CLASS CRUSTACEA. 
137 
number of moultings. With the exception of a small num- 
ber, in which the changes of the skin exercise a trifling influ- 
ence on their primitive form, modify or augment their locomo- 
tive organs, these animals are, when born, size excepted, such 
as they will remain for the whole of their existence. 
DIVISION OF THE CRUSTACEA INTO ORDERS. 
The situation and form of the gills, the manner in which 
the head is articulated with the trunk, the mobility or fixed- 
ness of the eyes, the masticatory organs, and the teguments, 
will form the basis of our divisions, and give rise to the fol- 
lowing orders : — 
We shall divide this class into two sections, the Malacos- 
traca and the Entomostraca. The first have generally 
very solid teguments, of a calcareous nature, and ten or four- 
teen feet, usually unguiculated ; the mouth, situated in the 
usual way, is composed of a tongue, a labrum, two mandibles 
(often bearing a palpus), two pairs of jaws covered by the 
jaw-feet; in a great number, the eyes are carried on an arti- 
culate and mobile pedicle, and the gills are concealed under 
the lateral edges of the testa or carapace ; in the others, they 
are usually placed under the post-abdomen. This section is 
composed of five orders, the Decapods, the Stomapods, 
the LjEMODJPODS, the Amphipods, and the Isapods ; the 
first four embrace the genus Cancer of Linnmus, and the last 
that which he names Oniscus. 
The Entomostraca, or insects with shells, of Muller, com- 
pose the genus Monoculus of Linnaeus. Here the teguments 
are corneous and very thin, and a testa in the form of a buck- 
ler, of one or two pieces, or in the form of a bivalve shell, 
covers or encases the body of the great majority ; the eyes are 
almost always sessile, that is fixed, and often there is but one; 
the feet, the number of which varies, are in the majority ex- 
clusively adapted for swimming, and without any claw at the 
