150 HUGH WATSON. 



lina) and Paryphanta.^ It must be remembered that at 

 least some of these authors believed that the pleural ganglia 

 never did give rise to nerves in the Pulmonata, a belief 

 that Amaudrut- has shown to be altogether erroneous. At 

 the same time it is unlikely that all these careful observers 

 would have figured the "nerves of the ueck" as arising 

 solely from the pedal ganglia, if in all carnivorous forms they 

 arise partly from the pleural ganglia, as they undoubtedly 

 do in Apera. Moreover the fact that the central roots of 

 these nerves arise in Apera exactly from the junction of 

 the pedal and pleural gauglia also suggests that the more 

 posterior " nerves of the neck " arise indifferently either 

 from the outer side of the pedal ganglia or from the plem-al 

 ganglia. This seems to uphold the theory that the outer 

 dorsal portions of the so-called pedal ganglia of most Gas- 

 tropods are in reality parts of the pleural nerve-centres 

 which have become united with the pedal ganglia, and that 

 all the ''nerves of the ueck" therefore spring from the 

 pleural division of the central nervous system. The composite 

 nature of the pedal ganglia has been shown to be specially 

 evident iti some of the most primitive Gastropods, such as 

 Pleurotomaria; -^ and the fact that the so-called pedal 

 commissure contains, as we have seen, nerve-fibres emanating 

 from the pleural ganglia affords additional evidence in favour 

 of this theory.'* 



The nerves arising from the visceral ganglia are fewer 



' For references, see p. 148. 



2 ' Ami. Sfi. Nat., Zool.,' 1898, vol. vii, p. 128. 



^ Bouvier and Fischer, ' Journ. de Concliyl.,' 1899, vol. xlvii, pp. lU9- 

 143. 



■* It is interesting to note that in Aplysiella and some species of 

 Aplysia, as well as in the Gymnosomata, the cervical nei-ves also 

 arise partly from the pleural and pai'tly from the pedal ganglia, although 

 they anastomose to form a single plexus (Pelseneer, " Mem. Couronne 

 Acad. Roy. Belg.,' 1893, ex vol. liii, p. 27, pi. x, fig. 81); while in the 

 AuriculidEe these nerves may arise either from the pleural ganglia or 

 from the pleuro-pedal connectives (Bonvier, ' Comptes Rendus Soc. de 

 Biologic' (9th ser.), 1892, vol. iv. p. 990). 



