THK LTFE-HISTORY OF NUCULA DELPHINODOKTA. 373 



commissure. The commissure that connects the two cere- 

 bral ganglia is a broad, somewhat ilattened band, that passes 

 between the oesophagus and the anterior adductor muscle, 

 and joins their anterior ends. The cerebral ganglia give rise 

 to a number of nerves. A large nerve leaves the ventral side 

 of each near its anterior end, passes ventrally along the 

 posterior and ventral surfaces of the anterior adductor 

 muscle, to which it sends branches, and is distributed to the 

 anterior and ventral portions of the corresponding lobe of 

 the mantle. Just posterior to the origin of the pallial nerve, 

 and a little closer to the median line, another nerve, about 

 equal to the pallial nerve in size, leaves each cerebral 

 ganglion. This nerve follows along the fold of tissue that 

 suspends the labial palps and is continued into the palp 

 appendage. Other nerves from these ganglia are distributed 

 to the visceral mass and to the dorsal portions of the foot 

 muscles. Posterior and still further toward the median line 

 than the palp nerve, the two portions of each cerebro-pedal 

 commissure leave each cerebral ganglion, one a little ante- 

 rior and ventral to the other. The two portions run poste- 

 riorly a short distance, and join to form a single commissure 

 that is continued to the pedal ganglion of the same side. A 

 nerve leaves each cerebro-pedal commissure dorsal to the 

 corresponding otocyst, and is continued to it. This nerve is 

 generally supposed to have its origin in the cerebral ganglion, 

 and the angle at which it issues from the commissure indi- 

 cates that this is probably the case. The otocystic nerve is 

 about equal in size to the posterior division of the cerebro- 

 pedal commissure. Stempell (18) finds that each otocystic 

 nerve of Solemya to gat a leaves the cerebral ganglion 

 direct, and runs an independent course to the otocyst. He 

 also finds that each cerebro-pedal commissure leaves the 

 cerebral ganglion as a single strand. He thinks that this is 

 a double commissure, because it receives fibres from what he 

 considers cerebral and pleural ganglia. 



It seems more likely to me that the nervous systems of all 

 molluscs have been derived from some such a generalised 



