The leech used for medical purposes is called the _hirudo medicinalis_
to distinguish it from other varieties, such as the horse-leech and
the Lisbon leech. It varies from two to four inches in length, and is
of a blackish brown colour, marked on the back with six yellow spots,
and edged with a yellow line on each side. Formerly leeches were
supplied by Lincolnshire, Yorkshire, and other fenny countries, but
latterly most of the leeches are procured from France, where they are
now becoming scarce.