1895.] ANATOMY or NAUTILUS POHPIIIUS. 683 



Pelseneer then goes on to combat the view that the brachial 

 ganglion has been derived from the fusion of a dovvngrowth on 

 each side from the cerebral ganglion. 



While protesting, in passing, against the statement that the 

 supra-oesophageal 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 gangUon of Cephalopods — i. e. the anterior sub- 

 oesophageal nerve-mass of Nautilus, which in the higher Cepha- 

 lopods has, 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-oesophageal nerve-mass, and yet be as 

 completely without evidence as we were before that the structures 

 supplied by it have anything whatever to do with the foot. 



In brief it appears to me 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 mentioned ^. 



IX. The Phylogenetic SelationsMps 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 me 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-hranchiate 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 highlv sensitive mass which surrounds the mouth in 

 (^hiton. 



