194 



BULLETIN OF THE 



than the pedal ganglia (Plate VIII. Fig. 107), and the viscera lie rather 

 more to the left side of the body (Figure G). 



The central nervous system (Figure F) now consists of five well devel- 

 oped pairs of ganglia and an azygos ganglion (Figure G). The cerebral 

 ganglia with their commissure form the dorsal portion of three nerve 

 rings, the remainder of which are completed respectively, (1) by the 

 cerebro-pedal connectives, the pedal ganglia, and their commissures ; 

 (2) by the cerebro-pleural connectives, the pleural ganglia, the pleuro- 



Fioure F. — Posterior face of a transverse section from an embryo of the sixteenth day. X 70. 



visceral connectives, the visceral ganglia, the viscero-abdominal connect- 

 ives, and the abdominal ganglion ; (3) by the cerebro-buccal connectives, 

 the buccal ganglia, and their commissure. The first and second rings 

 are further joined to each other by means of the pleuro-pedal connect- 

 ives. Each of these three rings encircles the oesophagus. The posterior 

 end of the radula sac in the earlier stages, up to the present one, is 

 usually found to occupy a position above the pedal ganglia and their 



