The most common brittle star is Ophionereis schayeri which may grow to 300 mm across and has long, thin, dramatically banded arms. Crinoids are different to other echinoderms because their mouth is in the centre of the top surface, and along the top of the each arm is a groove along the midline. When food particles are caught by the arms, they are propelled along the channels in the arms towards the mouth. Crinoids may occasionally be found on rocky shores when the spring tide is at its lowest. Sea Urchins , Heart Urchins and Sand Dollars are surrounded by calcareous skeletal plates called ossicles, which combine to form a globular test.
All wall plates carry moveable spines, hinged onto knob shaped tubercles.
Sea Urchins move on hydraulically driven tube feet. Sea cucumbers also have a dorsal and a ventral surface. Respiratory water is both brought in and expelled through the anus by muscular contraction.