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Show 682 MR. J. GRAHAM KERR 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 wit, 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 w e 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)- a fortiori, in asserting here is an organ innervated by the pedal ganglion, therefore it is morphologically part of the foot. Tet 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 w e know of Nautilus, to consist primarily o f - (1) A supra-cesophageal mass, connected with (2) A n anterior sub-cesophageal, and (3) A posterior sub-cesophageal mass. To these is added in the Dibranchiata 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. 6 5 ) : - " Eegarding (1) there is no disagreement as to its nature, all recognizing in it the fused cerebral ganglia. (2) " H as been universally regarded as constituted by the pedal ganglia. (3) " Corresponds to the combined visceral ganglia of other Mollusca. "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." |
