208 J. GRAHAM KERR. 



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 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 quite 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). 



(4) The traces of metamerism exhibited by the heart in 

 some forms, there existing several pairs (four in 

 Chiton magnificus) of auriculo-ventricular openings. 



(5) General relations of coelom, nephridia, &c. 



(6) Eggs developed within follicles. 



In regard to (5), fig. 6 indicates diagrammatically the 

 relationships of the parts concerned. In the case of Chiton (B) 

 two coelomic chambers are shown, one lying in front of the 

 other — the genital coelom and the pericardium. The pericar- 

 dium communicates with the exterior by a pair of functional 

 nephridia; the genital ccelom by the pair of genital ducts 

 which from their relations can hardly be otherwise than 

 morphologically a pair of nephridia too. In Chcetoderma 

 (A), a less primitive animal, a less primitive arrangement has 



