included by Linnzus in his genus Cancer; the fourth form- 
ed the genus Oniscus, of that author. 
