SEGMENTED WORMS OR ANNELIDA. 



Nervous system and sense organs. — The nervous system 

 mainly consists of a pair of dorsal ganglia lying above the 

 pharynx, and of a double nerve-cord, with twenty-three 

 ganglia, lying along the middle ventral line. The dorsal (or 

 supra-cesophageal) ganglia are connected with the most 

 anterior (or sub-oesophageal) pair on the ventral chain, by a 



Fig. 91. — Transverse section of leech. — After Bourne. 



£., Cuticle ; e., epidermis ; cm., dermis and outer muscles (circular 

 and oblique) ; l.m., longitudinal muscles (the peculiar connective 

 tissue is hardly indicated) ; r.m., radial muscles; l.v., lateral 

 blood vessel ; d.s., dorsal sinus ; v.s. t ventral sinus enclosing 

 nerve-cord (m);^-., median part of crop, with lateral pockets (/.) ; 

 r., testis;/;, nephridial funnels ; v.d., vas deferens. 



narrow nerve-ring surrounding the beginning of the gut. 

 From the dorsal centres nerves proceed to the " eyes " and 

 anterior sense spots, from the ventral centres the general 

 body is innervated, and from the beginning of the ventral 

 chain special nerves supply the alimentary canal, forming 

 what is called a visceral system. 



The sense organs of the leech are ten so-called " eyes," 

 besides numerous sense spots usually occurring on every 



