232 PROFESSOR r. m. balfour. 



which appears to supply the integument of the dorsal part of 

 the head (PI. XV, fig. 8 ; PI. XVI, figs. 11—14 d n). 



Sympathetic System. — In addition to the nerves just de- 

 scribed there are two very important nerves which arise near 

 the median ventral line, close to the hind end of the supra- 

 oesophageal ganglia. The origin of these two nerves is shown 

 in the surface view (fig. 22 s y, and in section in fig. 11). They 

 at first tend somewhat forwards and pass into the muscles near 

 the epithelium lining the groove on each side of the tongue. 

 Here they suddenly bend backwards again and follow the 

 grooves into the pharynx. 



The two grooves are continuous with the two dorsal angles 

 of the pharynx ; and embedded in the muscles of the pharynx, 

 in juxtaposition with the epithelium, these two nerves may 

 easily be traced in sections. They pass backwards the whole 

 length of the pharynx till the latter joins the oesophagus. 

 Here they at once approach and shortly meet in the median 

 dorsal line (fig. 16). They can only be traced for a very 

 short distance beyond their meeting point. These nerves are, 

 Avithout doubt, the homologues of the sympathetic system of 

 Chsetopods, occupying as they do the exact position which 

 Semper has shown to be characteristic of the sympathetic 

 nerves in that group, and arising from an almost identical part 

 of the brain. 1 



Histology of the Nervous System. 



Ventral Cords. — The histology of the ventral cords and 

 oesophageal commissures is very simple and uniform. They 

 consist of a cord nlmost wholly formed of nerve-fibres, placed 

 dorsally, and a ventral layer of ganglion cells (figs. 16 and 20). 



The fibrous portion of the cord has the usual structure, being 

 foi-med mainly of longitudinal fibres, each probably being a 

 bundle of fibres of various sizes, enveloped in a sponge-work 

 of connective tissue. The larger bundles of fibres are placed 



1 Vide Spengel, ' Oligognatlius Bouellitr, Naples Mittbeilungen,' Bd. iii, 

 pi. iv, fig. 52. 



