222 ESSAY ON CLASSIFICATION. 



great types of the animal kingdom ; or perhaps, still more 

 distinctly, the classes are characterized by the different 

 ways in which life is maintained, and the different means 

 employed in establishing these ways. An example will 

 suffice to show that this distinction implies a marked dif- 

 ference between class and ordinal characters. 



Let us compare the Polyps and Acalephs as two classes, 

 without allowing ourselves to be troubled by the different 

 limits assigned to them by different authors. Both are 

 constructed upon the same plan, and belong, on that 

 account, to the type, of Radiata. In establishing this fact, 

 we do not consider the actual structure of these animals, 

 whether they have a nervous system or not, whether they 

 have organs of the senses or not, whether their muscles 

 are striated or smooth, whether they have a solid frame 

 or an entirely soft body, whether their alimentary cavity 

 has only one opening or two opposite openings, whether 

 it has glandular annexes or not, whether the digested 

 food is distributed in the body one way or another, 

 whether the undigested materials are rejected through 

 the mouth or not, whether the sexes are distinct or not, 

 whether they reproduce themselves only by eggs, or by 

 budding also, whether they are simple or not : all we 

 need know, in order to refer them to the branch of 

 Radiata, is whether the plan of their structure exhibits a 

 general radiated arrangement or not. But, when we 

 would distinguish Polyps, Acalephs, and Echinoderms as 

 classes, or rather, when we would ascertain what are the 

 classes among the Radiata, and howmany there are, we must 

 inquire into the manner in which this idea of radiation, 

 which lies at the foundation of their plan of structure, is 

 actually expressed in all the animals exhibiting such a 

 plan, and we easily find that, while in some (the Polyps) 



