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 dijffers from the rest, except in the presence of the 

 mouth and the principal organs of the senses. The branchise 

 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 

 setae 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 exter- 

 nal 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 arti- 

 culated 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 ven- 

 tricle placed in the back, which receives it from the branchiae, 

 situated on the sides of the body, or under its posterior por- 

 tion, and to which it returns by a ventral and sometimes 

 double canal. In the last or lower species, the heart or dor- 

 sal ventricle is itself extended into a tube. They all have 

 antennse 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 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 sim- 

 ple eyes, that vary as to number, but the antennse are always 

 wanting. Their circulation is eifected by a dorsal vessel, 



