Echinoderm
Marine animals with five-pointed radial symmetry and unique water vascular system.
Echinoderms are animals of the phylum Echinodermata, a group that includes starfish, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies. While their larvae are bilaterally symmetrical and free-swimming, adult echinoderms are characterized by a usually five-pointed radial symmetry, known as pentamerous symmetry, an adaptation to a sessile or slow-moving lifestyle. They inhabit the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after chordates and the largest phylum restricted entirely to marine environments. The first definitive echinoderms appeared near the start of the Cambrian period.
Ecologically, echinoderms are exceptionally abundant in both deep-sea and shallower ocean environments. Geologically, they are significant because their ossified dermal endoskeletons, composed of calcite-based plates called ossicles with a porous sponge-like structure known as stereom, are major contributors to many limestone formations and offer valuable clues about past geological environments. Most echinoderms can reproduce asexually and regenerate tissue, organs, and limbs; some can even undergo complete regeneration from a single limb. They were the most used species in regenerative research during the 19th and 20th centuries, and some scientists believe their radiation drove the Mesozoic Marine Revolution.
The name Echinodermata was originated by Jacob Theodor Klein in 1734, initially only for echinoids, and was expanded to the phylum level by Jean Guillaume Bruguière in 1791. The phylum contains about 10,500 extant species and roughly 13,000 known extinct species. Five extant classes are generally recognized: Asteroidea (starfish, over 1,900 species), Ophiuroidea (brittle stars, around 2,300 species), Echinoidea (sea urchins and sand dollars, about 900 species), Holothuroidea (sea cucumbers, about 1,430 species), and Crinoidea (feather stars and sea lilies, around 580 species). While most echinoderms have five-part symmetry, some species have six or more arms, and certain crinoids and starfish can possess up to fifty or even two hundred arms. Genetic studies suggest that a starfish body can be considered to consist largely of a head, with anterior-developing genes expressed along the ambulacra and trunk genes expressed
- field
- Marine biology, paleontology
- known_for
- Pentamerous radial symmetry, water vascular system, regenerative ability
- first_appearance
- near the start of the Cambrian
Quick Facts
- Taxon
- Echinodermata
- Type Genus
- Echinus
- Type Genus Authority
- Linnaeus, 1758
- Subdivision
- See taxonomy
Facts from the source article.
Lore & Background
Echinoderms evolved from bilaterally symmetrical ancestors. As larvae, they are free-swimming and bilaterally symmetrical, but during metamorphosis the left side grows at the expense of the right, resulting in pentaradial symmetry in adults. Most echinoderms have five-part symmetry, though some starfish and brittle stars have six or more arms. The name 'echinoderm' comes from Ancient Greek for 'hedgehog skin', referencing their spiny appearance. Their skin covers a mesodermal skeleton made of calcite plates called ossicles, which have a porous, sponge-like structure known as stereom. These ossicles can fuse into a rigid test, as in sea urchins, or form flexible joints in the arms of starfish and brittle stars. External spines, granules, or warts often project from the ossicles. Specialized skeletal structures include the chewing organ called Aristotle's lantern in sea urchins, the supportive stalks of crinoids, and the lime ring of sea cucumbers. The epidermis contains pigment cells that produce vivid colors such as deep red, black-and-white stripes, and intense purple. Echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after chordates and the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian. Their ossified skeletons contribute to limestone formations and provide geological clues. Most echinoderms can reproduce asexually and regenerate tissue, organs, and limbs; some can regenerate entirely from a single limb. They were the most used species in regenerative research in the 19th and 20th centuries.
Reader's Guide
Echinoderms are significant for their ecological abundance in marine environments and their geological role in forming limestone deposits. Their ossified endoskeletons, composed of calcite with a porous stereom structure, fossilize readily and help paleontologists interpret ancient environments. They were the most used species in regenerative research in the 19th and 20th centuries, as many can reproduce asexually and regenerate tissue, organs, and limbs—some can regenerate entirely from a single limb. Some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution. Their unique water vascular system, used for locomotion, feeding, and gas exchange, is a defining feature. The phylum includes five extant classes: Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, and Crinoidea.
Did You Know?
- The name 'echinoderm' means 'hedgehog skin' in Ancient Greek.
- Some echinoderms can regenerate an entire body from a single limb.
- The first definitive echinoderms appeared near the start of the Cambrian.
More in Invertebrates 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record