194 



GASTROPODA. 



stomodaeum and the radular sac and form the rudiments of the 

 buccal ganglia. 



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 Bythinia, as two lateral 

 ectodermal thickenings lying somewhat ventrally behind the velum 



FIG. 88. Transverse sections through the anterior region of two embryos of Palitdina 

 at the stage represented in Fig. 59, A and 7J, p. 139 (after v. ERLANGBR). bg, buccal 

 ganglia ; eg, cerebral ganglia ; mes, mesodermal tissue ; oes, oesophagus ; ot, otocyst ; 

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

 us, primitive sinus ; v. velum. 



(Fig. 88 A, pi), and the two visceral ganglia also arise ventrally 

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

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



