CRUSTACEA. 291 



great many instances, become true feet; and ten feet properly so called, 

 all terminated by a single small nail. The mouth, as in Insects, 

 presents a labrum and a ligula, but no lower lip, properly so called, or 

 comparable to that of the latter; the third pair of foot-jaws, or the 

 first, closes the mouth externally, and replaces that part. 



Their envelope is usually solid, and more or less calcareous. They 

 change their skin several times, and generally preserve their primitive 

 form and natural activity. They are mostly carnivorous and aquatic, 

 and live several years. They do not attain their adult state until after 

 casting their skin a certain number of times. With the exception of a 

 few in which these changes somewhat influence their primitive form 

 and modify or augment their locomotive organs, they are at birth, size 

 apart, such as they are always to remain. 



Division of the Crustacea into Orders, 



The situation and form of the branchiae, the mode in which the head 

 is articulated with the trunk, the mobility or fixedness of the eyes, the 

 organs of manducation, and the teguments, constitute the basis of our 

 divisions, and give rise to the following orders. 



We divide this class into two sections, the MALACOSTRACA, and the 

 ENTOMOSTRACA. 



The first are usually furnished with very solid teguments, of a calca- 

 reous nature, and with ten to fourteen feet, generally unguiculated. 

 The mouth, situated in the ordinary place, is composed of a labrum,' 

 tongue, two mandibles (frequently furnished with palpi), and two pairs 

 of maxillae covered by the foot-jaws. In a great number each eye is 

 placed on an articulated and moveable 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, Lamodipoda, Amphi- 

 poda, 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 Muller, 

 is formed of the genus MONOCULUS, Lin. Here the teguments are 

 horny and very thin, while a shell, resembling a buckler, composed of 

 from one to two pieces, covers or incloses the body of the greater num- 

 ber. The eyes are almost always sessile, and frequently there is but 

 one. The feet, the number of which varies, are mostly fitted for nata- 

 tion, and without a terminal nail. Some of them, having an anterior 

 mouth, composed of a labrum, two mandibles rarely furnished with 

 palpi, a tongue, and one, or at most two pairs of jaws, of which the 



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