The phylum Echinodermata is an extremely large, well-known phylum containing many familiar organisms, including starfish, sea urchins, and sand dollars. All echinoderms have several things in common, making members of this phylum easily recognizable. First off, all echinoderms live in the ocean, with no known exceptions from present day or the fossil record. They can be found in every ocean living on the ocean floor, whether 1000 ft below ocean level or 2 ft below ocean level. In most cases there are two definite sexes, and to mate they breed externally. The larvae are symmetrically bilateral, and constitute a large percentage of the zooplankton. The larvae undergo a radical metamorphosis, becoming symmetrically radial with 5 symmetrical sections. In some species, the whole digestive system is changed during metamorphosis, and the organism acquires a new mouth and anus. Internally, echinoderms have a thickened vessel which acts as the heart, pumping blood through a closed circulatory tract. All species lack brains, although some may have ganglia. They lack specialized sense organs (nose, eyes, ears, etc). There are four basic things that undeniably make an echinoderm an echinoderm. First, they all have calcitic skeletons, being composed of calcium carbonate crystals. In most cases, a thin tissue surrounds the skeleton, making it an endoskeleton, but for all intents and purposes the skeleton acts as an exoskeleton. Secondly, echinoderms have a water vascular system, which allows locomotion, respiration and feeding. The tubefeet are a part of this system, and contain most of the organisms sensory neurons. Thirdly, the ligaments are primarily composed of collagen. These ligaments are usually rigid, but can be temporarily loosened, allowing for a wide variety of body postures to occur with minimal use of muscles. Lastly, the body of the adult is pentaradially symmetrical (5-fold organization of the skeleton in which each of the 5 sections are symmetrical). The best example of a pentaradially symmetrical echinoderm is the starfish, with its 5-armed star-shaped body.
There are 13 000 species of echinoderms known by the fossil record, and they are divided into 20 classes. Today, most of these classes are extinct. There are now five known classes with a sixth one that is rather controversial. There are approximately 7000 living species in these six classes:
Asteroidea (starfish)
Conantricycloidea (sea daisies)
Crinoidea (sea lilies, featherstars)
Echinoidea (sea urchins, sand dollars)
Holothuroidea (sea cucumbers)
Ophiuroidea (basket stars, brittle stars)
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