296 ESSAY ON CLASSIFICATION. 



the Brachiopods, alluded to above, 1 are fully understood, 

 no doubt will remain of their true relation to Mollusks. 

 As it is not within the limits of my plan to illustrate 

 here the characters of all the classes of the animal 

 kingdom, I will only state further, that the branch of 

 Mollusks appears to me to contain only three classes, as 

 follows : 



1st Class: Acepliala; with four orders, Bryozoa, (in- 

 cluding the Vorticellse) Brachiopoda, Tunicata, and Lamel- 

 libranchiata. 



2nd Class : Gasteropoda; with three orders, Pteropoda, 

 Heteropoda, and Gasteropoda proper. 



3rd Class : Cephalopoda ; with two orders, Tetra- 

 branchiata and Dibranchiata. 



The most objectionable modification introduced in the 

 general classification of the animal kingdom, since the 

 appearance of Cuvier's Kegne Animal, seems to me to be 

 the establishment of a distinct branch, now very generally 

 admitted under the name of VERMES, including the 

 Annulata, the Helminths, the Eotifera, and, as Leuckardt 

 would have it, the Bryozoa also. It was certainly an im- 

 provement upon Cuvier's system to remove the Helminths 

 from the type of Eadiates ; but it was at the same time 

 as truly a retrograde step to separate the Annelides from 

 the branch of Articulata. The most minute comparison 

 does not lead to the discovery of a distinct plan of 

 structure, uniting all these animals into one natural 

 primary group. What holds them together and keeps 

 them at a distance 2 from the other Articulate groups is 

 not a different plan of structure, but a greater simplicity 

 in their organization. 3 In bringing these animals to- 



1 Chap. I, Sect. 18, p. 108. 2 Chap. II, Sect. 7, pp. 261-263. 



3 See above, Chap. I, Sect, 18, pp. 112, 113. 



