442 ARTICULATA. 



The system of organs in which the Articulata resemble each 

 other the most, is that of the nerves. 



Their brain, which is placed on the esophagus, and fur- 

 nishes nerves to the parts adhering to the head, is very small. 

 Two cords wliich embrace the esophagus 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 how- 

 ever of the organs of hearing, the existence, number and form 

 of those of sight, the product and mode of generation(l), 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 inter- 

 nal or external. 



The Annelides, Lam., or Red-blooded Worms, Cuv., 

 constitute the first. Their blood, which is generally red, like 

 that of the Vertebrata, circulates in a double and closed sys- 

 tem of arteries and veins, sometimes furnished with one or 

 several visible hearts or fleshy ventricles. Respiration is per- 

 formed in organs which are sometimes developed externally, 



(1) M. Herold has made a remarkable discovery on this subject, viz. that in the 

 ovum of the Crustacea and Arachnides, the vitellus communicates with the inte- 

 rior by the back. See his Dissert, oh the ovum of Spiders, Marburg, 1824, and 

 that of M. Rathke on that of the Astaci, Leipzig-, 1829. 



