206 J. GRAHAM KERR. 



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-homo- 

 logy. 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 

 eusheathed in a continuous layer of nerve-cells ; and any depar- 

 ture from this condition, in the direction of collecting and 

 centralizing these nerve-cells into ganglia to fulfil local require- 

 ments, is a process which has taken place independently within 

 each of the two stems of descent. 



It follows, from this independence in phylogenetic develop- 

 ment of these secondarily formed ganglia, that we are not 

 justified in taking any one of the ganglia of the higher Cepha- 

 lopods and saying this is the "pedal" ganglion (implying in the 

 term "pedal" accurate homology with the so-named ganglia of 

 Gasteropoda) — a fortiori, in asserting here is an organ inner- 

 vated by the pedal ganglion, therefore it is morphologically part 

 of the foot. Yet it is precisely this latter line of argument which 

 modern 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-oesophageal, and 



(3) A posterior sub-oesophageal mass. 



To these is added in the Dihranchiata a separate nervous 

 mass lying in front of (2) — the branchial ganglion ; and it is 

 this which innervates the arms. 



To quote Pelseneer (Ghall. Rept. p. 65): — 



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

 all recognising 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 Mollusca. 



