ON THE PRINCIPLE OF CEPHALIZATION. 179 



Applying these terms to the classes or groups of Sac- 

 cata, we have the following: 



C Alphatypic, Cephalopoda. 



Holozoic. < Betatypic, Gasteropoda. 



( Garnniatypic, Lamelllbranchiata. 



C -r, 14-- i ( TUNICATA. 



Phytozoic. ) Degradation^, £ Brachiopoda . 

 ( Hemiphytoid, Polyzoa. 



Prof. Danahas pointed out many interesting parallel- 

 isms between the groups of the different branches. Let 

 us now look at the parallelisms between the groups above 

 indicated, and the other branches. Cephalopods approach 

 nearest the Vertebrates through their lowest class, the fishes, 

 and already many interesting analogies have been point- 

 ed out between them. 



Gasteropods may be likened to Articulates, through 

 their lowest class, the Worms, through certain resemb- 

 lances many forms bear to the Leeches, Planarians, and 

 Trematodes. Lamellibranchiates may be considered the 

 essential embodiment of the branch to which they belong. 

 Tunicates and Polyzoa may be compared to Radiates. 



Or, in considering their freedom or fixedness in life, we 

 have Cephalopods free, as in all Vertebrates ; Gasteropods, 

 a few fixed, as in Articulates ; Lamellibranchiates, many 

 fixed as in Saccata, with relation to the other Branches. 

 Tunicates, the greater portion fixed, though they do not 

 compare so well with the Radiates in this respect, but 

 Brachiopods and Polyzoa fixed as in the lowest class of 

 Radiates, the Polyps. 



We would thus have 



axphatypic, 

 ga5en1atypic, 

 Betatypic, 



Degradational, 



Hejiiphytoid, 



Cephalopds, 



Vertebrates, 



Fishes. 



Gasteropods, 



Articulates, 



Worms. 



Lamellibranchiates. 



, Saccates. 





Tunicates, > 

 Brachiopods, ) 



Badiates. 





Polyzoa, 



Badiates, 



Polyps. 



