Hirudinea is a subclass of annelids that consists of the leeches. These annelids do not have bristles and many species feed on blood. They are used medically to drain blood, as well as provide several molecules, such as anticoagulants.
In fact, the saliva of parasitic leeches contains anticoagulant, anaesthetic, vasodilator and anti-inflammatory substances that allows them to suck the blood of their victims without being noticed.
A leech (Hirudinea). © One Tree Hill Studios CC by-sa 2.0