The leech is an aquatic worm with a flattened body, tapering end terminating in circular flattened disc, the hinder one being larger of two. It swims with a vertical undulating motion and moves, when out of water by means of these discs or suckers, flattening itself first by one and then by other and alternately stretching out and contracting its body. The mouth is situated in the centre of the anterior disc and furnished with 3 cartilaginous lens shaped jaws. These jaws are lined at their edges with fine sharp teeth and meet so as to make a triangular incision in the flesh. The head is furnished with small raised points, supposed by some to be eyes.
The leech belongs to the kingdom (Animalia); Phylum (Annelida); Class (Hirudinea); Order (Gnathobdellida); Family (Hirudidae).Leech can ingest an amount of blood close to that of eight times of their own weight. The leeches can be primarily found in the freshwater lakes, ponds or rivers ranging from 5cm to nearly 25 cm. Leeches are hermaphrodite having both male and female parts, but they still need to come together to meet with each other.After mating 15-50 eggs are laid in one spongy case or cocoon above the waterline often under stones.
Leech eggs hatch in 3 to 5 weeks and the young leeches need two seasons of feeding before they are ready to breed themselves. There are about 45 species of leeches belonging to 22 genera. However the species commonly cultured for medicinal use are Hirudinaria granulosa and Hirudo medicinalis. H. granulosa is abundantly found in the state of Madras, Kerala, M.P, U.P. and Punjab, whereas Hirudo medicinalis is commonly used in the western world. Leeches are known by following vernacular names: Alaq ( Arabic); Zalu ( Persian ); Jonk ( Urdu ); Salook ( Turkish); Leech ( English ); Hirudo (Latin); Bdella (Greek); Jaluka (Sanskrit); Jalu ( Hindi); Jalagalu ( Telugu); Attai ( Tamil); Jiganey ( Kannada)