176 MORSE, CLASSIFICATION OF MOLLUSCA 



anus opening anteriorly; the arms surrounding the mouth 

 only. Two rough diagrams, alike in form, but reversed 

 in one case, would represent each class as we have it here. 



In Brachiopoda (Series II, B) we have the sac widen- 

 ing laterally, and correspondingly depressed dorsally ; 

 mouth and anus opening posteriorly. In Gasteropoda 

 (Series II, G) we have the same features, except that the 

 parts are reversed again. In Tunicata (Series II, T) the 

 sac is lengthened and swollen. Lamellibranchiata (Series 

 II, L) the same. The relative diameters of the Monomy- 

 arians are unlike those of any other class, as before point- 

 ed out. 



It is confidently believed that when these relations, or 

 polarities, between the ascending, and descending, or, as 

 Professor Dana terms them, the Holozoic and Phytozoic 

 classes, have been farther studied, new and interesting 

 features will be revealed. Thus , the resemblances be- 

 tween the Tunicates and Lamellibranchiates are too obvi- 

 ous to indicate. 



Among the Brachiopods and Gasteropods, beside what 

 has been pointed out, we have unlooked for similarities, 

 as for instance Descina and Calyptrsea, or Terebratula and 

 HyalEea. Among the Polyzoa and Cephalopoda, though 

 no polarities are brought to mind, except those given 

 above, yet we cannot help remarking how strong the re- 

 semblance is between the Polyzoa and Protozoa, through 

 Vorticella : and if Vorticella belongs to Polyzoa, as Pro- 

 fessor Agassiz appears inclined to believe, a few steps more 

 bring us to the Ammonitic forms of the Rhizopods. This 

 is speculative (though suggestive), as it is now considered 

 by many that the Protozoa forms a fifth Sub-Kingdom. 



In considering a transverse section of the sacs, as shown 

 in Series III, we obtain a like order of polarity. Thus the 

 highest orders in Polyzoa and Cephalopoda present a cir- 

 cular section. Brachiopoda and Gasteropoda are trans- 

 versely oval ; Tunicates and Lamellibranchiates are longi- 

 tudinally oval, or in lower forms circular ; while the Mono- 

 myarians have the dorso-ventral diameter in excess, and 

 the transverse diameter reduced to the minimum. 



In considering the position, or angle of the antero-pos- 

 terior axis of each class in nature, we obtain similar re- 

 sults (Series IV). 



