ARTICUL/vTED AXIMALS. 



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 exteraally, 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 diflerent 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 setcB^ 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 



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