334 ESSAY ON CLASSIFICATION. 



CL. 13. ACEPIIALA. Ord. Lamellibranchiata (Cormopoda Nitzsch, 



Pelecypoda Car.) and Brachiopoda. 

 CL. 14. GASTEROPODA. Ord. Heterobranchia (Pteropoda, Infero- 



branchia, aad Tectibranchia), Dermatobranchia (Gymno- 



branchia aud Phlebeuterata), Heteropoda, Cteuobranchia, 



Pulmonata, and Cyclobrauchia. 

 CL. 15. CEPHALOPODA. 

 VI. VEKTEBBATA. (Not considered.) 



I need not repeat here what I have already stated, in 

 the first section, respecting the primary divisions adopted 

 by Siehold and Leuckart. As to the classes, I may add 

 that his three classes of Echinoderms exhibit only ordinal 

 characters. Besides Birds and Cephalopods, there is not 

 another class so well defined, and so little susceptible of 

 being subdivided into minor divisions presenting any- 

 thing like class characters, as that of Echinoderms. Their 

 systems of organs are so closely homological, (compare 

 p. 293,) that the attempt here made by Leuckart, to sub- 

 divide them into three classes, can readily be shown to 

 rest only upon the admission, as classes, of groups which 

 exhibit only ordinal characters, namely, different degrees 

 of complication of structure. With reference to the 

 classes of Worms, the same is equally true, as shown 

 above. The arrangement of these animals proposed by 

 Burnieister is certainly more correct than those of von 

 Siebold and of Leuckart, inasmuch as he already rightly 

 refers the Rotifera to the class of Crustacea, and does 

 not, like Leuckart, associate the Bryozoa with the Worms. 

 I agree, however, with Leuckart, respecting the propriety 

 of removing the Nemertini and Hirudinei from among 

 the true Annelides. Again, Burmeister appreciates also 

 more correctly the position of the whole type of Worms, 

 in referring them, with De Blainville, to the branch of 

 Articulata. 



The common fault of all the anatomical classifications 



