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The phylum Echinodermata , which contains about 6000 species, gets its name from the Greek, literally meaning "spiny skin." Many echinoderms actually do have "spiny" skin, but others do not. This phylum exists exclusively in the sea, and cannot be found on land or in fresh water. All echinoderms have one thing in common: radial symmetry. This means that the creatures have appendages (or body construction) which point outward from the center of the body like the spokes on a bicycle wheel. Furthermore, these appendages usually occur in multiples of five, although there are a few exceptions. There are several well known members of this group, like sea stars and sea urchins. The radial symmetry is obvious in these creatures.
Perhaps not as obvious is the water vascular system, another trait common to all echinoderms. By examining the oral underside of a sea star, one will be able to see hundreds of tiny feet usually arranged into several rows on each ray (appendage) of the star. These are called tube feet, or podia, and are filled with sea water in most echinoderms. The water vascular system within the body of the animal is also filled with sea water. By expanding and contracting chambers within the water vascular system, the echinoderm can force water into certain tube feet to extend them. The animal has muscles in the tube feet which are used to retract them. By expanding and retracting the right tube feet in the proper order, the creature can walk. Many echinoderms can also form suckers on the ends of their tube feet. These suckers can be used to capture and hold prey, or to hold onto rocks in a swift current or tide.
Interestingly, although most mature echinoderms live on the bottom, the larvae are usually planktonic with bilateral symmetry. During the process of maturing, the echinoderm will change its body shape from bilaterally symmetrical to radially symmetrical, and in the process, settle down on the sea floor.
Sea stars are capable of regenerating limbs in the event that one or more is severed or damaged. The wound first closes off, and in time, the new limb will begin to grow. In a few species, the severed limb can regenerate a new sea star, but in most species, the severed limb dies. Sea stars eat a variety of different things, including barnacles, clams, mussels, snails, sea urchins, and in some cases, other sea stars! Many sea stars, such as the Northern Sea Star, eat mussels and clams in a fascinating way. The sea star first surrounds its intended victim. Then it applies outward force (with its suction cup equipped tube feet) on the two mussel shells (called valves), to pull them apart. Contrary to popular belief, the sea star does not need to apply force for a long time in order to tire out the mussel. The sea star can apply so much force to the mussel valves (7 or more pounds!) that it will bend the shell. Seizing the moment, the sea star then everts its stomach out through its mouth, and into the mussel (only a 1/100th of an inch opening is required). Once the sea star begins to digest its victim within the victim's own body, the victim dies. The sea star then finishes the meal by consuming the rest of the mussel. When the star is finished with the mussel, nothing remains but a shell.