194 



principal modifications are produced among the Vertebrata and Arti- 

 culata, seems almost absent in the Mollusca; the changes among 

 them being produced by an asymmetrical development of the prima- 

 rily symmetrical archetype, a process comparatively rare among the 

 Articulata and Vertebrata. 



5. The part thus asymmetrically developed is invariably a portion 

 of the hsemal surface, and may be called an abdomen or a post-abdo- 

 men, according as it is placed before or behind the anus. 



6. The intestine is found to be bent in two directions among the 

 Mollusca, hsemad or neurad, and these flexures correspond vi^ith the 

 development of a post-abdomen or abdomen, respectively. 



7. The process of development demonstrates that the Tectibran- 

 chiata, Nudibranchiata and Pectinibranchiata (in part at least) belong 

 to the former division, and that the Cephalopoda and Pulmonata 

 belong to the latter. 



8. Anatomical evidence show^s that the Heteropoda have a haemal 

 flexure of the intestine, the Pteropoda a neural flexure ; and it is 

 almost certain that when their development is traced, the former will 

 be found to have a post-abdomen, the latter an abdomen ; there will 

 then be two great divisions of the cephalous mollusca. 



a. Those which develope an abdomen : — Cephalopoda, Pteropoda, 

 Pulmonata. 



b. Those v^^hich develope a post-abdomen : — Heteropoda, Pectini- 

 branchiata, Tectibranchiata, Nudibranchiata. 



9. Prosobranchism and Opisthobranchism may occur as secondary 

 results of either course of development. 



10. The principal nervous centres are similar in number and posi- 

 tion throughout, and diff'er only in their arrangement and degrees of 

 concentration. The amount of the latter does not correspond with 

 the complexity of organization of the moUusk, but rather the re- 

 verse. 



11. The organization of the vascular system is equally uniform; 

 its completeness or incompleteness is no mark of complexity or sim- 

 plicity of the rest of the organization. 



12. The cephalous Mollusca are characterized by the possession of 

 a peculiar organ, the dentigerous " tongue," whose mode of action 

 resembles that of a chain-saw. 



13. The locomotive apparatus, when fully developed, consists of 

 four parts, the propodium, raesopodium, metapodium and epipodium. 

 These parts are least modified in such mollusks as Atlanta or Pneu- 

 modermon ; most altered and disguised in such as Cleodora or Oc- 

 topus. 



14. The term "mantle" should be confined to the surface of the 

 abdomen ov post-abdomen, and to the prolonged edges of that surface. 



15. It is of great importance to recollect that the "shells" are 

 probably not homologous organs in all the diflferent forms of mollusks. 



The shells of Sepia, Spirula (?), Limax, Clausilia and Helix are 

 developed in the thickness of the mantle. 



The shells of Nautilus (?), Pectinibranchiata, &c., are developed 

 from the surface of the mantle by a quite distinct process. 



