216 ESSAY ON CLASSIFICATION. 



The best evidence I can produce that the idea of 

 distinct plans of structure is the true pivot upon which 

 the natural limitation of the branches of the animal 

 kingdom must ultimately turn, lies in the fact that every 

 great improvement, acknowledged by all as such, which 

 these primary divisions have undergone, has consisted in 

 the removal from among each,*of such groups as had been 

 placed with them from other considerations than those of 

 a peculiar plan, or in consequence of a want of informa- 

 tion respecting their true plan of structure. Let us 

 examine this point within limits no longer controvertible. 

 Neither Infusoria nor Intestinal Worms are any longer 

 arranged by competent naturalists among theRadiata. Why 

 they have been removed may be considered elsewhere ; 

 but it was certainly not because they were supposed to 

 agree in the plan of their structure with the true Eadiata, 

 that Cuvier placed them in that division, but simply 

 because he allowed himself to depart from his own prin- 

 ciple, and to add another consideration, besides the plan 

 of structure, as characteristic of Kadiata, the supposed 

 absence of a nervous system, and the great simplicity of 

 structure of these animals; as if simplicity of execution 

 had any necessary connection with the plan of structure. 

 Another remarkable instance of the generally approved 

 removal of a class from one of the types of Cuvier to 

 another, w T as the transfer of the Cirripeds from among the 

 Mollusks to the branch of Articulata. Imperfect know- 

 ledge of the plan of structure of these animals was here 

 the cause of the mistake, which was corrected without any 

 opposition, as soon as they became better known. 



From a comparison of what is here stated respecting- 

 the different plans of structure characteristic of the pri- 

 mary divisions of the animal kingeom with what I have 



