Leeches are annelids that are bloodsucking parasitic worms. They are found in ponds, rivers and lakes. They are measured from as small as 0.2 inches to 18 inches in length. They consume blood from animal hosts. They use outer circular muscles and inner longitudinal muscles to create a looping movement. They are very good swimmers. They flatten their body into a ribbon-like shape and propel themselves forward in a series of vertical wavelike motions.