HIRUDO MEDICINALIS LINN.^US 107 



Their function is antagonistic to that of the circular muscles. 

 Traversing both layers at right angles are the radial and the 

 dor -so-ventral muscles the function of which is to flatten the 

 animal. In the posterior sucker the muscular system is rep- 

 resented by radial and circular fibres. In reality the entire 

 muscular system is still more complicated. Between the muscles 

 we find connective and botryoidal tissue. 



Digestive system. The alimentary canal begins with the 

 mouth on the ventral surface of the first segment. The mouth 

 is surrounded by the mouth sucker. In the mouth cavity are 

 three jaws one of which is median and dorsal in its position, 

 while the other two are oblique and ventral. The edge of each 

 jaw is provided with a row of teeth for the perforation of the 

 skin. The jaws are operated by a muscular system of their 

 own. Behind the jaws the mouth cavity leads into a short, 

 muscular pharynx into which the ducts of the salivary glands 

 open. It is the secretion of these glands which prevents the 

 coagulation of the blood of the animal attacked by the leech 

 and causes prolonged bleeding from the wound. Behind the 

 pharynx begins the midgut which is sharply divided into two 

 sections. The anterior section or stomach, called also crop, is 

 by far the most powerfully developed part of the alimentary 

 canal. It possesses eleven pairs of cozca the last of which is 

 the longest. It is a storage place for the food. The resorption 

 of the food takes place in the second section or intestine which 

 is a much shorter and thinner tube connected with the stomach 

 (crop) by what is termed the funnel. The hindgut or rectum 

 terminates in an anus situated on the dorsal surface at the 

 base of the posterior sucker. 



The circulatory system consists of two lateral blood-vessels, 

 one dorsal and one ventral sinus and their ramifications. The 

 lateral vessels have muscular walls. Anteriorly and posteriorly 

 they are directly connected with each other, so that the blood 

 can pass from one lateral vessel into the other. A further con- 

 nection between the lateral vessels is established by the presence 



