CuAP. II. CLASSES OF ANIMALS. 147 



we find easily, that while in some (the Polypi) the body exhibits a large cavity, 

 divided by radiating partitions into a number of chambers, into which hangs a .sac, 

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

 the main cavity, whence it is circulated to and fro in all the chambers, by the 

 agency of vibratmg cilia; in others, (the Acalephs,) 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 com- 

 munication 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 sj^stems of organs, etc. 



Without giving here a full description of these classes, I only ^vish to show, 

 that what truly characterizes them, is not the complication of their structure, (for 

 Hydroid Medusas are hardly more complicated 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 tentacles, an equally large number of internal partitions, etc., while in 

 Halc3onarians the eight tentacles are lobed and complicated, and all the parts are 

 combined in pairs, in definite numbers, etc., differences which establish a dis- 

 tinct stixnding between them in their class, assigning 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 executed, 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 ultimate 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 charac- 

 teristics of classes. These great divisions, so important in the study of the animal 

 kingdom, that a knowledge of their essential features is rightly considered as the 

 primary object of all investigations in comparative anatomy, are generally repre- 

 sented as exhibiting each some essential modification of the type to which they 



