ON THE RIORPHOLOGV OF THE CEPHALOUS MOLLUSCA 1 75 



branchial apertures. However, as T have endeavoured to show else- 

 where,! whatever be the position of the anus in the Ascidians, the 

 first bend of the intestine is always haemal. I have already referred 

 to their probable analogy with Spinalis in this respect. 



Having now determined the changes which take place in the axis 

 of S}'mmetry of the Mollusca, let us examine into those undergone by 

 their principal external organs. 



^^'hether we have to do \\\i\\ a Cephalopod or with an ordinary 

 Mollusk, the first step in development is the separation of the blasto- 

 derm into a central elevation, the mantle, and certain lateral portions. 

 Now these portions become in the Gasteropoda, the head and foot ; in 

 the Cephalopoda, the head and arms. It follows, therefore, that the 

 arms of the Cephalopod are homologous with the foot of the Gasteropod. 



Again, in the Cephalopod an eminence becomes developed along two 

 lines, which run on each side of the upper part of the " lateral expan- 

 sions " and meet behind the head ; along the anterior portion of this 

 line the eminence remains as a slight ridge, which afterwards becomes 

 one of the muscles of the funnel ; along the posterior portion of the 

 line a considerable process is developed, and, uniting with its fellow, 

 becomes the funnel. 



In the Gasteropod, it is along the anterior halves of two corre- 

 sponding lines that processes are developed, \vhich become the ciliated 

 ala; or vela of the embryo. The line in question I propose to call the 

 epipodial line, and whatever is developed along that line I consider to 

 be the epipodium, or a portion of it. I do not venture upon such a 

 refinement at present, but I think it probable that, as we have distin- 

 guished three portions in the foot, so it will be necessary to distinguish 

 three portions in the epipodium ; anterior, middle, and posterior. For 

 instance, in the Cephalopoda the posterior portion only is developed as 

 the funnel ; in the Gasteropod larva: the ciliated vela are the homo- 

 logues of its anterior portion. The palmated lobes of the Turbinida;, 

 the " lobes of the mantle " of Aplysia, appear to be developed from the 

 whole epipodial line, while it is apparently the middle epipodium alone 

 which is developed into the " wings " of the Pteropoda. 



All traces of the epipodium appear to have vanished in the majority 

 of the Pectinibranchiata.^ 



1 Upon the Anatomy and Physiology of Salpa and Pyrosoma, Philosophical Transactions, 

 1851, and in a "Report upon the Structure of the Ascidians," read before the British 

 Association, September, 1852. The law as regards the Polyzoa was first enunciated by 

 Professor AUman, " On the Homology of the Organs of the Tunicata and Polyzoa," Trans. 

 Royal Irish Acad. vol. xxii. 



2 Leuckart and Loven have enunciated very different views with regard to the homologies 

 of the external organs of the Mollusca, to which it seems proper I should refer. 



