ARTICULATA. 125 
The Crustacea constitute the second form or class of articulated 
animals. They are provided with articulated and more or less com- 
plexed limbs, attached to the sides of the body. Their blood is 
white : it circulates by means cf a fleshy ventricle placed in the 
back, which receives it from the branchiae, 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 com- 
pound eyes. A distinct ear is only to be found in some species. 
The Arachnid ES 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 enclosed 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 antennae are always wanting. Their circidation 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 stig- 
mata. 
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 nu- 
merous 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 of a heart 
only is perceptible, consisting of a dorsal vessel, which experiences an 
alternate contraction and dilatation, but to which, no branch has ever 
been discovered, 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 In- 
