40 THE LEECH 



Hold the leech in your hand with the ventral surface up- 

 wards. ' Dry it with a cloth, and then gently squeeze it : 

 minute drops of fluid will exude from the nephridial apertures, 

 ivhich are thus rendered clearly visible. 



II. DISSECTION OF THE LEECH. 



Stretch the leech slightly with tJie fingers and fix it under 

 water with the dorsal surface upwards, putting the pins 

 through the sides of the suckers so as to avoid injury to the 

 nerve-ganglia. 



Make a longitudinal incision through the skin a little to 

 one side of the middle line, taking care not to damage the 

 alimentary canal, which lies close beneath the skin. Carefully 

 dissect off the integument from the alimentary canal along the 

 whole length of the animal, and pin out the flaps right and 

 left. Wash thoroughly under the tap. Note at once the 

 dorsal blood-sinus, which runs along the dorsal wall of the 

 alimentary canal. 



A. The Digestive System. 



The alimentary canal of the leech runs straight from 

 mouth to anus. Along the greater part of its length it is a 

 wide tube, whose capacity is much increased by paired lateral 

 diverticula, and is further capable of great distension. 



A leech can draw as much as three times its own weight 

 of blood, a great part of which is often spontaneously dis- 

 charged. The digestion of a full meal may take as long as 

 nine months in an adult animal. 



Dissect clean the dorsal wall of the alimentary canal 

 along its whole length, taking care not to damage the supra- 

 ORSOphageal ganglia. If the canal is empty its walls are white 

 and readily distinguished ; if full, it appears red from the 

 contained blood, and is less easy to dissect. Wash frequently 

 under the tap during the dissection. 



1. The mouth is a conical depression in the anterior sucker, 

 at the bottom of which are the jaws. These are three 

 laterally compressed muscular cushions arranged in 



