THE MORPHOLOGY OF THE MOLLUSC A. 49 



Development of the pleural ganglion. — That the pleural 

 ganglion is essentially distinct from the pedal is, I think, 

 sufficiently clear from the facts of development. Although 

 these ganglia are placed so close together and are so inti- 

 mately connected in the lower Gastropods there is not a 

 single case on record in which the pleural ganglion has been 

 observed to arise from the pedal ganglion, or from a 

 common pleuro-pedal rudiment in the embryo. It is 

 equally true on the other hand that Sarasin's derivation of 

 the cerebral and pleural ganglia from a common rudiment 

 in Bithynia (the cephalic sense-plate) has been opposed by 

 v. Erlanger, who shows that all the great ganglionic centres 

 arise separately, and do not become connected with one 

 another until after their differentiation (7). 



A renewed investigation of the origin of the cerebro- 

 pleural ganglion in Pelecypoda would be of great interest 

 in this connection. Pelseneer's ( 1 8) observations on Nuctila 

 have placed the fact of the composite nature of this ganglion 

 in Pelecypoda beyond all doubt ; and still, to the best of 

 my knowledge, no one has yet observed the appearance in 

 the embryo of a pleural element distinct from the main body 

 of the ganglion. This apparent community of origin of the 

 cerebral and pleural ganglia in Pelecypoda may be compared 

 with the direct continuity of the cerebral and pleural 

 elements of the nervous system in Amphineura. 



Development of the visceral ganglia. — Sarasin en- 

 deavoured to show that the visceral ganglia of Bithynia, 

 together with the pedal and abdominal ganglia, arise in the 

 embryo from a common ventral proliferation of the ectoderm 

 which he compares with the ventral ganglionic chain of 

 Annelida. On this point also Sarasin has been corrected 

 by v. Erlanger, who shows that all these ganglia arise 

 separately from one another in Bithynia (7), as well as in 

 Palndina (6). 



The visceral ganglia are also quite distinct from the 

 pleural ganglia in their origin, as v. E Hanger's observations 

 show. In one important respect, however, the visceral 

 ganglia and the pleural ganglia betray a marked similarity, 

 the significance of which seems, however, to have escaped 



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