ARTICULATED ANIMALS. 
Distribution of the Articulated Animals into Four Classes . 
The articulated animals, whose inter-relations are equally 
numerous and varied, nevertheless present themselves under 
four principal forms, whether internal or external. 
The Annelida, or Red-blooded Worms, constitute 
the first. Their blood, generally of a red colour, like that 
of vertebrated animals, circulates in a double and closed sys- 
tem of arteries and veins, which has sometimes one or many 
hearts or fleshy ventricles, sufficiently marked. They respire 
by organs which are sometimes developed externally, and 
sometimes remain at the surface of the skin, or sink into its 
interior. Their body, more or less elongated, is always divided 
into numerous rings, the first of which, named the head, is 
scarcely different from the others, except in the presence of the 
mouth, and the principal organs of sensation. Many have 
their gills uniformly distributed along their body or its middle ; 
others, which generally inhabit tubes, have them all at the an- 
terior part. Those animals never have articulated feet, but 
the greater number, instead of feet, have seta, or bundles of 
stiff* and moveable hairs. They are generally hermaphrodites, 
and some of them have need of a reciprocal intercourse. 
The organs of their mouth consist sometimes of jaws, more or 
less strong, sometimes of a simple tube ; those of the external 
senses, of fleshy tentacula, and sometimes articulated ; and of 
some blackish points, which have been regarded as eyes, but 
which do not exist in all the species. 
The Crustacea constitute the second form or class of 
articulated animals. They have articulated, and more or less 
complicated 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 gills, situated 
on the sides of the body, or under its posterior part, and 
