806 AFFINITIES. 



It is difficult to know how to regard the Aplacophora {Neomenia, Chaetoderma), but 

 as for Chiton itself, the constant octomerism of the shells seems to indicate that it is 

 a highly finished type which has retained primitive relations of main axis and bilateral 

 symmetry. 



Kerr holds that the Amphineura are the nearest living allies of the Cephalopoda, 

 and he gives instructive diagrams of their coelomic properties. In respect of the mutual 

 relations between coelom and haemocoel. Nautilus is particularly interesting, and in respect 

 of its possession of a spacious perivisceral coelom incompletely divided into two halves 

 by a median mesentery, the genito-intestinal ligament, Nautilus seems to present a more 

 primitive organisation than any other existing mollusc. 



Grobben (1884-1886) was of opinion that the Scaphopoda {Dentalium) were the 

 nearest living allies of Cephalopoda, but this view has not been sustained by Simroth 

 in his most recent treatise on the former group in Bronn's Thierreick. 



For my part I have found, during a somewhat limited experience of malacological 

 matters, that a comparison of Nautilus with the prosobranchiate Gastropods, more especially 

 with Haliotis, seems to throw considerable light upon the organisation of the former genus 

 and indirectly of the dibranchiate Cephalopods. 



The torsion of the palho-visceral region of Haliotis with the correlated streptonexirism 

 of the visceral commissure, in spite of the profound effect it has had upon the facies of the 

 animal, may be regarded from a morphological standpoint as an incidental phenomenon 

 equally with the pallio-visceral flexure of Nautilus. 



If abstraction be made of the torsion of Haliotis and the flexure of Nautilus the 

 general resemblance between the two genera can be more readily conceived, and the 

 relation of the Cephalopod to the Gastropod would appear in the same light as that 

 which a Hexapod insect bears to a MjTiapod, or a Decapod Crustacean to a Phyllopod. 



If we take into consideration the analogous phenomena of cephalisation in different 

 groups of coelomate animals, it becomes evident that the general tendency is to evolve a 

 composite head and a concentrated central nervous system. In this sense the cephalopodium 

 of the most highly organised group of MoUusca is analogous with the cephalothorax of 

 a crab or a lobster. There is thus undoubtedly a prima facie case for the composite 

 nature of the cephalopodium, tending to exclude the idea that this structure can be 

 merely a derivative of the head without the co-operation of any part of the trunk. 



I have endeavoured to show that Nautilus presents certain features of organisation 

 which have become obliterated in the Dibranchs, but which are of crucial significance 

 in regard to the interpretation of the organisation of Cephalopoda, and this without 

 regard to the diplomerism and perivisceral coelom of Nautilus. Chief among these 

 nautiline characteristics, I reckon the remarkable tilting of the cephalopodial axis at an 

 angle to the skeletal axis (PI. LXXXI. fig. 1). If this is a sound description of facts 

 it must be of importance in itself, and especially because no such conflicting axes can be 

 observed in the Dibranchs. I interpret it as meaning that a tentaculiferous epipodium 

 has coalesced with the head and grown round the buccal cone both above and below, 

 while the protopodium has suffered a greatly inferior modification of structure, simply 

 having its margins folded over to form a fuimel. 



In the commonly accepted orientation of Cephalopoda, no difference is recog;nised 



