Radially symmetrical animals have no front or back ends but only a top and a bottom. Even the terms 'ventral' and 'dorsal' are not easily applied to the echinoderms, for some of them are orientated mouth uppermost while in others the mouth faces downwards. Therefore the term 'oral' is used to describe the surface that bears the mouth, while 'aboral' is used to refer to the opposite surface.

The second uniquie feature is their skeleton. Embedded in the skin is a layer of tiny platelets (ossicles) which gives the skin a spiny texture.

The third unusual feature possessed by echinoderms is their hydraulic system of locomotion, which is most easily understood in the starfish. Extending arounf the mouth is an internal tubular fluid-filled ring, from which a canal extends into each of the animals five arms. Along these canals arise numerous tiny tube feet which pass through the skeleton and the skin to form rows of external 'legs', each terminating in a sucker that grips onto the substratum. Associated with each tube foot is a balloon-like ampulla, lying inside the body waal. Contraction of the circular muscles around the ampulla drives its fluid contents down into the tube foot, thus extending it. Longitudinal muscles within the tube foot can then bend the foot in any direction or withdraw it once again. The tube feet possess tiny valves so that it is possible to isolate them from their interconnecting canals, but these vales may also be opened to allow the fluid to be diverted from one part of the body to another.