32 CATEGORIES OF CLASSIFICATION. 



confirms this result ; and I shall attempt to 

 diow that there arc really four, and only four, 

 such structlOnal ideas at the foundation of the 

 animal kingdom, and that all animals are in- 

 cluded under one or another of them. But it 

 does not follow, that, because we have arrived 

 at a sound principle, we are therefore unerring 

 in our practice. From ignorance we may mis- 

 place animals, and include them under the 

 wrong division. This is a mistake, however, 

 which a better insight into their organization 

 rectifies ; and experience constantly proves, that, 

 whenever the structure of an animal is perfectly 

 understood, there is no hesitation as to the head 

 under which it belongs. We may consequently 

 test the merits of those four primary groups on 

 the evidence furnished by investigation. 



It has already been seen that these plans may 

 be presented in the most abstract manntr with- 

 out any reference to special animals. Radiation 

 expresses in one word the idea on which the 

 lowest of these types is based. In Radiates we 

 have no prominent bilateral symmetry, such as 

 exists in all other animals, but an all-sided 

 symmetry, in which there is no right and left, 

 no anterior and posterior extremity, no above 

 and below. It is true that in some of them 

 there are indications of tfiat bilateral symmetry 

 which becomes a law in the higher animals ; but 



