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 setz 
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 GS 
leech to fasten on the arm. As the Fic. 123. — Alimentary 
blood passes down the pharynx, it is system of leech. —After 
influenced by the secretion of glandu- _ Moain-Tandon. 
lar cells which lie among the muscles He ae 2 ae eeons 
of the seventh, eighth, and ninth Doe yy SERNA thy 
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 
