EXTERNAL CHARACTERS. 51 



b. The anus is a very small aperture on the dorsal 

 surface in, or just in front of, the constriction 

 separating the posterior sucker from the body. 



c. The genital apertures. The leech is hermaphrodite : 

 the male and female apertures are separate, and 

 are both on the ventral surface in the median line. 



i. The male aperture is a small hole with tumid 

 lips at the hinder border of the twenty-fourth 

 annulus, i.e. in the second annulus of the sixth 

 somite. From it the muscular penis is often 

 seen protruding. 



ii. The female aperture is less conspicuous than 

 the male, and is placed one somite further 

 back, i.e. at the hinder border of the twenty- 

 ninth annulus, the second annulus of the 

 seventh somite. 



d. The apertures of the nephridia, or excretory organs, 



are minute paired openings on the ventral surface 

 of the body in the hindmost annulus of each 

 somite from the second to the eighteenth in- 

 clusive. 



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

 ward. Dry it with a cloth, and then gently squeeze it : minute 

 drops of fluid will exude from the nephridial apertures, which 

 are thus rendered clearly visible. 



II. DISSECTION OF THE LEECH. 



Stretch the leech slightly with the fingers and fix it under 

 water with the dorsal surface upward, 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 ali- 

 mentary canal, which lies close beneath the skin. Carefully 

 dissect off the integument from the alimentary canal along the 



