It seems that students of Mollusca quite commonly hold that 

 the lamellibranch ganglia have been derived from a gastropod- 

 like type, a type that possess at least one pair of ganglia, the 

 pleural, that are not commonly found in lamellibranchs. This 

 view seems to be based largely upon the acceptance of a hypo- 

 thetical type for a primitive mollusk that seems to me to be a 

 much better ancestor for the gastropods than for the other classes 

 of tfye Mollusca. The discussion of this hypothetical form may 

 be left for another place, but the discussion of the nervous sys- 

 tem properly belongs here. About all of the actual evidence 

 that we have of the presence of pleural ganglia in lamellibranchs 

 is that in Nucula (22) and some other forms the anterior gan- 

 glionic mass is so shaped that it is possible to consider it as 

 two ganglionic masses, and further that the connective that runs 

 from this mass to the pedal ganglion is connected with this 

 mass by two roots. The interpretation (22) that has been put 

 on this is that the two apparent divisions of the ganglion repre- 

 sent respectively the cerebral and pleural ganglion, and that 

 the roots of the connective represent the cerebro-pedal and 

 pleuro-pedal connectives that have become fused before reach- 

 ing the pedal ganglion. My own view, discussed in another 

 paper (7) is that the apparent division into two ganglionic 

 masses is superficial, and due to the swellings accompanying the 

 origins of nerves, and that one of the cerebral ends of the con- 

 nective may be the central end of the otocystic nerve which 

 is fused for the greater part of its length with the connective, 

 but, unlike most forms, is free near the ganglion. This view 

 seemed to me most reasonable as Stempell (30) has found that 

 in Soleyma togata, a supposed near relative of Nucula, the oto- 

 cystic nerve arises directly from the cerebral ganglion and is 

 separate from the connective throughout its length. So far as 

 I know, the instance given by Stempell is the only one that has 

 heretofore been reported where the otocystic nerves originate 

 from the cerebral ganglia, and are free from the cerebro-pedal 

 connectives throughout their length. Pecten tenuicostatus has 

 the same arrangement. In this form the position of the ganglia, 

 connectives and otocysts is such that it is a very simple matter 

 for the otocystic nerves to make direct connection with the cere- 

 bral ganglia, but they do not join the ganglia at their nearest 



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