TETHYS CERVINA DALL AND SIMPSON 47 



The fourth, or optic nerve (c. 4) is slender, long and un- 

 branched. It arises from the dorsal margin of the ganglion and 

 passes outward and upward to the eye. No special optic gan- 

 glion can be made out at its base without serial sections. 



The fifth cerebral nerve (c. 5) arises from the postero-lateral 

 face of the ganglion, immediately above the origin of the cerebro- 

 buccal connectives. It is a rather slender nerve, passing forward 

 and ramifying to the muscles of the mouth region. That of the 

 right side sends, in addition, a branch to the penis. 



The sixth cerebral, or auditory nerve, is closely associated 

 with the cerebro-pedal connectives and is not visible in dorsal 

 view. It arises close to the base of the connective, and follows 

 it to the upper face of the pedal ganglion where it terminates in 

 the otocyst, being throughout its course inclosed in the connective 

 tissue sheath of the cerebro-pedal connective (c. p. con.). 



Three sets of connectives arise from the cerebral ganglia, 

 the cerebro-buccal, the cerebro-pleural and the cerebro-pedal. 

 The first named pair is the longest, arising from the outer ventral 

 margins of the ganglia, and encircling the anterior end of the 

 esophagus to unite with the buccal ganglia beneath. They are 

 not visible in fig. 34. The cerebro-pedal connectives (c. p. con.), 

 pass obliquely outward and backward from their origins upon 

 the postero-ventral margin of the cerebral ganglia to the pedal 

 ganglia (ped. g.). They are short and thick, and are inclosed 

 in a strong connective tissue sheath with the cerebro-pleural con- 

 nectives (c. pi. con.), which are of nearly equal diameter, but less 

 long. 



Pedal ganglia. The pedal ganglia (ped'. g.), are rounded 

 flattened structures, measuring 1.4 mm. in diameter and 0.6 mm. 

 in maximum thickness. They are connected below the esophagus 

 by a strong commissure (p. com.), i.o mm. long and 0.15 mm. 

 wide. A much longer and quite slender parapedal commissure 

 (p. p. com.), arising from the lower posterior margin of each 

 ganglion also unites the two. Upon the upper margin of the 

 pedal ganglia are received the cerebro-pedal connectives and just 

 behind them the very short pleuro-pedal ones. From each gan- 

 glion ten nerves are given off. These will be described for the 

 right side, any difference which may exist upon the opposite one 

 being noted. The nerves are taken in order and numbered in the 



