LEECH, the common name of members of the Hirudinea, a division of Chaetopod worms. It is doubtful whether the medicinal leech, Hirudo medicinalis, which is rarer in England than on the continent of Europe, or the horse leech, Aulastoma gulo, often confused with it, has the best right to the original possession of this name.
But at present the word "leech" is applied to every member of the group Hirudinea, for the general structure and classification of which see Chaetopoda.
They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits.
Thus the body cavity is divided into a sequence of chambers by transverse septa; and even among the Hirudinea, Spadella cephaloptera (Busch).
Goodrich), that among the Hirudinea the coelom, which is largely broken up into narrow tubes, may be confluent with the tubes of the vascular system.