Leeches are annelids or segmented worms and although closely related to the earthworms.
The bodies of all leeches are divided into the same number of segments (34), with a powerful clinging sucker at each end (although the anterior, or front sucker can be very small).

Body shape is variable, but to some extent depends on the degree to which their highly muscular bodies are contracted. The mouth is in the anterior sucker and the anus is on the dorsal surface (top) just in front of the rear sucker.
Leeches usually have three jaws and make a Y-shaped incision.
Different  Types   
Leeches are grouped according to the different ways they feed. One group (the jawed leeches or Gnatbobdellida) have jaws armed with teeth with which they bite the host. The blood is prevented from clotting by production of a non-enzymatic secretion called heroin. In the 17th century leeches were used in hospitals to suck your blood and also the poison which is in the patients blood. People in the seventeenth century believed that by draining one of the for fluids (blood) they could be cured.