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In the Freshwater Biological Association key to British leeches (Elliott & Mann 1979) it says:

Temporary immobilisation of all but the largest leeches is achieved by covering them in soda water. They will be anaesthetised for about 15 minutes. Larger leeches may be immobilised with chloroform in water.

Just the right amount of relaxation is important for critical examination of leeches. If dropped alive into 70% alcohol or 4% formalin they contract too strongly. Excessive relaxation causes the furrows between annuli to disappear. A good method is to add 70% alcohol to the water containing live leeches, gradually increasing the concentration over about 30 minutes until movement ceases. The leeches are then removed, passed between the fingers to straighten them and remove excessive mucus (mmm) 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 Fleming's fixative should be used for histological studies.

John Bratton