HIRUDINEA OR DISCOPHORA. 



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 

 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, in part 

 tactile and in part visual, — a series which 

 is full of suggestiveness in regard to the 

 evolution of sense organs (cf. the series of 

 sensitive setce in the crayfish). The visual 

 organs of the leech are not able to form 

 images of external objects, but the animals 

 are exquisitely sensitive to alterations of light. 



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 pro- 

 truding three chitinous tooth-plates 

 which lie within. Each of these 

 tooth-plates is worked by muscles, 

 and is like a semicircular saw, for the 

 edge bears from 60 to 100 small 

 teeth. Rapidly these saws cut a 

 triangular wound, whence the flowing 

 blood is sucked into the muscular 

 pharynx. The process may be fig. 

 observed and felt by allowing a 

 hungry leech to fasten on the arm. 

 As the blood passes down the 

 pharynx, it is influenced by the 

 secretion of glandular cells which 

 lie among the muscles of the seventh, eighth, and ninth 

 segments, and exude a ferment which prevents the usual 

 clotting. The blood greedily sucked in gradually fills the 

 next region of the gut — the crop — which bears on each 

 side eleven storing pockets. These become wider and 



92. — Alimentary 

 system of leech. — After 

 Moquin-Tandon. 



>t. , Mouth ; cr 6 . , sixth crop- 

 pocket ; cr 11 ., last crop- 

 pocket ; v., rectum; s., 

 posterior sucker. 



