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SUPPLEMENT 
The simple eyes of the Crustacea, when they do exist, are 
always sessile, i. e. fixed. The complex eyes are often 
'pedunculated, and mobile, and this character is exclusively 
peculiar to the present class. The peduncle of these eyes is 
usually formed of a single cylindrical piece, varies in its 
dimensions, and is lodged in a fosset, sometimes very deep. 
The branchipes have pedunculated eyes, but not placed in a 
particular fosset. 
The principal parts of the mouth, most frequently destined 
for the operations of grinding and lacerating, are in pairs, and 
placed laterally, as in the masticating insects. But some- 
times, united to other parts, which might be termed lips, they 
are modified so as to form a sort of bill, or sucker. 
In the ordinary Crustacea, or malacostraca, the parts of the 
mouth present variations pretty frequent as to dimension and 
form, so that the most exterior among them are often similar 
to feet, and perform the functions of those organs. Tn the 
entomostraca, those pieces which are less numerous, present 
such varied modifications that it is impossible to describe 
them in a general manner. A clear understanding, however, of 
this subject is of so much importance, that even at the hazard 
of repeating some of thesubstance of the text, we must venture 
on a few details respecting the composition of the mouth in 
the different orders of the Crustacea. 
In general, the pieces which compose it, are attached to the 
edges of an emargination, presented by the testa underneath, 
which has received the name of buccal aperture, and is 
sometimes regularly quadrilateral, sometimes in the figure of 
a trapezium or triangle. This aperture is not distinct, except 
in the species which are provided with a calcareous testa of 
greater or less solidity. 
The decapod and short-tailed Crustacea are provided, 1st, 
with a transverse upper lip, articulated with the anterior edge 
of the buccal aperture : 2d, with a pair of mandibles, or thick, 
