Tube-feet are seen here extending beyond the spines of a regular echinoid. They have a hollow cylindrical stem and a small circular suckered disc for adhesion (in this case for attaching to the surface of the aquarium tank). Tube-feet allow sea urchins to climb, and are used for grip, for example when the animal is rasping algae from rock surfaces. They can also be used to pick up and manipulate particles.
Each tube-foot is connected to the interior radial water vessel through a single or double pore that pierces the ambulacral plates. There is an internal sac, the ampulla, that acts as an internal fluid reservoir to each tube-foot.
Tube-feet can become highly specialised in structure and function. Aboral tube-feet are often modified into flattened, leaf-like structures for gaseous exchange. In the tube-feet around the mouth of heart urchins the disc is covered in a mass of tiny finger-like projections and these are very efficient at gathering fine organic particles.