456 Royal Society. 



fall under one or other of the great types of which these have been 

 taken as exemplifications. 



After a discussion of the various theories of the homology of the 

 organs of cephalous Mollusca proposed by Loven, Leuckart, &c., the 

 following general conclusions are set forth : — 



1. The cephalous Mollusca are all organized after the same fun- 

 damental form or archetype. 



2. The arrangement of the systems of organs within this arche- 

 type is essentially the same as in the Vertebrata and Annulosa ; that 

 is to say, supposing the digestive system to form the axis of the body, 

 the nervous centre lies on one side of that axis ; the blood-vascular 

 centre upon the opposite ; and furthermore, the archetype is sym- 

 metrical with regard to a longitudinal vertical plane, passing through 

 these three. 



3. The molluscous archetype differs from the vertebrate in the 

 circumstance — 1, that the mouth opens upon the neural surface; 2, 

 that the embryo commences its development upon the hsemal side. 



It differs from the articulate archetype in the latter circumstance, 

 and from both in the fact, that tbe proper appendicular system (re- 

 presented by the epipodium) is almost rudimentary, and that the 

 locomotive function is mainly performed by a development of the 

 neural surface. 



4. The process of concentration and fusion of parts by which the 

 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 with 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 shows 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 j^ost-abdomen, the latter an abdomen ; there will 

 then be two great divisions of the cephalous mollusca. 



a. Those M'hich develope an abdomen : — Cephalopoda, Pteropoda, 

 Pulmonata. 



b. Those which 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- 



