MEDICINAL LEECH. 



239 



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 setae 

 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 ob- 

 served 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 glandu- 

 lar 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 more capacious towards the hind end, the largest 

 terminal pair forming two great sacs on each side of the 

 comparatively narrow posterior part of the gut. As all the 



FIG. 123. Alimentary 

 system of leech. After 

 M oquin-Tandon. 



ftf., Mouth; cr^. t sixth crop- 

 pocket; cr 11 ., last crop- 

 pocket; v., rectum; s. t 

 posterior sucker. 



