Pedicellariae are very small skeletal appendages in the shape of forceps that include a stem and a head. The head consists of valves that come together or move further apart through the action of a muscular device. The valves are used to clean the organism, to keeps predators away and to prevent the larvae of other invertebrate from taking hold. There are five types of pedicellariae. Tridactyl pedicellariae have very long valves and a more or less large limb, which is often quite big. Ophicephal pedicellariae are essentially present on the oral membrane and have short and strong valves, with a very well developed hard articular arch at their base. Trifoliate pedicellariae are very small and their valves are flat and slender. Globiferous pedicellariae have thin and narrow valves, which, on their exterior, feature a venom gland, which opens outside of the valve end. Rostra are found only among irregular echinoids and have narrow, slightly curved valves that come together at the ends.