10 LEECHES 



saw; one is median dorsal in position, the other two are ventro- 

 lateral. They have a covering of cuticle, and along the free edges 

 of the discs this is thickened to form a row of numerous minute 

 teeth. In making the incision, the muscles of the jaws rock them 

 so that the teeth move with a sawing action. The result is a 

 Y-shaped incision. Numerous unicellular salivary glands open 

 onto the jaws and secrete an anticoagulin which prevents the 

 clotting of the blood exuded from the wound. The blood is sucked 

 into the alimentary canal by the pumping action of the pharynx, 

 an oval sac about 5 mm long behind the buccal cavity. Its walls 

 are deeply folded when at rest, but unfold when the pharynx is 

 dilated by the action of the radial muscles which run out to the 

 body wall. The space between the radial muscles is almost 

 entirely occupied by salivary gland cells. 



Immediately behind the pharynx is a short narrow tube, the 

 oesophagus, through which the blood is passed to the crop. The 

 crop is the largest part of the alimentary canal and is adapted for 

 the storage of a considerable volume of blood by the possession 

 of eleven pairs of diverticula, one pair in each of segments VIII- 

 XVIII. The last pair run back to the hind end of the body. In 

 segment XVIII the crop leads by a narrow pore to the intestine. 

 This is a thin-walled tube, slightly swollen into a heart-shaped 

 chamber in segment XIX, which runs straight back between the 

 last pair of crop diverticula to segment XXIII where it becomes 

 constricted and leads to the rectum. The latter, when full, is 

 distended to form a rectal bladder which opens to the exterior on 

 segment XXVI at the anus. 



4. Reproductive System 



The testes are contained in ten pairs of coelomic sacs situated 

 in segments XII to XXI. The number is not absolutely constant, 

 there may be one pair more or less. Short vasa efferentia connect 

 the testis sacs to the vasa deferentia of each side. These run 

 forward to segment XI where they become enlarged and coiled 

 to form storage organs known as epididymes or sperm vesicles. 

 Beyond these are thick-walled ejaculatory ducts which in turn lead 

 to a median organ, the atrium. The atrium consists of two parts, 

 a basal bulb covered with several layers of unicellular prostate 



