What Is an Echinoderm? Five-Part Animals With Hydraulic Feet

Three characteristics identify the group. The adult body is arranged in five parts around a centre, which is unusual among animals since most are built on a left-right plan with a front and a back. The skeleton consists of calcite plates and spines formed inside the tissue, each plate being a single crystal with a sponge-like internal structure that makes it light and strong, an arrangement found nowhere else. And a water vascular system, a network of internal canals filled with fluid, operates hundreds or thousands of small tube feet by hydraulic pressure, which is used for movement, for feeding, for gripping and for gas exchange. That hydraulic arrangement is the group's characteristic solution to problems other animals solve with muscles, and it works well enough to have persisted for over half a billion years.

Five groups survive and they look unalike:

• •Starfish, which move on tube feet and feed in several ways including everting the stomach onto prey
• •Brittle stars, with slender arms moved by muscle rather than by tube feet, and by far the most numerous group
• •Sea urchins and sand dollars, with a rigid test of fused plates and a complex jaw apparatus
• •Sea cucumbers, elongated and soft with reduced plates, which include the deep sea's most abundant large animals
• •Sea lilies and feather stars, attached or formerly attached filter feeders with a long fossil record
• •Numerous extinct classes, since the group was far more varied in the Palaeozoic than it is now

The five-part arrangement is secondary rather than original, which is established by development. The larvae are bilaterally symmetrical, free-swimming and unremarkable, and the adult form develops through a dramatic metamorphosis in which the larval body is largely reorganised or discarded and the radial adult grows from a portion of it. That means the group descends from bilateral ancestors and acquired the radial arrangement afterwards, which the fossil record supports, since several early extinct groups show asymmetric, three-part and spiral arrangements rather than five, and the five-part plan appears to have been arrived at rather than being present from the start. The bilateral larva also places the group among the animals more closely related to vertebrates than to insects and molluscs, which is settled by developmental and genetic evidence and surprises people who expect relationship to follow appearance.

The capacity of these animals to regrow lost parts is extreme even by the standards of invertebrates and has attracted substantial research interest. Starfish regrow lost arms routinely, and in several species a detached arm with a portion of the central disc regenerates an entire animal, which means an attempt to destroy them by cutting them up multiplies them instead, a fact discovered expensively by shellfish farmers. Sea cucumbers eject their internal organs as a defence and regrow them within weeks. Brittle stars shed arms deliberately to escape and regenerate them. Sea urchins regrow spines and tube feet. Some species reproduce by splitting. The mechanisms involved are studied for what they might indicate about regeneration generally, since the animals are closer relatives of vertebrates than most invertebrates used as models.

The group is ecologically important out of proportion to how much attention it receives. Sea urchins graze algae and can transform habitats, with documented cases where removing their predators produced urchin population explosions that destroyed kelp forests and converted them to barren rock, which is one of the clearest demonstrations of a trophic cascade. Starfish include a species whose removal from an experimental shore let mussels exclude everything else, which produced the keystone species concept. Sea cucumbers process enormous volumes of sediment and are the dominant animals by mass on much of the deep sea floor. Crown of thorns starfish outbreaks damage coral reefs substantially. And the calcite skeletons contribute to sediment and to limestone, with some rocks composed largely of the fragmented remains of sea lilies.

Five-part symmetry, a skeleton of single-crystal calcite plates formed within the tissue and hundreds of tube feet worked by water pressure define the group. The symmetry is secondary, since the larvae are bilateral and the adult form develops through a drastic metamorphosis. They are closer relatives of vertebrates than of insects, and their grazing has repeatedly transformed whole habitats.