Echinoderms are all marine animals with a skeleton composed of many small plates. The skeleton encloses the main soft parts of the body and is covered by a thin layer of skin. On the outside of the body are numerous tentacle-like structures called tube-feet, which are connected to a system of water-filled canals inside the body. The pressure of water in these canals enables the animal to extend or retract the tube-feet. The tube-feet have many uses, including moving the animal around, capturing food and passing it to the mouth, extracting oxygen from the seawater, and sensing the surrounding environment. Most echinoderms have radially symmetrical bodies arranged in five sections, but this is not true of some fossil groups and may not be obvious in some living ones.