Echinoderms Temporal range: Late Ediacaran-Recent
Sea urchin
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Deuterostomia
Phylum:	Echinodermata

Fossil crinoid crowns.

Echinoderms are a successful phylum of marine animals. They include sea stars, brittle stars, sea urchins, sea cucumbers and their relatives.

Echinoderms have these features:

1. A skeleton of plates. These are formed from calcite, a mineral made of calcium carbonate. The plates are usually spiny, and the skeleton is covered outside and in by a layer of skin.
2. Five-rayed (pentameral) symmetry.
3. A water-vascular system. This is an internal system of tubes and bladders filled with water.
4. Tube feet. These are extensions of the water-vascular system. The tube feet poke out from the skeleton, and are used in walking, respiration and feeding. The animal handles the world outside by using its tube feet.
5. Stenohaline. They can't handle big changes in the salinity of the water. No doubt this is why they are:
6. Entirely marine.

Echinoderms live in all parts of the ocean, but mostly on the sea floor. Some are filter feeders, and others (starfish) are important predators of molluscs and other shell-fish. They are extremely common near the shore, and on reefs.

They have a long and abundant fossil record. This phylum appeared in the early Cambrian period; it contains about 7,000 living and 13,000 extinct species. The four or five main groups are called sub-phyla by some authorities, and classes by others.