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Show 1895.] ANATOMY OP NAUTILUS POMPILIUS. 681 are pedal in their nature, was based upon three different sets of evidence-to wit, those derived from (1) Their ontogenetic development; (2) Their innervation ; (3) Their homology with the sucker-bearing processes of the larval Pneumoderma. Of these (3) derived its force from the supposed pedal nature of the sucker-bearing appendages. However, it has n o w been satisfactorily shown1 that they are purely cephalic, and therefore this argument, if it be argument at all, tells precisely in the opposite direction. At present, therefore, the view that the Cephalopod arms are parts of the foot rests upon (1) and (2). In regard to (1), however, although it must be admitted that the facts of embryology do tend to bear up the view that the crown of arms is formed by an upgrowth from each side of the foot, it must be borne in mind how extremely unreliable any evidence, as to topographical relations, must be which is based on the phenomena exhibited in the development of enormously yolk-laden eggs. Therefore it appears that the only one of the three classes of evidence adduced above which can be considered of real weight, is that resting upon the innervatiou of the parts under consideration, and that this opinion is shared by other workers, is shown by its tendency in more recent writings to supplant the evidence derived from embryology. It appears, therefore, not inadvisable to submit this portion of the evidence to a short critical examination, to endeavour to ascertain whether it is equal to bearing the strain of acting as main support to a view which we have seen to be inherently improbable, on the evidence afforded by gross anatomical relations. And as a preliminary it may be well to look into the general ideas n o w held and taught by zoologists as to the general character of the Cephalopod central nervous system. In the latest text-book of Zoology (Lang, p. 722) one reads, "Das symmetrische Nervensystem aller Cephalopoden zeichnet sich durch die sehr starke Concentration der typischen Mollusken-ganglien, auch derjenigen der Visceralconnective, aus;"2 and this I think I may venture to say fairly represents the views held and taught by zoologists generally: that the Cephalopod central nervous system consists typically of three pairs of ganglia aggregated round the oesophagus, which ganglia are homologous with the three similar pairs of, say, a Gasteropod. That a certain rough resemblance does exist between the arrangement of the ganglia round the oesophagus of a Dibranchiate Cephalopod and that met with in many Gasteropods may be at once admitted; but when it comes to be a question of precisely homologizing the individual ganglia in the one case with those in the other, one has to do with a very different matter. Supposing, for a moment, the homology to hold, then one ought to find the resemblance most marked in those Cephalopods which phylogenetically most nearly 1 ' Challenger ' Reports: Pteropoda, Anatomy, p. 39. 2 The italics are mine, |
