682 MB. J. GEAHAM KEBB. ON THE [June 18, 



approach the common ancestral forms of Gasteropods. But what 

 are the actual anatomical facts ? — that in the Nautilus, the most 

 primitive and oldest Cephalopod now existent, such division into 

 three pairs of ganglia is completely absent. And then one might 

 turn to that Gasteropod (I here use the term in its wide sense) 

 which other evidence points to as having similarly to the greatest 

 extent retained such common ancestral conditions — to \\\t, Chiton. 

 And here again one finds a complete absence of segregation of the 

 central nervous system into its three pairs of ganglia, and in its 

 stead a central nervous system showing in many respects a strong 

 and fundamental resemblance to that of Nautilus. The facts of 

 Anatomy, then, are strongly opposed to any rough-and-ready 

 homologizing of the various ganglia of the higher Cephalopod 

 with those of the higher Gasteropod. One might go so far as to 

 say that they demonstrate their non-homology. The common 

 ancestor of Gasteropods and Cephalopods, so far as we can see, 

 possessed, as did and do so many other primitive forms, a nervous 

 system consisting of thick strands ensheathed in a continuous layer 

 of nerve-cells ; and any departure from this condition, in the 

 direction of collecting and centralizing these nerve-cells into 

 ganglia to fulfil local requirements, is a process which has taken 

 place independently -within each of the two stems of descent. 



It follows, from this independence in phylogenetic development 

 of these secondarily formed ganglia, that we are not justified in 

 taking any one of the ganglia of the higher Cephalopods and saying 

 this is the " pedal " ganglion (implying in the term " pedal " 

 accurate homology with the so-named ganglia of Gasteropoda) — 

 ci fortiori, in asserting here is an organ innervated by the pedal 

 ganglion, therefore it is morphologically part of the foot. Yet 

 it is precisely this latter line of argument which modem exponents 

 of the " pedal " hypothesis use as their mainstay. 



The central nervous system of Cephalopoda may be said, 

 according to what we know of Nautilus, to consist primarily of — 



(1) A supra- oesophageal mass, connected with 



(2) An anterior sub-cesophageal, and 



(3) A posterior sub-cesophageal mass. 



To these is added in the Bihrandiiata a separate nervous mass 

 lying in front of (2) — the brachial ganglion ; and it is this which 

 innervates the arms. 



To quote Pelseneer (Chall. Eept. p. 65) : — 



" Eegarding (1) there is no disagreement as to its nature, 

 all recognizing in it the fused cerebral ganglia. 



(2) " Has been universally regarded as constituted by the 

 pedal ganglia. 



(3) " Corresponds to the combined visceral ganglia of other 

 MoUusca. 



"All observers are agreed as to the interpretation of the 

 supra-oesophageal and the two posterior sub-cesophageal 

 masses {i. e. (2) and (3)). The disagreement relates only 

 to the brachial ganglia, which are regarded by one party as 

 pedal and by the other as cerebral." 



