THE MORPHOLOGY OF THE MOLLUSC A. 57 



while the latter became more and more extended pari passu 

 with the elongation of the trunk. Such a separation is to 

 some extent paralleled in the development of Holothurians 

 from the Auricularia larva, as described by Semon. On 

 this theory alone can I explain to myself the absence of the 

 ancestral nerve-ring in the trochospheres of Mollusca, and 

 I find some support for this view in the ontogeny of Nemer- 

 tines. The lateral nerve-cords in this group have the same 

 relation to the gut and brain as have the pleuro-visceral 

 cords of Chiton, since they form a dorso-lateral ring, the 

 posterior commissural portion passing above the rectum. 

 In Nemertines there can be very little doubt that this 

 nerve-ring has been derived phyletically by the elonga- 

 tion of a nerve-ring which underlay the ciliated band of a 

 more or less Pzlidzum-like ancestor, as it underlies the 

 ciliated band of the Pi/idzum-laxva., although this phyletic 

 origin is disguised by the profound metamorphosis which 

 breaks the continuity of the ontogenetic record in Nemer- 

 tines. On this theory of course the lateral cords of Nemertines 

 do not correspond to the ventral cords of Annelids. The 

 latter are represented by the general ventral plexus of 

 Nemertines and by the pedal plexus or cords of Mollusca. 

 These ventral nervous systems appear to bear relations to 

 the dorso-lateral ring-nerve similar to those of the subum- 

 brellar plexus of Medusae to the circumferential nerve-ring. 

 It will be recognised from these remarks that the 

 conclusions to which I have arrived present distinct points 

 of agreement with those of Balfour (1, p. 37&) and Sedg- 

 wick (21) on the same subject, although attained throughout 

 by an independent series of inductions. With both these 

 writers I agree in tracing back the Molluscan nervous 

 system to a primitively annular type, such as might be 

 expected to exist in a Ccelenterate ancestor. Balfour 

 derives the whole Molluscan nervous system from a 

 peripheral nerve-ring which followed the course of a hypo- 

 thetical ciliated ring: ; Sedgwick derives it from a broad 

 plexus surrounding an elongated blastopore, such as occurs 

 in existing Actinians. Sedgwick's theory was practically 

 an alternative to Balfour's, but I find myself able to give a 



