386 Royal Society. 



The Heteropoda and Pteropoda, then, may be considered to repre- 

 sent two opposite phases of the modification of the molluscous 

 archetype. 



In the second part of the paper, the author endeavours, by care- 

 fully collating the known facts of the development of the Mollusca, 

 to ascertain (a) the primary form of all cephalous Mollusca, and (b) 

 the mode in which,, in the course of development, this embryonic 

 form becomes metamorphosed into the adult form ; in order, if pos- 

 sible, to account, on the safe basis of ascertained morphological laws, 

 for the peculiar modifications of structure which have been found, 

 anatomically, to obtain among the Heteropoda and Pteropoda. 



He finds that it is possible not only to deduce the structure of the 

 Heteropoda and Pteropoda from a simple and symmetrical archetype 

 by such morphological laws, but that all the cephalous Mollusca 

 fail 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 ail 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 difl^'ers from the vertebrate in the 

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

 that the embryo commences its development upon the haemal side. 



It differs from the articulate archetype in the latter circumstance, 

 and from both in the fact, that the 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 develojjed is invariably a portion 

 of the haemal 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, haemad or neurad, and these flexures correspond with the 

 development of a post-abdomen or abdomen, respective^. 



7. The process of development demonstrates that the Tectibran- 

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



