This slide shows a stained whole mount of a common freshwater leech (Helobdella stagnalis) with its proboscis extended.  Although they are known for their blood sucking habits, many leeches are actually predators that feed on soft-bodied invertebrates.  During feeding, the proboscis is extended out of the mouth where it is used to penetrate the surface of the prey and suck out its body fluids.  In the region of the neck a dark, circular structure can be seen. The function of this structure (which is unique to this species) is unknown!