306 CRUSTACEA. 



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 or- 

 gans, they are at birth, size apart, such as they are always to re- 

 main. 



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 Malacostbaca, and 

 the Entomosteaca. 



The first are usuajly furnished with very solid teguments, of a 

 calcareous nature, and with ten or fourteen feet, generally unguicu- 

 lated. The mouth, situated in the ordinary place, is composed of 

 a labrum, tongue, two mandibles (frequently furnished with palpi), 

 and two pairs of maxillas covered by the foot-jaws. In a great 

 number each eye is placed on an articulated and movable pedicle, 

 and the branchiee 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 Deca- 

 foda, Stomapoda, Lasmodipoda, Amphipoda, 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 Monocuxus, 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 

 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 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 external ones are naked or are not covered by the foot- 

 jaws, approximate to the preceding Crustacea. In the other Ento- 

 mostraca, which seem to approach the Arachnides in several 

 particulars, the organs of manducation are sometimes simply formed 

 by the coxae of the feet, projecting and arranged like lobes bristling 



