CLASSIFICATION OF ANIMALS. 15 



and anus, suspended in a distinct abdominal cavity : the well-defined 

 nerves govern a corresponding development of the muscular system. 

 Generation is by impregnated ova, rarely by spontaneous fission or 

 gemmation. The Echinodermata, Rotifera^ Ccelehnmtha, and Bryo- 

 zoa, are the classes of Cuvier's zoophytes, which were grouped 

 together by positive characters, under the title Nematoneura. 



But a filamentous condition of the nervous system is not to be de- 

 nied in the rest of the zoophytes ; each day brings with it testimony 

 of its presence in animalcules, where it had not before been detected. 

 Nevertheless, in those classes in which the condition of the nervous 

 system is most obscure, we find that the digestive cavity is generally 

 excavated in the common parenchyma of the body, is devoid of free 

 parietes, and seldom has an anal outlet : particular organs are often 

 indefinitely multiplied, as the digestive cavities in the Polygastria, 

 the generative organs in the Tcenice, the prehensile mouth in the 

 Polypi. Parthenogenesis by gemmation and spontaneous fission is 

 the rule in this lowest division of the animal kingdom, to which I 

 applied the name Acrita, and of which the Hydrozoa, Sterelmintha, 

 and Polygastria are examples. 



The groups called Acalephce and Anthozoa stand in an inter- 

 mediate position between the Acrita and Nematoneura ; and most 

 of the classes in the lowest division of the Radiata lead by 

 more or less gentle gradations into those of the higher one. Nor 

 is this surprising : the radiated animals are closely analogous 

 to the embryo forms of the higher classes ; and as the earlier changes 

 of such embryos succeed each other more rapidly than the later ones, 

 so also each class of the Acrita more closely approximates some class 

 of the Nematoneura than is observed in the classes of the higher 

 groups, and the characters of the lowest or acrite classes are the 

 least definite and fixed. 



Moreover, the positive characters assigned to the Nematoneura do 

 not link together groups in other respects so naturally allied as those 

 which are united together by the molluscous and articulate characters. 

 The Rotifera^ e. g., are more nearly related to the lower Crustacea 

 than they are to the Echinodermata ; and the Codelmintha are more 

 nearly allied to the Epizoa and Anellata than to any nematoneurous 

 group. Where Nature, therefore, seems not to have intrenched herself 

 within recognisable bounds, it is vain to attempt to impose them upon 

 her : and I shall, therefore, content myself with indicating, in the 

 subjoined Table of the Provinces and Classes of the Animal King- 

 dom, the subordinate groups of Cuvier's Zoophytes*, which appear 

 to be best defined by the light of embryonal development. 



* XII. torn. iv. (1817). 



