56 SCIENCE PROGRESS. 



only by the decentralisation of the primitive 

 pleuro-visceral nervous system, and its separa- 

 tion into special ganglia and nerves, the latter 

 being formed ontogenetically as fibrous out- 

 growths from the ganglionic centres. 

 Venturing now, in conclusion, upon more speculative 

 ground, I believe that the embryonic relations, to which I 

 have drawn attention, between the pleural and visceral 

 ganglia and the ciliated band are of phylogenetic importance. 

 It has long puzzled me that the larval forms (trochospheres) of 

 two groups so closely allied as the Annelida and Mollusca, 

 while presenting a close similarity in general structure, 

 should differ so remarkably in regard to their nervous 

 system. The Annelid trochosphere has a nerve-ring 

 beneath its ciliated band, while the Molluscan trocho- 

 sphere has none. In this respect the Molluscan trocho- 

 sphere appears to be less primitive than that of the 

 Annelida. The explanation of this now appears to me to 

 be as follows. In the evolution of the Annelida the proto- 

 troch and nerve-ring remained for a long time unmodified, 

 and did not share in the elongation of the postero-ventral 

 region of the body which gave rise to the trunk of the 

 Annelid. This would explain the absence of the dorsal 

 nerve-ring in the adult Annelid, provided that the nerve- 

 ring, together with the prototroch, came to have merely a 

 larval significance, — as actually happens in the ontogeny of 

 Annelids to-day. On the other hand, in the evolution of 

 the Mollusca from the same simple type of ancestor, the 

 whole body must have shared in the elongation — the proto- 

 troch and nerve-ring as well as the more ventrally placed 

 parts of the body. This elongated nerve-ring I identify 

 with the pleuro-visceral ring of Amphineura, although the 

 phyletic connection between the nerve-ring and the ciliated 

 band is inferred from the development of certain Gastro- 

 pods rather than from the Amphineura themselves. As a 

 larval adaptation for conveniences of natation I imagine 

 that a separation became gradually effected in embryonic 

 life between the ciliated ring and the nerve-ring, the former 

 becoming restricted to the anterior end of the larval body, 



