202 ESSAY ON CLASSIFICATION 



it belongs to the cycle of Distoma; and the only link in the meta- 

 morphosis of that genus which was not known is now supplied, since, 

 as I have stated above, the embryo which is hatched from the egg 

 laid by the perfect Distoma is found to be Opalina. 



All this leads to the conclusion, that a division of the animal king- 

 dom to be called Protozoa, differing from all other animals in pro- 

 ducing no eggs, does not exist in nature, and that the beings which 

 have been referred to it have now to be divided and scattered, partly 

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

 the classes of Acephala (Vorticellce) of Worms (Paramecium and 

 Opalina), and of Crustacea (Rotifera); Vorticellas being genuine 

 Bryozoa and therefore Acephalous Mollusks, while the beautiful in- 

 vestigations 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 ani- 

 mals 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 con- 

 sideration of their peculiar plan, the element of simplicity of their 

 structure as an essential feature in the typical character of these ani- 

 mals, 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 re- 

 peat here what I have already stated of the Infusoria when consider- 

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

 priety 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, constitute really three natural classes in this great di- 

 vision, inasmuch as they exhibit the three different ways in which 

 the characteristic plan of the type, radiation, is carried out in distinct 

 structures. 



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

 homologous in their structure with Acalephs and Polypi, it must 



