ARTICULATA. 277 



Their brain, which is placed on the oesophagus, and furnishes 

 nerves to the parts adhering to the head, is very small. Two cords 

 which embrace the O3sophagus are extended along the abdomen, and 

 united at certain distances by double knots or ganglia, whence arise 

 the nerves of the body and limbs. Each of these ganglia seems to 

 fulfil the functions of a brain to the surrounding parts, and to preserve 

 their sensibility for a certain length of time, when the animal has 

 been divided. If to this we add, that the jaws of these animals, when 

 they have any, are always lateral, and move from without inwardly, 

 and not from above downwards, and that no distinct organ of smell 

 has hitherto been discovered in them, we shall have expressed all that 

 can be said of them in general. The existence, however, of the organs 

 of hearing, the existence, number, and form of those of sight, the kind 

 of respiration, the existence of the organs of circulation, and even the 

 colour of the blood, present great differences, which must be noticed 

 in the various subdivisions. 



Distribution of the Articulata into Four Classes. 



The Articulata, whose mutual relations are as varied as numerous, 

 present, however, four principal forms, either internal or external. 



The ANNULATA, Lam., or RED-BLOODED WORMS, Cuv., constitute 

 the first. Their blood, which is generally red, like that of the Verte- 

 brata, circulates in a double and closed system of arteries and veins, 

 sometimes furnished with one or several visible hearts or fleshy ven- 

 tricles. Respiration is performed in organs which are sometimes 

 developed externally, 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 differs from the rest, except in the presence of the mouth and 

 the principal organs of the senses. The branchiae 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 moveable 

 hairs. The organs of their mouth sometimes consist 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 articulated 

 animals. They are provided with articulated and more or less com- 

 plex limbs, attached to the sides of the body. Their blood is white : it 



