274 



PHYLUM ANNELIDA 



Special nerves from the dorsal ganglia supply the ali- 

 mentary 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^blackTspots. Microscopic 



Fig. 147. — Transverse section, of leech. — After Bourne. 



c, 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., ventral sinus enclosing nerve-cord (n.) ; g., 

 median part of crop, with lateral pockets (p.) ; t., testis ; /., nephridial 

 ' funnels ; v.d., vas deferens. 



examination shows them to be definite cups, surrounded by 

 connective tissue with black pigment, and containing clear, 

 strongly refracting cells, each in connection with a fibre of 

 the optic nerve. 



It has been shown (Whitman) that the eyes of leeches are serially 

 homologous with the segmental sense organs. At the one extreme 

 there are purely tactile organs, at the other extreme there are purely 

 visual organs, and between these there are compound sense organs. 



