Chap. II. BRANCHES OF THE ANIMAL KINGDOM. 143 



true Eadiata, that Cuvier placed thorn in that (hvision, but simply because he 

 allowed himself to depart from his own princi2)le, and to add another consideration, 

 besides the plan of structure, as characteristic of Radiata, — the supposed absence of 

 a nervous system, and the great siraplicitj- 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, was the transfer of the Cirripeds from 

 among the Mollusks to the branch of Articulata. Imperfect knowledge of the plan 

 of structure of these animals was here the cause of the mistake, which was cor- 

 rected without any opposition, as soon as they became better known. 



From a comparison of what is stated here respecting the different plans of 

 structure, characteristic of the primary divisions of the animal kingdom, with what 

 I have to say below about classes and orders, it will appear more fully, that it 

 is important to make a distinction between the plan of a structure and the man- 

 ner in which that plan is carried out, or the degrees of its complication and its 

 relative perfection or simplicity. But even after it is understood that the plan of 

 structure should be the leading characteristic of these primary groups, it does not 

 3"et follow, without further examination, that the four great branches of the animal 

 kingdom, first distinguished by Cuvier, are to be considered as the primary divisions 

 which Nature points out as fundamental. It will still be necessary, by a careful 

 and thorough investigation of the subject, to ascertain what these primary groups 

 are ; but we shall have gained one point with reference to our system.s, — that what- 

 ever these primary groups, founded upon different plans, which exist in nature, may 

 be, when they are once defined, or whilst they are admitted as the temporary ex- 

 pression of our present knowledge, they should be called the branches of the animal 

 kingdom, whether they be the Vertebrata, Articulata, MoUusca, and Eadiata of Cuvier, 

 or the Artiozoaria, Actinozoaria, and Amorphozoaria of Blainville, or the Vertebrata 

 and Invertebrata of Lamarck. The special inquiry into this point must be left for 

 a special paper. I will only add that I am daily more satisfied, that, in their 

 general outlines, the primary divisions of Cuvier are true to nature, and that never 

 did a naturalist exhibit a clearer and deeper insight into the most general relations 

 of animals than Cuvier, when he perceived, not only that these primary groups are 

 founded upon differences in the plan of their structure, but also how thej- are 

 essentiallv related to one another. 



Though the term type is generally employed to designate the great fundamental 

 divisions of the animal kingdom, I shall not use it in future, but j)refer for it the 

 term branch of the animal kingdom, because the term type is employed in too 

 many different acceptations, and quite as commonly to designate any group of any 

 kind, or any peculiar modification of structure stamped Avith a distinct and marked 



