152 LEECHES 



in weak alcohol, chloroform, chloretone, magnesium sulphate or 

 soda water. Excessive relaxation leads to the annuli being difficult 

 to make out and practice is needed before the right degree of 

 narcotization is achieved. The method favoured by the author is 

 to add 70% alcohol to the water containing living leeches, gradu- 

 ally increasing the concentration over a period of about 30 min 

 until movement ceases. The leeches are then removed, passed 

 rapidly between the fingers to straighten them and remove excess 

 mucus, and then laid out and kept flat while the fixative is poured 

 on. Alcohol or formaldehyde is suitable for simple morphological 

 work, but Bouin's or Flemming's fixative should be used for 

 histological studies. 



The characters which are fundamental in leech identification 

 are the annulation, the number and arrangement of eyes and the 

 positions of the male and female genital pores. To determine the 

 number of annuli per segment look for segmentally repeated 

 features such as colour pattern and sensillae (see Chapter 2). To 

 see the eyes, flatten the head of a narcotized leech between two 

 glass slides. If the leech has been fixed and the eyes are hidden 

 by pigment, decolorize the head by immersion in 5% caustic 

 potash. The genital pores are in the mid- ventral line about one- 

 third of the distance from anterior to posterior suckers. The male 

 pore is normally anterior and the more prominent. The female 

 pore is often small and difficult to see. It is seen most easily 

 immediately after narcotization, its position often being revealed 

 by some colour diflFerence which is lost during fixation. 



A Key to the Freshwater Leeches of Central Europe, 

 THE British Isles, and North America north of the 



Rio Grande 



The species to which this key refers are those included in the 

 following works: 



AuTRUM, H. 1958. Die Tierwelt Mitteleuropas. I, 7b. Hirudinea. 

 Mann, K. H. 1954. A key to the British Freshwater Leeches, with 



Notes on their Ecology. Freshwater Biological Association 



Scientific Publication 14. 



