THE MORPHOLOGY OF THE MOLLUSC A. 53 



a pair of post-anal prolongations of the visceral nerves 

 precisely resembling those described by Kerr in Nautilus ; 

 yet in Dentalium, owing to the smaller degree of concen- 

 tration or cephalisation which has taken place in the 

 nervous system, it is easy to see that the typical sub-intes- 

 tinal visceral commissure exists as in Gastropods and 

 Pelecypods. The posterior sub-cesophageal nerve-mass 

 of Cephalopods has clearly been produced, not, as Mr. 

 Kerr suggests, by a secondary fusion of the pleuro- visceral 

 nerve-masses of the two opposite sides, but by a simple 

 shortening of the visceral loop as it occurs in Dentalium. 

 This would bring the visceral ganglia into continuity 

 with the pleural ganglia and with one another, — a process 

 of condensation with which we are already familiar in the 

 Tenioglossa and the Euthyneura among Gastropoda. 



It may here be mentioned that Willey's simultaneous 

 account (26) of the visceral nerves of Nautilus, while con- 

 firming Mr. Kerr's observations as to the existence of post- 

 anal prolongations of a pair of visceral nerves, differs from 

 his statement as to their origin. Willey states that the 

 nerves supplying the post-anal papilla arise independently 

 from the sub-cesophageal visceral loop, although at their 

 origin they are adjacent to the branchial nerves and for a 

 large part of their course are actually contiguous with them. 

 The significance of this separation is not remarked upon by 

 Willey ; but if the separation really exists it is certainly a 

 difficulty in the way of his contention that the post- 

 anal papilla represents an approximated posterior pair of 

 branchial sense-organs, since the anterior osphradium and 

 both gill-plumes are all innervated from the outer visceral 

 nerve. 



Etithyneurism. — Since the publication of Spengel's paper 

 on the olfactory organ and nervous system of Mollusca, a 

 division of the Gastropoda into two groups, the Strep- 

 toneura and the Euthyneura, has been generally adopted. 

 This classification has been accepted, moreover, not merely 

 as an expression of the anatomical facts concerning the 

 condition of the visceral loop in the two groups, but as a 

 classification of phylogenetic significance. It is to be in- 



