292 ESSAY ON CLASSIFICATION. 



it have now to be divided, and scattered, partly among 

 plants in the class of Algce, and partly among animals in 

 the classes of Acephala, (Yorticelloe,) of Worms, (Para- 

 rnecium and Opalina,) and of Crustacea (Rotifera) ; the 

 Vorticellse being genuine Bryozoa, and therefore Acepha- 

 lous Mollusks ; while the beautiful investigations of Dana 

 and Leydig have proved the Rotifera to be genuine 

 Crustacea, and not Worms. 



The great type of Radiata, taking its leading features 

 only, was first recognized by Cuvier, though he associated 

 with it many animals which do not properly belong to it. 

 This arose partly from the imperfect knowledge of those 

 animals at the time, but partly also from the fact that he 

 allowed himself, in this instance, to deviate from his own 

 principle of classification, according to which types are 

 founded upon special plans of structure. With reference 

 to Radiata, he departed, indeed, from this view, so far as 

 to admit, besides the consideration of their peculiar plan, 

 the element of simplicity of their structure as an essential 

 feature in the typical character of these animals, in con- 

 sequence of which he introduced five classes among 

 Radiata : the Echinoderms, Intestinal Worms, Acalephs, 

 Polypi, and Infusoria. In opposition to this unnatural 

 association I need not repeat here what I have already 

 stated of the Infusoria, when considering the case of 

 Protozoa ; neither is it necessary to urge again the pro- 

 priety of removing the Worms from among the Radiata 

 and connecting them with the Articulata. There would 

 thus remain only three classes among Radiates, Polypi, 

 Acalephs, and Echinoderms, which, in my opinion, con- 

 stitute really three natural classes in this great division, 

 inasmuch as they exhibit the three different ways in 

 which the characteristic plan of the type, radiation, is 

 carried out, in distinct structures. 



