PRINCIPAL SYSTEMS OF ZOOLOGY 205 



to Bryozoa there can be no doubt that their association with Polypi 

 or with Worms is contrary to their natural afTinities. The plan of 

 their structure is in no way radiate; it is, on the contrary, distinctly 

 and essentially bilateral; and as soon as their close affinities with 

 the Brachiopods, alluded to above, are fully understood, no doubt 

 will remain of their true relation to Mollusks. As it is not within the 

 limits of my plan to illtistrate 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: Acephala; with four orders, Bryozoa, including the Vor- 

 ticellse, Brachiopods, Tunicata, and Lamellibranchiata. 



2d Class: Gasteropoda; with three orders, Pteropoda, Heteropoda, 

 and Gasteropoda proper. 



3d Class: Cephalopoda; with two orders, Tetrabranchiata and Di- 

 branchiata. 



The most objectionable modification introdticed in the general 

 classification of the animal kingdom since the appearance of Cuvier's 

 Regne 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 Rotifera, and, as Leuck- 

 art would have it, the Bryozoa also. It was certainly an improvement 

 upon Cuvier's system to remove the Helminths from the type of 

 Radiates, 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 from 

 other gToups is not a common plan of structure, but a greater sim- 

 plicity in their organization. In bringing these animals together 

 naturalists make again the same mistake which Cuvier committed 

 when he associated the Helminths with the Radiates, only in another 

 way and upon a gxeater scale. The Bryozoa are, as it were, de- 

 pauperated Mollusks, as Aphanes and Alchemilla are depauperated 

 Rosacea^. Rotifera are in the same sense the lowest Crustacea; while 

 Helminths and Annelides constitute together the lowest class of Ar- 

 ticulata. This class is connected by the closest homology with the 

 larval states of Insects; the plan of their structure is identical, and 

 there exists between them only such structtiral differences as consti- 



