OCR Text |
Show 8 CRUSTACf~A. rection; and that finally, this approximation in all directions is carried to its extreme point, when it is reduced to a single nucleus in the thorax-as in Cancer properly so called, or the Brachyura. · Of all the Decapoda Macroura examined by Messrs Audouin and Edwards, the Palinurus was found to have the venous system most centralized; and in fact, that animal in our system is but little removed from the Brachyura. But this should not be the case with Palremon and the Astacini, for according to them the former approximates more closely in this respect to Palinurus than the latter, while in our arrangement the second precede the first, a disposition which appears to us· to be founded on several very natural characters. The Crustacea are apterous or deprived of wings, furnished with compound eyes, though rarely with simple ones, and usually with four antennre. They have mostly-the Precilopoda excepted-three pairs of jaws, the two superior ones, designated by the name of mandibles, included ; as many footjaws( I), the last four of which, however, in a great many instances, become true feet; and ten feet properly so called, all terminated by a single small nail. When the last 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 comparable 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 0) .!luxiliary jaws, as they are termed by M. Savigny, at least when speaking of the Crustacea Dccapoda. As the two superior ones, in the Amphipoda and lsop.oda, _form a so1·~ of lip, he there calls them the auxiliary li'p. lie distinguishes the Jaws m PhalangiUm, a genus of Arachnides, as p1·incipal jaws; those which are attached ~o the palpi-false palpi, according to him; and as supernum~rary jaws, tho.se whtch are attac~ed to the first four feet. Those parts of the same animals whtch have been cons1dered as mandibles, are his mandibz~les succedanes. He admits of two auxiliary lips in the Scolopendra:. CRUSTACEA. 9 called the tail, and never posteriorly. Their envelope is usually solid, ancl 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 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 branchire, the mode in which the head is articulated with the trunk( I), the mobility or fixedness of the eyes(2), the organs of manducation, and the teguments, 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), generally unguiculated. The mouth, situated in the ordinary (1) With respect to tlds term, and that of thorax, which are frequently employed in an arbitrary manner, see our general observations on the class of Insects. (2) These organs are either pediculated and movable, or sessile and fixed. It is from this ch:u·actcr that Lamarck has divided the Crustacea into two grt>at sections, the Pediocles and the Sessiliocles,· for which denominations, but restricting its application to the Mnlacostraca, Doctor Leach has substituted those of Podopthalma and Edriopthalma. Gronovius was the first who had recourse to this distinction. (3) Although we possess but few observntions 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, according to the presence or absence of the mandibles. Jurine, Jun., has already p1·oposcd these divisions in his excellent Me moire sur 1' Argule foliace. (5) The font· ante1•ior, when there are fourteen, are formed by the last four posterior foot-jaws. In the Decapoda, the six foot-jaws belong to the mouth, and perform the office of maxilla:. VoL. III.-B |