146 CRUSTACEA. 



founded upon the difference in structure and form of the bran- 

 chiae, the manner in which the head is connected with the trunk, 

 and the organs of mastication. The first three were included 

 by Linnaeus in his genus Cancer ; the fourth formed the genus 

 Oniscus of that author. 



Order I. DECAPODA. With palpi at the mandibles, move- 

 able eyes, and the head not distinct from the trunk; branchiae 

 pyramidal, in leaflets or plumes, placed at the exterior base 

 of the feet-jaws, and the feet properly so called, and conceal- 

 ed under the lateral margins of the shell. 

 Order II. STOMAPODA. With palpi at the mandibles and 

 moveable eyes, but the head distinct from the trunk and di- 

 vided into two portions, of which the anterior bears the an- 

 tennae and eyes ; branchiae plumose, suspended under the tail. 

 Order III. AMPHTPODA. Palpi at the mandibles and eyes 

 immoveable ; head distinct from the trunk and in one piece ; 

 branchiae vesicular, and placed at the interior base of the feet, 

 with the exception of the anterior pair. 



Order IV. ISOPODA. Mandibles without palpi, and the mouth 

 always composed of many jaws, of which the two under ones re- 

 semble a lip with two palpi ; branchiae generally under the 

 abdomen ; feet simple, and only proper for locomotion or 

 prehension ; head for the most part distinct ; no shell ; and 

 the eyes granulated. 



Order V. BRAKCHIOPODA. Mouth in the form of a beak, some- 

 times composed of many jaws ; feet ki the form of fins, and 

 the branchiae attached between them ; body generally cover- 

 ed with a shell, not distinct from the head. This order in- 

 cludes the Monoculi of Linnaeus. 



In a very elaborate work by Desmarest, entitled Considera- 

 tions Generales sur la classe des Crustaces, published at Paris 

 in 1825, the method of division followed is chiefly that propos- 

 ed by Dr Leach in the Edinburgh Encyclopaedia and Tran- 

 sactions of the Linnaean Society. And in a work published by 

 Latreille in the same year, entitled Families Naturelles du 

 Regne Animal, the arrangement is nearly the same as that 

 given in the third volume of Cuvier's work, but with some of 

 the groups arranged as families in that work raised into orders. 

 In this last work, the arrangement, so far as regards the Crusta- 

 cea, stands thus : 



