CLASSES OF ANIMALS. 223 



the body exhibits a large cavity, divided by radiating par- 

 titions into a number of chambers, into which hangs a sac 

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

 digested 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 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 communica- 

 tion 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 what truly characterizes them 

 is not the complication of their structure, (for Hydroid 

 Medusae are hardly more complicated in their structure 

 than Polyps,) but the manner in which the plan of the Ra- 

 diata 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 ; and 

 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 Polyps, as 

 orders, are distinguished, the first by having a larger and 

 usually indefinite number of simple tentacles, an equally 

 large number of internal partitions, etc.; the second by 

 having the eight tentacles lobed and complicated, and all 

 the parts combined in pairs in definite numbers, etc. ; dif- 

 ferences which establish a distinct standing between them 



