CRUSTACEA. 9 



called the tail, and never posteriorly. 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 in- 

 fluence 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 branchisB, the mode in which 

 the head is articulated with the trunk(l), the mobility or fix- 

 edness of the eyes(2), the organs of manducation, and the tegu- 

 ments, constitute the basis of our divisions, and give rise to 

 the following orders(3). 



We divide this class into two sections, the Malacostraca, 

 and the Entomostraca(4). 



The first are usually furnished with very solid teguments, 

 of a calcareous nature, and with ten or fourteen feet(5), gene- 

 rally unguiculated. The mouth, situated in the ordinary 



(1) with respect to this term, and that of thorax, which are frequently em- 

 ployed in an arbitrary manner, see ourg-eneral observations on the class of Insects. 



(2) These organs are either pediculated and movable, or sessile and fixed. It 

 Is from this character that Lamarck has divided the Crustacea into two great sec- 

 tions, the Pediocles and the Sessiliocles; for which denominations, but restricting its 

 application to the Malacostraca, Doctor Lead) has substituted those of Podop- 

 thalma and Edriopthalma. Gronovius was the first who had recourse to this dis- 

 tinction. 



(3) Although we possess but few observations on the nervous system of the 

 Crustacea, all those which have been made support the truth of our divisions. 



(4) They might be still further divided into the Dentata and the Edentata, ac- 

 cording to the presence or absence of the mandibles. Jurine, Jun., has already 

 proposed these divisions in his excellent Memoire sur I'Argule foliace. 



(o) The four antei-ior, when there are fourteen, are formed by the last four pos- 

 terior foot-jaws. In tlie Decapoda, the six foot-jav. s belong to the mouth, and per- 

 form the office of maxill?E. 

 Vol. in.— B 



