278 ARTICULATA. 



circulates by means of a fleshy ventricle placed in the back, which receives 

 it from the branchiae, situated on the sides of the body, or under its pos- 

 terior 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 forepart 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 ABACHNIDKS 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 prin- 

 cipal 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 antennae 

 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 trachese, like insects. In both of them, however, we 

 observe lateral openings or true stigmata. 



The INSECTA constitute the fourth class of the Articulate, 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 

 portions : 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 receive air 

 through stigmata pierced on their sides, and distribute it by infinite 

 ramifications to every part of the body. A vestige only of a heart is 

 perceptible, consisting of a dorsal vessel which experiences an alternate 

 contraction and dilatation, but to which no branch has ever been dis- 

 covered, so that we are forced to believe that nutrition is effected in this 

 class of animals by imbibition. It is, probably, this sort of nutrition 

 which necessitated the kind of respiration proper to insects; for as the 

 nutritive fluid is not contained in vessels, and could not be directed 

 towards pulmonary organs in search of air, it was requisite that this 

 air should be diffused throughout the body to reach the fluid. This 



