Chap. III. MODERN SYSTEMS. 185 



of the branch of Molhi.sks. Those difference.s, however, coastitute only class charac- 

 ters and exhibit in no way a different plan. It is, indeed, by no means difficult 

 to homologize all the systems of organs of the Cephalopods with those of the 

 other Mollusks, and with this evidence, the proof is also furnished that the Cepha- 

 lopods constitute only a class among the Mollusks. 



As to the differences in the development of the Cephalopods and the other 

 Mollusks, the type of Vertebrata teaches us that partial and total segmentation 

 of the yolk ai'e not inconsistent with unity of type, as the eggs of Mammalia and 

 Cyclostomata undergo a total segmentation, while the process of segmentation is 

 more or less limited in the other classes. In Birds, Reptiles, and Selachians, the 

 segmentation is only superficial ; in Batrachians, and most Fishes, it is much deeper ; 

 and yet no one would venture to separate the Vertebrata into several distinct 

 branches on that account. With reference to Bryozoa, there can be no doubt, 

 that their association Avith Polypi or with Woi-ms is contrary to their natural 

 affinities. The plan of their structure is in no way radiate ; it is, on the con- 

 trary, distinctly and essentially bilatei'al ; 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 

 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: Acephala; with four ordei's, Bryozoa, including the Yorticella?, Bra- 

 chiopods, Tunicata, and Ljimellibranchiata. 



2d Class : Gasteropoda; with three orders, Pteropoda, HeterojDoda, and Gas- 

 teropoda proper. 



3d Class: Cephalopoda; with two orders, Tetrabranchiata and Dibranchiata. 



The most objectionable modification introduced 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 

 imder the name of Vermes, including the Annulata, the Helminths, the Rotifera, 

 and as Leuckardt would have it, the Bryozoa also. It was certainly an improve- 

 ment 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 sejjarate 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 grouj). What holds them together and keeps them at a distance " from 

 other groups is not a coiniuou plan of structure, but a greater simplicity in their 



» Cliap. I., Sect. 18, p. 72. » Chap. IT.. Sect. 7. p. 171, 172. 



2i 



