Annelida

          The Phylum Annelid consists of many types of worms. Annelids (or ringed worms) are a group of segmented worms including over 22,000 species. They are bilaterally symmetrical, and can live in either marine or terrestrial environments.

http://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Sv%C3%B8mmende_blodigle.JPG/220px-Sv%C3%B8mmende_blodigle.JPG

          One species of annelids is the medicinal leech or "Hirudo Medicinalis". This particular annelid has proved useful to humans in medicine. Being that it feeds on mammals blood, the leech has been the perfect tool to release pressure or restore blood flow. This works because the leech excretes an anti-coagulant to keep blood flowing which can release or destroy blood clots in areas with trauma.

          The leech moves by contracting the longitudinal muscles in it's body to expand and contract pushing itself through the water. Leeches reproduce sexually, all leeches are hermaphroditic, so each leech can reproduce. Leeches lay eggs that hatch in 14 days, and leeches hatch fully formed.