THE MEDICINAL LEECH (Hirudo medidnalis) 



External Features. 



1. Make a drawing of the ventral surface of the Leech showing : — 



(a) The body divided by a series of transverse furrows into a definite ntimber 

 of annuli (about ninety-five). The annuli are more numerous than 

 the true segments of the body ; and, except at the two extremities, 

 each segment contains five annuli. The anterior annulus of each seg- 

 ment bears six sensory papillae, both dorsally and ventraUy. A pair of 

 nephridiopores occurs on the posterior annulus of each segment, from 

 the *6th to the 22nd inclusive. Thus the sensory papillae detepnine 

 the anterior border of each segment of the body, while the n^^ridio- 

 pores determine the posterior border of segments 6 to 22. There are 

 five segments in front of the one bearing the first pair of nephridio- 

 pores, and four behind the one bearing the last pair, giving a total of 

 26 true segments, of which each of the first six and the last four has 

 less than five annuli. 



{b) The anterior sucker, oval in shape, at the anterior end of the body. It 

 has a cup-like hollow, in the middle of which is a small aperture, the 

 mouth, with the three jaws surrounding it. 



(c) The blind posterior sucker, at the posterior end of the body, circular in 



shape and larger than the anterior one. 



(d) The male genital aperture, situated in the middle of the second annulus 



of the 10th segment. 

 {e) The female genital aperture, situated in the middle of the second annulus 

 of the 11th segment. 



2. Make a drawing of the dorsal surface of the first seven segments to show : 



(a) The five pairs of small eyes on the dorsal surface of the anterior sucker. 



(b) The sensory papiUse, six in the anterior annulus of each segment, except 



in those annuli containing the eyes. Here there are only four, the 

 eyes taking the place of the other two in annuli 1, 2, 3, 5 and 8. There 

 are five pairs of eyes. 



3. Make a drawing of the dorsal surface of the last four segments to show : 



(a) The anus, situated in the middle of the last annulus of the body, just 



in front of the junction between the posterior sucker and the end of 

 the body. 



(b) The sensory papillae, occupying the same positions as in the other 



segments. 



General Structure. 



Pin the animal down under water, with the dorsal surface uppermost, by 

 inserting pins through the sides of the suckers. Cut along the mid-dorsal line, 



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