OCR Text |
Show 442 ARTICULATA. The system of organs in which the Articulata resemble each other the most, is that of the nerves. T heir brain which is placed on the esophagus, and fur-nishes nerves t'o the parts adhering to the hea d, I.S very small. Two cords which embrace the esophagus are extended along the abdomen and united at certain distances by double knots or ganglia, ~hence arise the nerves of the ?ody and li~bs. Each of these ganglia seems to fulfil the functiOns of abram to the surrounding parts, and to preserve their sensibility for a certain length of time, when the animal has been divided. If to this we add, that the jaws of these animals, when they have any, are always lateral and move from wi~h~ut, inwardly, and not from above, downwards, and that no d1stmct organ of smell has hitherto been discovered in them, we shall have expressed all that can be said of them in general. The existence however of the organs of hearing, the existence, number and form of those of sight, the product and mode of generation(!), the kind of respiration, the existence of the organs of circulation, and even the colour of the blood present great differences, which must be noticed in the various subdivisions. Distribution of the .llrticulata into four Classes. The Articulata, whose mutual relations are as varied as numerous, present however four principal forms, either internal or external. The ANNELIDEs, Lam., or RED-BLOODED WonMs, Cuv., constitute the first. Their blood, which is generally red, like that of the Vertebrata, circulates in a double and closed system of arteries and veins, sometimes furnished with one or several visible hearts or fleshy ventricles. Respiration is performed in organs which are sometimes developed externally, (1) M. H~rold has made a remarkable discovery on this subject, viz. that i~ the ovum of the Crustacea and Aracbnides, the vitellus communicates with the mte· rior by the back. See his Dissert. on the ovum of Spiders, Marburg, 1824, and that of M. Rathke on that of the Astaci, Leipzig, 1829. ARTICULATA. 443 and at others remain on the surface of the skin or dip into its interior. Their body, more or less elongated, is always divided into numerous rings, the first of which, called the head, scarcely differs from the rest, except in the presence of the mouth and the principal organs of the senses. The branchire of several are uniformly distributed along their body or on its middle; in others, which are generally those that inhabit tubes, they are all placed anteriorly. They never have articulated feet, but most of them, in lieu thereof, are furnished with setre or fasciculi of stiff and movable hairs. They are mostly hermaphrodites, and some of them require a reciprocal coitus. The organs of their mouth sometimes consist in jaws, more or less strong, and at others of a simple tube, those of the external senses in fleshy, and sometimes articulated tentacula, and in certain blackish points, considered as eyes, but which do not exist in all the species. The CRUSTACEA constitute the second form or class of articulated animals. They are provided with articulated and more or less complex limbs, attached to the sides of the body. Their blood is white = it circulates by means of a fleshy ventricle placed in the back, which re<;eives it from the branchi~, situated on the sides of the body, or under its posterior portion, and to which it returns by a ventral and sometimes double canal. In the last or lower species, the heart or dorsal ventricle is itself extended into a tube. They all have antennre or articulated filaments inserted in the fore-part of the head, usually four in number, several transverse jaws and two compound eyes. A distinct ear is only to be found. in some species. The AnACHNIDEs form the third class of the Articulata. Their head and thorax, as in many of the Crustacea, are united in one single piece, furnished, on each side, with articulated limbs; but their principal viscera are inclosed in an abdomen connected to the posterior portion of that thorax. Their mouth is armed with jaws, and their head furnished with simple eyes, that vary as to number, but the antennre are always wanting. Their circulation is effected by a dorsal vessel, |