Leeches belong to the phylum Annelida and are placed in the subclass Hirudinea. They may be freshwater, marine or terrestrial animals. Like earthworms they bear clitellum and are hermaphrodites but few are hematophagous. The European medicinal leech Hirudo medicinalis and some related species have been used for clinical bloodletting since long time ago. There are few species also who do not feed on human blood but they feed on some invertebrates. Haemophagic leeches attach to their hosts until they are fully fed and then they fall off to the ground. The body of leech is made up of 34 segments. The anterior end of body is surrounded by a oral sucker which comprises about six segments. The oral sucker is used for attaching to the host's body and for feeding followed by releasing an anesthetic which prevents the host from feeling the leech. They use a combination of mucus and suction to remain attach to host's body and secrete an anticlotting enzyme known as hirudin in the blood of the host.
Leeches are hermaphrodites. Reciprocal fertilization occurs in them and sperm transfer takes place during copulation. During the act of reproduction, the leech playing the part of male will develop a sperm sac at the base of the tail and the leech acting as female will bite it off. They also use their clitellum like earthworms in order to hold their eggs and forming cocoons. During reproduction they use spermatophore which stores sperm. During the act of copulation one leech will shoot its spermatophore into the clitellar region of the opposing leech where the sperm will make their way towards the female reproductive organs. The digestive system starts from the jaws situated on the ventral surface of body at the anterior end. The jaws are attached to the pharynx which joins oesophagus extending to crop and then gizzard. The gizzard then joins the intestinum which finally terminates into the posterior sucker. Crop is actually the extension of stomach which acts like storage chamber. Leeches can store blood five times more than its actual body size and since they produce an anticoagulant the blood remains there in liquid state. The medicinal leeches can stay alive for about two years with blood stored in their crop.
Predatory leeches bear a retractile proboscis in their mouth. These species are ambush predators which lie in wait for the prey and then use their proboscis in a spear-like fashion. Earlier it was believed that they carry gut bacteria for carrying out the breakdown of blood in the gut but studies have clearly demonstrated that their gut secretes exopeptidases. Not all species suck blood of animals but 90% of the leeches are known to feed on the dead and decaying bodies of amphibians, reptiles and waterfowl.
The use of leeches in medicine started about 2,500 years ago. The saliva of leech contains a protein known as hirudin which acts as an anticoagulant. It is also produced by recombinant technology in the present scenario. Injections of recombinant hirudin are given to those individuals who are allergic or are not able to tolerate heparin.
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