ALIMENTARY CANAL OF LEECH 



275 



in part tactile and in part visual — a series which is full of suggestive- 

 ness in regard to the evolution of sense organs (of. 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 protruding 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 flow- 

 ing 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 glan- 

 dular 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 gradu- 

 ally 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 pockets 

 point more or less backwards, it is 

 evident that a leech to be emptied of the blood 

 which it has sucked must be pressed from behind 

 forwards. The pockets filled, the leech drops off its 

 victim, seeks to retire into more private life, and digests 

 at leisure. The digestion does not take place in the 

 pockets, but in a small area just above the beginning of 



Fig. 148. — Alimentary 

 system of leech. — 

 After Moquin-Tandon. 



m., Mouth ; cr^., sixth crop- 

 pocket ; cyii., last crop- 

 pocket ; v., rectum; s., 

 posterior sucker. 



