232 SUPPLEMENT 



The simple eyes of the Crustacea, when they do exist, are 

 always sessile, i. e. fixed. The complex eyes are often 

 pedunculaied, 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 

 theiii 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 the substance 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, 



