Homologies of the Cephalopoda. 307 



cavity, and meet near its base. Thus the first portion of the 

 alimentary canal, as soon as it is formed, has the same direction 

 with respect to the mantle as in the Gastropods. The direc- 

 tion, therefore, that is normal is this one, namely direct into 

 the mantle-cavity, and not parallel to the edges, as Huxley's 

 diagram would make it ; and we must place the Cephalopod 

 for comparison with the Gastropod with the oesophagus in the 

 same direction, either both horizontal or both vertical. As 

 the line parallel to this on the cerebral side of the latter is 

 called dorsal, and the basis of the foot ventral, so in the 

 former the os sejrice lies on the dorsal surface and the funnel 

 along the ventral, while the shell of the Nautilus comes " be- 

 hind." 



And now as to the foot. There is this essential difference 

 between the foot of a Gastropod and the arms of a Cephalopod, 

 strongly insisted on by Grenadier (Zeitsch. fur wiss. Zool. 

 vol. xxiv., 1874), that the foot is an unpaired organ, being 

 situated in the median line. It shows a tendency to spread 

 forward and backward, but not laterally ; and where it is 

 divided the several parts succeed each other in a longitudinal 

 direction. This character is seen even in Lamellibranchs which 

 have a paired shell. Only the anterior portion in any mollusk 

 shows itself slightly bilobed. The arms of the Cephalopod, an 

 animal with a single shell, are, on the contrary, from their very 

 commencement, paired ; and they are thus lateral in a very dif- 

 ferent sense from that in which the foot is so. This is to me, 

 as it is to Grenadier, conclusive against their homology. The 

 one can only be compared (with Huxley) to the dorsal fin of a 

 fish ; the others with its paired fins. But we must seek light 

 also on this question from the relations of the nerve-ganglia. 

 On this point, too, there seems to be a conflict of opinion ; but 

 the testimony appears to me conclusive against the homology 

 I am disputing. In the first place, it might be said, the pedal 

 ganglia are paired, therefore the foot itself is in its nature 

 paired ; yet the buccal ganglia and some of the visceral gan- 

 glia are often paired ; and no one will assert the alimentary 

 canal to be any thing but a single organ. In the next place, 

 the normal arrangement of nerves in a Gastropod consists 

 of two cerebral ganglia above the oesophagus and two pedal 

 ganglia below, with which may be more or less united a pair 

 of splanchnic ganglia behind. The auditory organs are in 

 connexion with the pedal ganglia when not directly supplied 

 from the cerebral. There is thus but one nervous ring. Now 

 in the Nautilus* the ring is subdivided, and there are two sets 



* Owen, ' Memoir on the Pearly Nautilus,' 1832 ; Macdonald, Phil. 

 Trans. 1855. 



