292 ARTICULATA. 



of 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 articula- 

 ted 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 receives it from the branchia3, 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 antennae 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 ARACHNIDES form the third class of tBe 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 ocelli, that vary as to number, but 

 the antennas are always wanting. Their circulation is effected by 

 a dorsal vessel, which gives off arterial branches, and receives venous 

 ones from them; but their mode of respiration varies, some of them 

 still having true pulmonary organs which open on the sides of the 

 abdomen, while others receive air by tracheae, like Insects. In both 

 of them, however, we observe lateral openings or true stigmata. 



The INSECTA constitute the fourth class of the Articulata, and 

 the most numerous of all the animal kingdom. With the exception 

 of some genera, the Myriapoda, in which the body is divided into 

 numerous and nearly equal parts, it is always divided into three por- 

 tions: the head, furnished with the antennae, eyes and mouth; the 

 thorax, to which are appended the feet and wings, when they exist; 

 and the abdomen, which is suspended behind the thorax and contains 

 the principal viscera. Those which have wings only receive them at 

 a certain age, and frequently pass through two more or less different 

 forms before they assume that of the winged insect. In all their 

 states they respire by tracheae; that is, by elastic vessels which re- 

 ceive air through stigmata pierced on their sides, and distribute it 





