238 



PHYLUM ANNELIDA. 



supra-oesophageal) ganglia are connected with the most 

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

 a narrow nerve-ring surrounding the beginning of the gut 

 The sub-cesophageal ganglia represent about five pairs of 

 ganglia fused together. From the dorsal ganglia nerves 

 proceed to the "eyes" and anterior sense spots; from the 

 ventral centres the general body is innervated. Special 



id. 



FIG. 122. Transverve section of leech. After Bourne. 



C., Cuticle; e., epidermis; c.m., dermis and outer muscles (circular 

 and oblique); /.*., longitudinal muscles (the peculiar connective 

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

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

 nerve-cord (.); g., median part of crop, with lateral pockets (/*)> 

 /., testis ]f. t nephridial funnels ; v.d. t vas deferens. 



nerves from the dorsal ganglia 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 

 fifth skin-ring. The eyes are arranged round the edge of 

 the mouth, and look like little black spots. Microscopic 

 examination shows them to be definite cups, surrounded by 



