52 SCIENCE PROGRESS. 



At the same time when Pelseneer added a paragraph to 

 the effect that the supra-rectal commissure of the Amphi- 

 neura is also a merely secondary junction of the pallial 

 nerves, he was probably not yet acquainted with Haller's 

 work on Lottia, and allowed his views upon the Polychsete 

 ancestry of the Mollusca to bias his interpretation of the 

 Molluscan nervous system. 



In a recent paper on the anatomy of Nautilus Mr. 

 Graham Kerr (13) also refers to the question of the supra- 

 rectal commissure. It will be remembered that in Nautilus 

 the pleuro-visceral ganglia of the two sides form a stout 

 ganglionic band encircling the oesophagus in the region of 

 the cerebral ganglia. The pallial nerves radiate from the 

 lateral portions of this half-ring, and the pair of visceral 

 nerves arise from the ventral portion. The visceral cords 

 pass backwards on either side of the vena cava, and, after 

 giving off the branchial nerves, are prolonged posteriorly as 

 far as the post-anal papilla, behind which Mr. Kerr has 

 recognised an apparent anastomosis. Mr. Kerr adds that 

 in this case " the homologue of the pleuro-visceral cord of 

 Chiton is not merely the posterior sub-cesophageal nerve- 

 mass, but rather the two lateral portions of this, together 

 with the post-branchial prolongations which run on either 

 side of the vena cava. The mesial part of the posterior 

 sub-cesophageal nerve-mass would therefore be a secondary 

 fusion between the nerve-masses of the two opposite 

 sides." 



In his suggested homology of this possible post-anal 

 {i.e., supra- rectal) commissure of the visceral nerves in 

 Nautilus with the supra-rectal " commissure " of Chiton, 

 Mr. Kerr has undoubtedly failed to appreciate the true 

 nature of the posterior sub-cesophageal loop of Nautilus, as 

 well as the relation of the visceral nerves to the pleuro- 

 visceral cords of Chiton. The explanation of the Cephalo- 

 pod nervous system is most readily found by comparing it 

 with that of Dentalium, whose organisation in many respects 

 supplies connecting links between that of the Cephalopoda 

 and that of the primitive prae-torsional Gastropod or 

 primitive Pelecypod. In Dentalium (22, p. 401) we find 



