194 



GASTKOPODA. 



stoniodaeuni and the radular sac and form the rudiments of the 

 buccaWganglia. 



The formation of these ganglia recalls to some extent that of the frontal 

 ganglion in the Insecta, which also arises from the stomodaeum (vol. iii., 

 p. 328). 



The pleural ganglia form, in Paludina and Byihinia, as two lateral 

 ectodermal thickenings lying somewhat ventrally behind the velum 



a. 



-'Mi nm . 



Fig 88 —Transverse sections through the anterior region of two embryos of Paludina 

 at the stage represented in Fig. 59, .1 and B, p. 139 (after v. Ehlangeb). bg, , buccal 

 cano-lia ■ eg, cerebral ganglia ; mes, mesodermal tissue ; oes, oesophagus ; ot, otocyst ; 

 p, pedal ganglia; pi, pleural ganglion; r, radular sac; t. rudiment ot tentacle; 

 us, primitive sinus ; ,-. velum. 



(Fig. 38 A, pi), and the two visceral ganglia also arise ventrally 

 and laterally, though farther back than the pleural ganglia (v. 

 Eklanger). They lie near the edge of the mantle, and near the 



