218 SEGMENTED WORMS OR ANNELIDA. 



tooth plates is worked by muscles, and is like a semi- 

 circular 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 mouth by the 

 muscular pharynx. The process may be observed and felt 

 by allowing a hungry leech to fasten on your arm. As the 

 blood passes down the pharynx, it is influenced by the secre- 

 tion of salivary cells which lie among the muscles, 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 pockets point more or less backwards, it is evident 

 why 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 the terminal part or rectum. This rectum, 

 running between the two last pockets, is separable from the 

 true stomach just mentioned by a closing or sphincter 

 muscle. It ends in a dorsal anus above the hind sucker. 



The Vascular System. 



The vascular system consists of four main vessels running 

 longitudinally, one above the gut, one round about and 

 obscuring the nerve cord, and one on each side of the body. 

 These are all connected with one another by looping vessels, 

 and all give off numerous branches which riddle the spongy 

 body. The main side vessels are most distinct, are con- 

 tractile throughout, and give off to the skin, gut, and excre- 

 tory organs, a rich supply of branches. The dorsal and 

 ventral vessels, though quite distinct, are less definite, 

 being rather blood spaces than well-formed vessels. That 

 the lateral vessels do most of the work of circulation is 

 certain, but the precise course of the blood is not satis- 

 factorily known. The blood itself is a red fluid with floating 

 colourless cells diverse in form. 



