Chap. III. MODERN SYSTEMS. 183 



to be divided, and scattered, partly among plants, in the class of Algiv, and partly 

 among animals, in the classes of Acephala, (Vorticella;,) of Worms, (Paramecium and 

 Opalina,) and of Crustacea (Rotifera) ; Vorticellae being genuine Bryozoa and there- 

 fore Acephalous 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 prhiciple 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 consequence 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 propriety of removing the Worms from among Radiata, 

 and connecting them with 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, masmuch as they exhibit 

 the three different ways in which the characteristic plan of the tyjje, radiation, 

 is carried out, in distinct structures. 



Since it can be shown that Echinoderms are, in a general way, homologous 

 in their structure wuth Acalephs and Polypi, it must be admitted that these classes 

 belong to one and the same great type, and that they are the only representa- 

 tives of the branch of Radiata, assuming of course that Bryozoa, Corallinje, Sponges, 

 and <all other foreign admixtures have been removed from among Polyps. Now, 

 it is this Cuvierian type of Radiata, thus freed of all its heterogeneous elements, 

 which Leuckart imdertakes to divide into two branches, each of Avhich he considers 

 coequal with Worms, Articulates, Mollusks, and Vertebrates. He was undoubtedly 

 led to this exaggeration of the difference existing between Echinoderms on one 

 side and Acalephs and Polypi on the other, by the apparently greater resemblance 

 of Medusce and Polypi,' and perhaps still more by the fact, that so many genuine 

 Acalephs, such as the Ilydroids, including Tubularia, Sertularia, Campanularia, etc., 

 are still comprised by most zoologists in the class of Polypi. 



' AVe .sec Irtc clearly how the oonsideration of overridden the prim.iry feature of branches, their 

 anatomical differences whicli characterize classes has plan, to exalt a class to tlie rank of a branch. 



