154 
CRUSTACEA. 
and two pairs of maxillae covered by the foot-jaws. In a great num- 
ber each eye is placed on an articulated and movable pedicle, and the 
branchiae are concealed under the lateral margins of the upper or 
lower shell ; in the others they are usually placed under the post- 
abdomen. This section consists of five orders : the Decapoda, 
Stomapoda, L.emodipoda, Amphipoua, and the Isopoda. The 
four first embrace the genus Cancer of Linnaeus, and the last 
his Oniscus. 
The second, the Entomostraca, or “Insects with shells” of Miiller, 
is formed of the genus Monocueus, Lin. Here the teguments are 
horny and very thin, while a shell, resembling a buckler, composed 
of from one to two i)ieccs, covers or incloses the body of the greater 
number. The eyes are almost always sessile, and frequently there is 
but one. The feet, the number of which varies, are mostly fitted for 
natation, and without a terminal tail. Some of them, having an 
anterior mouth composed of a labrum, two mandibles — rarely fur- 
nished with ])alpi, a tongue, and one, or at most two pairs of jaAvs, of 
which the external ones are naked or are not covered by the foot-jaws, 
approximate to the preceding Crustacea. In the other Entomostraca, 
Avhich seem to appproach the Arachnides in several particulars, the 
organs of manducation arc sometimes simply formed by the coxse of 
the feet, i)rojecting and arranged like lobes bristling Avith small spines 
round a large central pharynx. At others, they cither compose a 
little siphon or beak, used for suction, as in several Arachnides and 
Insects, or they are Avholly (or nearly so) invisible externally, either 
because the siphon is internal, or because the suction is produced in 
the manner of a cup. 
The Entomostraca arc thus dentated or edentated. The first Avill 
form our order of the Branchiopoda *, and the second that of the 
PiEciLOPODA, which, in the first edition of this Avork, Avere a mere 
section of the preceding order. 
The singular fossils called Trilobites, of Avhich M. Brongniart 
has given an excellent Monograph, being considered by him, as aa’cII 
as by many otlier naturalists, as Crustacea allied to the Entomos- 
traca, Ave Avill briefly speak of them after Ave have done Avith the 
latter. 
* In my Avork entitled Fomilles Nat. dii Regne Animat, the Entomostraca are 
divided into four orders : the Lophyrofoda, Phyllopoda, Xiphosura, and the 
SiPIlONOSTOMA. 
