LEADING GROUPS OF EXISTING SYSTEMS 151 



food into the main cavity, whence it is circulated to and fro in all 

 the chambers by the agency of vibrating cilia; in others (the Aca- 

 lephs) the body is plain and full, not to be compared to a hollow 

 sac, traversed only in its thickness by radiating tubes which arise 

 from a central cavity (the digestive cavity) without a free communi- 

 cation with one another for their whole length, etc., etc.; while in 

 others still (the Echinoderms) there is a tough or rigid envelope to 

 the body, inclosing a large cavity in which are contained a variety 

 of distinct systems of organs, etc. 



Without giving here a full description of these classes, I only wish 

 to show that Avhat truly characterizes them is not the complication 

 of their structure (for Hydroid Medusae are hardly more compli- 

 cated in their structure than Polyps), but the manner in which the 

 plan of Radiata is carried out, the ways in which life is maintained 

 in these animals, the means applied to this end; in one word, the 

 combinations of their structural elements. But the moment we would 

 discern what are the orders of these classes these considerations no 

 longer suffice; their structure has to be viewed in a different light; 

 it is now the complication of these apparatus which may guide us. 

 Actinarians and Halcyonarians among Polypi, as orders, differ, the 

 first by having a larger and usually indefinite number of simple ten- 

 tacles, an equally large number of internal partitions, etc., while in 

 Halcyonarians the eight tentacles are lobed and complicated, and all 

 the parts are combined in pairs, in definite numbers, etc., differences 

 which establish a distinct standing between them in their class, as- 

 signing the latter a higher rank than the former. 



It follows, then, from the preceding remarks that classes are to be 

 distinguished by the manner in which the plan of their type is ex- 

 ecuted, by the ways and means by which this is done, or, in other 

 words, by the combinations of their structural elements, that is to 

 say, by the combinations of the different systems of organs building 

 up the body of their representatives. We need not consider here the 

 various forms under which the structure is embodied, nor the ulti- 

 mate details, nor the last finish which this structure may exhibit, as 

 a moment's reflection will convince any one that neither form nor 

 structural details can ever be characteristic of classes. 



There is another point to which I would call attention respecting 

 the characteristics of classes. These great divisions, so important in 



