The phylum Echinodermata consists of about 7000 living species but there are a further 13,000 extinct species dating back to the Cambrian. They are basically pentaradially symmetric which means that they are five-sided, and all live in the sea. They include the sea lilies, the starfish, the sea daisies, the brittle stars, the sea urchins and the sea cucumbers.
The class Stelleroidea or Asteroidea are the starfish, with about 1500 species. They have star-shaped bodies with 5 or more arms attached to a central disc with no obvious joint. They travel with their many tube feet. Most are scavengers or opportunistic predators and some can prise open the tightly closed shells of a clam and digest the contents.
Dorsally-flattened sea urchins, the sand dollars and sea biscuits.
The Crinoids include about 625 living species of sea lilies and feather stars. Some have stalks and wave their feathery arms to catch fragments and work them towards their mouths with their tube feet and cilia. The feather stars walk on the tips of their arms, or swim by moving their arms alternately so that half are up and half down at any one time.
The Echinoids consist of the sea urchins and sand dollars, with bodies shaped like globes or discs. Characteristically they have a hard shell and spines. Many are herbivores, feeding on algae, but others are predators. Sea urchins move by their podia or tubular feet and also their movable spines and many burrow. Sand dollars also burrow into soft sediments and may bury themselves completely, digging with their movable spines.
Holothuroideans are the sea cucumbers. They are sausage-shaped and have fleshy bodies with the skeleton reduced to a few ossicles in the body wall. Tentacles surround their mouths and help them to eat, pushing food towards their mouths. They crawl or burrow into soft sediments using muscular movements of the body wall.
The brittle stars, characterized by a central disc and long slender arms.