194 WORMS. 



a ferment which prevents the usual clotting. The blood 

 greedily sucked in gradually fills the next region of the gut, 

 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, seems 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, 

 lying 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 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 regular 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. 



The excretory system includes seventeen pairs of excreting 

 tubules or nephridia, opening laterally on the ventral surface, 

 ending blindly within the body, but extracting waste-pro- 

 ducts from the blood-vessels which cover their walls. Each 

 consists of two parts, a twisted horse-shoe-shaped glandular 

 region where the actual excretory function is discharged, and 

 a spherical, internally ciliated bladder opening to the exterior. 

 Within the latter there is a whitish fluid in which microscopic 

 examination shows numerous waste crystals. 



TTie Reproductive System. — The leech, like many other 

 Invertebrates, is hermaphrodite, containing both male and 



