CRUSTACEA. 



153 



two pairs of foot-jaws exercise the same functions, the number of 

 feet is increased to fourteen. The mouth, as in insects, presents a 

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

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

 closes the mouth externally, and replaces that part. 



The sexual organs, at least those of the males, are always double, 

 and situated on the breast or at the inferior origin of that posterior 

 and abdominal portion of the body commonly 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 some- 

 what 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 f, the organs of manducation, and the teguments, constitute the 

 basis of our divisions, and give rise to the following orders t- 



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

 Entomostraca §. 



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

 careous 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). 



« With respect to this term, and that of thorax, which are frequently employed in 

 an arhitrary manner, see our general observations on the class of Insects. _ 



t These organ'^ are either pediculated and movable, or sessile and fixed. It is 

 from this character that Lamarck has divided the Crustacea into two great sections, 

 the Pediocles and the Sessiliocks ; for which denominations, but restricting its 

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

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



^°* AUhough we possess but few observations on the nervous system of the 

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



S^Thev might be still further divided into the Berdata and the Edentufa, accord- 

 ing to the presence or absence of the mandibles. Jurine, jun., lias already proposed 

 these divisions in his excellent Mi^moiresurl'Argulefohac.'. 



11 The four anterior, when there are fourteen, are formedby the Inst four posterior 

 foot-jaws. In the Decapoda, the six foot-jaws belong to the me-uth, and perform the 

 office of maxillK. 



