| OCR Text |
Show 12 CRUSTACEA. FIRST GENERAL DIVISION. MALACOSTRACA. The Malacostraca naturally divide themselves into those whose eyes are placed on a movable pedicle, and those in which they are sessile and fixed. a. Eyes placed on a movable and articulated pedicle. Eyes(l) placed on a movable pedicle composed of two articulations, and received into fossulre, distinguish the Decapoda and Stomapoda from all the others. Anatomically considered, they appear to be still further removed from them,-Le<;ons d' Anat. Compar., Cuv.; Ann. des Sc. Nat., t. XI,-inasmuch as they are the only ones that present sinuses in which the venous blood is collected previous to its transmission to the branchire on its return to the heart. The Decapoda and Stomapoda resemble each other in several characters common to both. A large plate called a shell covers a greater or less extent of the anterior portion of their body. They all have four antennre(2), the middle ones of (1) Behind the cornea, according to Blainville, is a choroides perforated with numerous holes; then a true crystalline, resting on a nervous ganglion, and divided into a multitude of little fasciculi. (2) We must distinguish the peduncle-stipea,-and the stem-caulis, funiculus. The peduncle is thicker, cylindrical, and composed of three joints, a number which seems peculiar to these organs in their imperfect or rudimentary state. The stem is setaceous, and divided into a variable number of very small joints. That of the external antennre is simple, but that of the interior ones consists of at least two filaments, and in several of the Decapoda Macroura, of three. Passing gradually from these latter to the Brachyura, the antennre become shortened, so that, in several of the Quadrilatera, the lateral ones, at least, are very small. In this case the two terminal divisions of the intermediate ones form a sort of bifurcated forceps, or uneq~;~al and articulated fingers. MALACOSTRACA.. 13 which are terminated by two or three filaments; two mandibles, each of which, at its base, bears a pal pus that is divided into three joints and usually laid on it; a bilobate tongue ; two pairs of jaws; six foot-jaws, the four posterior of which, in some, are transformed into claws ; and ten feet, or fourteen, in those where the four foot-jaws have that form. In the greater number the branchire, of which there are seven pairs, are concealed under the lateral margin of the shell: the two anterior pairs are situated at the origin of the four last foot-jaws, and the others at that of the feet properly so called. In the other Crustacea they are annexed, in the shape of tufts, to :five pairs of paddles (feet) placed under the post-abdomen. The under part of this posterior portion of the body is similarly furnished, in the others, with four or five pairs of bifid appendages. -"\. ORDER I. DECAPOD A. The head, in the Decapoda, is closely joined to the thorax, and covered with it by a shell, entirely continuous, but that most frequently exhibits deep lines dividing it into various regions which indicate the places occupied by the principal internal organs( I). The mode of their circulation presents characters which distinguish them from the other Crustacea. ( 1) M. Desmarest, in his Risto ire Naturelle dea Crustacea Fossiles, and in his Comiderations Generales sur Ia Classe des Crustacea, has presented us, in relation to this point, with an ingenious nomenclature, based on the concordance of the portions of the external surface of the shell with the organs they cover. llut, in addition to the fact that the shell of several Decapoda presents no impressions, or has them nearly obliterated, these denominations may be replaced by others more simple, more familiar, and relating to these same organs; as the middle or centre, the anterior and posterior extremities, the sides, &c.: it appears useless to increase our nomenclature in this case. |
