CRUSTACEA, 
153 
two pairs of foot-jaws exercise the same functions, the number of 
feet is increased to fourteen. The mouth, as in insects, i)resents a 
labrum and a ligula, but no lower lip properly so called, or com- 
])arable 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, ^^'^ith the exception of a few in which these changes some- 
Avhat influence their primitive form and modify or augment their 
locomotive organs, they are at birth, size apart, such as they arc 
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^. 
AVe 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 arbitrary manner, see our general observations on the class of Insects. 
-f These organs are either pedicxxlated 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 Sessiliocles ; 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- 
tinction. 
+ 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. 
§ They might be still further divided into the Dentata and the Edentata, accord- 
ing to the presence or absence of the mandibles. Jurine, jun., has already proposed 
these divisions in his excellent Mtmoire sur I’Argule foliacth 
II The four anterior, when there are fourteen, are formed by the last four posterior 
foot-jaws. In the Decapoda, the six foot-jaws belong to the me-uth, and perform the 
office of maxilla:. 
