ANNELIDA, (a generally adopted, but barbarous latinization of the French term Annelides,' from Annellus,' a little ring ; ought rather to be written Annulata 'or An nellata!)—The natural group of Annelida comprehends all the invertebrated animals which have a soft body divided into transverse segments or rings ; a distinct central nervous system disposed in the form of a longitudinal gangliated chord, blood coloured (generally red), and contained in a system of appropriate and very distinct vessels ; and, lastly, organs of locomotion, consisting either of fleshy appen dages provided with bristles, or of bristles only ; or of a prehensile cavity situated at each ex tremity of the body ; but never of articulated members, as in the Arachnida, Crustacea, and Insecta.
The establishment of this class is due to Cuvier. Prior to him, Pallas, Miiller, and Otho Fabricius, had made observations of great interest on the animals of which it is com posed ; and we find in the writings of the author of the Miscellanea Zoologica the most happy ideas respecting the natural relations which these animals bear to one another. Nevertheless, these works had at first but little influence on the classification of the Inverte brata, and for a long time naturalists persisted in following the method of Linnaeus, who united under the term Verines, the Mollusca, Zoophyta, and Annelida, and dispersed the latter in three different sections of that great class ; confound ing some with the Entozoa (intestinal worms), others with the Acephalous Mollusca, and others again with the Testacea.
It was in the work entitled " Tableau Ele mentaire de l'Histoire Naturelle des Animaux," published in the years 1797-8, that M. Cuvier laid the first foundation of a natural distribution of invertebrated animals. He collected together in the class Vermes the species which more lately have constituted the groups of Annelida and Entozoa, and established in it the two divisions corresponding to those which are generally adopted at the present day. Having subse quently determined the presence of red blood in the leech, and having investigated the circu lating apparatus in these animals, Cuvier sepa rated the " red-blooded" from the " intestinal" worms, and constituted for the former a distinct class, to which Lamarck afterwards gave the name of " Annelides," which has been gene rally adopted, and is used at the present day by most naturalists.*
This classification being based essentially on anatomical structure, has been adopted by Lamarck, Dumeril, Savigny, Leach, Latreille, &c., but is not received by all zoologists of the present day. M. De Blainville, in his metho dical distribution of the animal kingdom, has adopted another plan. Taking the exterior organs for the base of his system, this naturalist divides the articulate animals, which he terms " Entomozoaires," into seven classes, of which the penultimate, viz., the " chetopodes," com prehends the Annelidans provided with loco motive bristles, and of which the last, viz, the cc apodes," is composed of the Annelidans des titute of those organs, together with the planaria and intestinal worms.* The general plan of organization exhibited in the animals which are grouped together by Cuvier under the name of " vers intestinaux," and the numerous affinities which connect the planarize and several helmintha to the Annelida, appear to us fully to justify a partial adoption of the innovations introduced by 1\1. De Blain ville, and to indicate that the natural position of the white-blooded worms is by the side of those with red blood, at the bottom of the sub kingdom of articulate animals ; whilst in the system of Cuvier the Annelida are placed at the head of that great division of the animal kingdom, and the entozoa are left among the zoophytes. But, on the other hand, similar reasons appear to us to oppose the adoption of the divisions which M. De Blainville has proposed for the articulate animals. That zoologist, in fact, establishes a distinction between his che topoda and apoda as wide as between the former and the insecta, arachnida and crustacea, and thus separates from the setiferous annelidans to place among the intestinal worms the hirudines, which approximate to the former and deviate from the latter in many of the most important points of their organization ; for example, in the existence of a gangliated nervous system.