|Sea urchins have a variety of external organs that can be found in among the spines. The photograph below is of the oral area of a regular sea urchin.|
The most important external organs are the tube-feet (tf). These are cylindrical tubes terminating in a circular sucking disc that are extensions of the water vascular system.
The mouth opens in the centre of the peristomial membrane (pm), a tough and flexible collagenous skin that acts as the lips. The white tips of the teeth are just visible in the opening. A similar membrane surrounds the anus. Both membranes usually have embedded spicules or platelets.
Pedicellariae (ped.) are tiny stalked valves that are used to ward off small parasites and keep the surface of the test clean.
Around the mouth there are five pairs of buccal sacs (bs). These are direct extensions from the internal body cavity and their primary role is as expansion sacs to compensate for internal volume changes when the jaws are moved in and out of the rigid test during feeding. Fluid from the internal coelom can be temporarily displaced into the external buccal sacs when the jaws are drawn inside the test and vice versa. These organs pass out at the edge of the peristome and their position is marked on the skeleton by small notches.
Spines (sp.) are external skeletal appendages covered in a thin skin.