BUR.NE : ON THE NERVOUS SYSTEM IN PELECYPODA. 47 



be observed in tbe most generalised living Pelecypods, I tbink tbere 

 can be no doubt tbat tbe early progenitors of tbe Pelecypods and 

 Gastropods were already distinct before tbere was any question of 

 specialised ganglia in the cerebro-visceral part of tbe nervous system. 

 This can be inferred from the occurrence at the present time, in each 

 of these groups, of genera (e.g. Pleiirotomaria, Nucula) in which this 

 part of the nervous system shows only the very slightest indication of 

 specialisation into individual ganglia.^ Now each of these forms is 

 far more specialised and typical of its class than its ancestors could 

 have been when first they diverged from the common molluscan stock. 

 So that it is legitimate to assume that at tbat time the cerebro-visceral 

 system was an entirely unspecialised ganglionic loop. 



Thus tbe facts at oiir disposal, and the inferences to be justly drawn 

 from them, seem strongly to favour Drew's general conclusion, that 

 the nervous systems of Gastropods and Pelecypods have arisen in 

 a common generalised form, probably of tbe Amphineuran type, and 

 tbat each class has developed ganglia independently, according to 

 its needs. 



To return finally to Mr. Latter's letter. It is clear that according 

 to Drew's views set forth above, ganglia appearing sporadically upon 

 tbe visceral connectives would have no direct homology to any 

 particular ganglion of normal occurrence in this or any other group 

 of molluscs, but would be a local retention of the ganglionic condition 

 once common to the whole cerebro-visceral system, and are thus 

 , a reversion, not to a more specialised, but to a more generalised 

 condition. 



^ These nervous systems certainly do not represent the degenerate condition of a once 

 more specialised type. Degeneration from a state with specialised and separate 

 ganglia could hardly result in the formation of continuously ganglionic cords. 

 Such are a sure sign of primitive generalisation. 



