193 



2. In the Heteropoda, the " foot,*' in its most perfect condition, 

 consists of three portions, a propodium, mesopodium and metapodium. 



3. The Heteropoda are more or less prosobranchiate, the degree 

 depending upon the amount of development of the post- abdomen. 



4. In the Pteropoda the intestine is bent towards the ventral or 

 neural side, in consequence of the development of the visceral " her- 

 nia" in front of the anus. It is therefore called an abdomen. 



5. In the Pteropoda, the foot, besides the parts mentioned above, 

 possesses an additional appendage, the epip odium, which forms the 

 expanded wing characteristic of the group. 



6. The Pteropoda are opisthobranchiate, prosobranchiate, or in- 

 termediate in character, according to the degree of development of 

 the abdomen. 



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 (6) 

 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 

 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 haemal side. 



It differs from the articulate archetype in the latter circumstance, 

 and from botii 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 



