150 ESSAY ON CLASSIFICATION 



establishing a proper difference between class and ordinal characters, 

 and have again and again raised orders to the rank of classes. For we 

 shall see presently that natural orders must be based upon the differ- 

 ent degrees of complication of structure, exhibited within the limits 

 of the classes, while the classes themselves are characterized by the 

 manner in which the plan of the type is carried out, that is to say, 

 by the various combinations of the systems of organs constituting the 

 body of the representatives of any of the great types of the animal 

 kingdom; or perhaps, still more distinctly, the classes are character- 

 ized by the different ways in which life is maintained, and the differ- 

 ent means employed in establishing these ways. An example will suf- 

 fice to show that this distinction implies a marked difference 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 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 open- 

 ings, whether it has glandular annexes or not, whether the digested 

 food is distributed in the body one way or another, whether the un- 

 digested 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 ar- 

 rangement or not. But when we would distinguish Polyps, Acalephs, 

 and Echinoderms as classes, or rather, when we would ascertain what 

 are the classes among Radiata, and how many there are, we must in- 

 quire 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 find easily that while in 

 some (the Polypi) the body exhibits a large cavity, divided by radiat- 

 ing partitions into a number of chambers, into which hangs a sac 

 (the digestive cavity), open below, so as to pour freely the digested 



