ARRANGEMENT AND CLASSIFICATION 65 



TMs class, notwithstanding its utility and inevitableness, has not yet 

 been adopted by naturalists. 



In my course in the year VII. (1799) I established the class of 

 crustaceans. At that time M. Cuvier, in his Tableau des animaux, 

 p. 451, still included crustaceans with insects ; and although this 

 class is essentially distinct, yet it was not till six or seven years later 

 that a few naturahsts consented to adopt it. 



The following year, that is to say, in my course of the year VIII. 

 ^1800) I suggested the arachnids as a class byitself, easy and necessary to 

 distinguish. From that time its characters have constituted a sure indi- 

 cation of an organisation pecuhar to these animals ; for it is impossible 

 to beheve that they arose from an organisation exactly similar to the 

 insects. Insects undergo metamorphosis, propagate only once in the 

 course of their hfe,and have only two antennae, two eyes with facets and 

 six jointed legs; while the arachnid-s never undergo metamorphosis, and 

 exhibit various characters besides which differentiate them from insects. 

 This fact has since been partly confirmed by observation. Yet this class 

 of arachnids is still not admitted into any other work than my own. 



M. Cuvier had discovered the existence of arterial and venous vessels 

 in various animals, which used to be confused under the name of 

 worms with other animals of very different organisation. I immediately 

 took this new fact into consideration for the improvement of my 

 classification ; and in my course in the year X. (1802) I established 

 the class of annelids, placing them after the molluscs and before the 

 crustaceans, as required by their organisation. 



By giving a special name to this new class I was able to keep the 

 old name of worms for the animals which have always borne it, and 

 whose organisation was remote from the anneUds. So I continued 

 to place the worms after the insects, and to distinguish them from the 

 radiarians and polyps with which they can never again be tmited. 



My class of annehds, published in my lectures and in my Recherches 

 sur les corps vivants (p. 24), was several years before being admitted 

 by naturahsts. For the last two years however this class has begun 

 to gain recognition ; but since it is held desirable to change the name of 

 it and to call it by the name of worms, they do not know what to do 

 with the worms properly so-called which have no nerves or circula- 

 tory system. In this difficulty they combine them with the class of 

 polyps, although their organisation is very different. ..» 



These instances of perfection at first attained in a classification,\ 

 then destroyed and subsequently re-estabhshed by the necessity of j 

 things, are not rare in natural science. 



linnseus in fact united several genera of plants which Tournefort 

 had formerly distinguished as in the case of Polygonum, Mimosa, 



