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THE MEDICINAL LEECH ; Hirudo medicinalis. 



A. Natural Movements. 



Observe that the leech crawls by fixing the anterior sucker as far as possible 

 ahead of the posterior, and then after detaching the posterior sucker by shorten- 

 ing itself, it brings the posterior sucker to within a short distance of the anterior, the 

 posterior having been fixed, the anterior is then detached, and the process repeated. 

 When ascending a vertical surface, note that the body is thrown into a loop by the 

 posterior sucker being fixed immediately behind the anterior. Observe further, 

 that it sometimes completely detaches itself, and after assuming a ribbon-Hke form 

 swims along in an undulating manner with the dorsal surface upwards when 

 moving in a vertical direction, but with one of the margins usually upwards when 

 moving in a horizontal direction. 



B. External Characters. 



Note— 



1. The dark olive colour of the slightly convex dorsal surface, and at each side 



of the middle line (a) an orange band, (b) a similar band interrupted 

 by dark elongated spots, (c) a dark, narrow band bordered with orange 

 on the one side and with yellow on the other. 



2. On the flattened ventral surface a number of dark patches scattered irregu- 



larly over a pale olive ground between two broad dark bands. 



3. That the body is made up of a large number of rings, and that the 1st, 



2nd, 3rd, 5th, and 8th rings are in some leeches provided with a pair 

 of simple eyes arranged around the anterior end ; from the presence of 

 eyes it may be inferred that the 1st and 2nd rings represent the 1st and 2nd 

 segments, the 3rd and 4th, the 3rd segment ; and that the 5th, 6th, and 

 7th rings represent the 4th segment ; the segments between the 4th and 

 the posterior sucker are probably made up of 5 rings each, the posterior 

 sucker seems to consist of 7 segments. 



4. The anterior sucker formed by the upper lip projecting backwards at each 



side of the mouth, and the large posterior sucker with the anus lying 

 between its dorsal margin and the last ring — apply the anterior sucker to 

 the skin, and note the presence of the three toothed ridges. 



5. On the ventral surface, the penis in the centre of the 24th ring behind the 



mouth, and the female genital opening between the 29th and 30th rings, 

 the openings, if visible, of the large mucous glands of the 39th, 40th, and 

 41st rings, and the small apertures of the segmental organs on every 5th 

 ring. 

 Make a sketch of the dorsal and another of the ventral surface to show 

 the external characters. 



