| OCR Text |
Show 1895.] ANATOMY OP NAUTILUS POMPILIUS. 683 Pelseneer then goes on to combat the view that the brachial ganglion has been derived from the fusion of a downgrowth on each side from the cerebral ganglion. While protesting, in passing, against the statement that the supra-cesophageal nerve-mass is formed of " the fused cerebral ganglia," when in reality it represents the primitive nerve-mass out of which " cerebral ganglia " have not yet become segregated, it is (2) the statements as to the " pedal" and brachial ganglia which concern most closely the point under discussion. The one fact of independence of evolution is enough to show that the so-called pedal ganglion of Cephalopods-i. e. the anterior sub-cesophageal nerve-mass of Nautilus, which iu the higher Cephalopods lias, in accordance with a very general law, become condensed into a definite ganglion, supplying the various organs originally in its neighbourhood-is not in the strict morphological sense the " pedal" ganglion at all. One may then accept with Pelseneer the development of the brachial ganglion by splitting off from this anterior sub-cesophageal nerve-mass, and yet be as completely without evidence as w e were before that the structures supplied by it have anything whatever to do with the foot. In brief it appears to m e that:-the general relations of the parts point undoubtingly to the arms of Cephalopods being processes of the head-region-that all the special evidence brought forward to support the pedal view is either erroneous, of little weight, or is permeated with fallacy-and that it therefore behoves us in the meantime to unhesitatingly accept the first mentioned1. IX. The Phylogenetic Relationships of the Cephalopoda. From its archaic character Nautilus might be expected to give valuable hints as to the phylogenetic relationships of the group to which it belongs. Upon the whole it appears to m e that its structure affords strong evidence that the nearest living allies of the Cephalopoda are to be found in the Amphineura. And it is interesting to note that amongst these it is the Chitons in which the points of resemblance are most striking, as they are apparently the oldest and most primitive members of the group. The number of really important morphological features in which the Chitons resemble Nautilus is really remarkable, e. g.- (1) Its bilateral symmetry. (2) The general characters of its nervous system. (3) Its possession of paired metamerically arranged ctenidia, of which in some species, believed to be phylogenetically younger, there is a tendency for those at the anterior end of the body to disappear-only those towards the posterior end persisting (mero-branchiate forms). 1 The forerunner of the hood-tentacle complex of Nautilus (and consequently of the arms of the Dibranchiata) we may probably see still persisting in the similarly innervated and highly sensitive mass which surrounds the mouth in Chiton. |
