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CLASS CRUSTACEA. 
end. Some, having an anterior mouth, composed of a labrum, 
of two mandibles (rarely furnished with palpi), of a tongue, of 
from one to two pair of jaws at most, the external ones, naked, 
or not covered with jaw-feet, approach the preceding Crusta- 
cea. In the other entomostraca, and which appear in many 
respects to border on the Arachnida, sometimes the masticat- 
ing organs are simply formed by the haunches of the feet, 
advanced and disposed in the manner of lobes, bristling with 
small spines, around a large central pharynx ; sometimes they 
compose a little siphon or bill, serving as a sucker, as in 
many Arachnidcs and many insects, or do not show themselves 
at all, or scarcely show themselves externally, whether the 
siphon be internal, or that suction is performed after the man- 
ner of a cupping-glass. Thus the Entomostraca are either 
denticulated or edenticulated ; the first form our order of 
Branchiopods, and the second that of Pcecilopods, 
which in the first edition of this work were only a section of 
the preceding order. 
The singular fossils called Trilobites, and of which M. 
Brogniart, our fellow-member of the Royal Academy of 
Sciences, has given an excellent monograph, being considered 
by him, as w T ell as by other naturalists, as Crustacea which 
border on the entomostraca, we shall treat of them succinctly 
at the sequel of the latter. 
FIRST GENERAL DIVISION. 
THE MALACOSTRACA. 
The Malacostraca are divided naturally into those whose 
eyes are on a moveable pedicle, and those in which these 
organs are sessile and immoveable. 
