STRUCTURE OF THE LEECH. 



217 



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, in part 

 tactile and in part visual, — a series which is full of suggestiveness in 

 regard to the evolution of sense organs (cf of the series sensitive set;e 

 in the crayfish. "The visual organs of the leech are not able to form 



Fig. 70. 



-Transverse section of Leech. 

 A. G. Bourne.) 



(Simplified from 



,:., Cuticle; c, epidermis; cm., dermis and outer muscles (cir- 

 cul.Vr and oblique) ; /.;«., longitudinal muscles (the peculiar connec- 

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

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

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

 t., testes ;/., nejjhridial funnel ; v.d., vas deferens. 



images of external objects, but the animals are exquisitely sensitive to 

 alterations of light. 



The Alimentary System. 



When the leech has firmly fastened itself to its prey by 



the hind sucker, it brings its muscular mouth into action, 



pressing the lips tightly on the skin, and protruding three 



chitinous tooth plates which lie within. Each of these 



