EXTERNAL CHAEACTEES. 41 



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 shin, a little to 

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

 alimentary canal, which lies close beneath the shin. Carefully 

 dissect off the integument from the alimentary canal along the 



