What is an echinoderm?

Echinoderms are a phylum of marine animals characterized by a hard, spiny covering or skin. Beginning in the Cambrian Period, they have a rich fossil history. Living echinoderms include sea lilies and feather stars , sea urchins , sea cucumbers , starfish , brittle stars , and sea daisies. They have a skeleton of mineral calcium carbonate. Part of their body cavity is a water-vascular system with fluid-filled vessels that are pushed out from the body surface as tube feet and other structures used in locomotion, feeding, respiration, and sensory perception. Though many of the earliest echinoderms lacked symmetry or were bilaterally symmetrical, living echinoderms exhibit radial pentamerous symmetry.

What are some examples of echinoderms?

Echinoderms are a diverse phylum of marine animals characterized by their hard, spiny skin. This group includes familiar creatures such as sea stars (also known as starfishes), sea urchins , sea cucumbers , sea lilies , and brittle stars . Echinoderms are exclusively marine animals that live in a variety of habitats, from rocky shores to deep-sea trenches. They play important roles in marine ecosystems, including scavenging decaying matter and preying on small organisms. They are found in the warmest and coldest seas. Most echinoderms are small, but some sea cucumbers can be as long as two meters, and some sea stars can be up to one meter in diameter.

Where do echinoderms live?

Echinoderms are exclusively marine animals that live in a variety of habitats, from rocky shores to sandy areas, and can be found in the warmest and coldest seas. Most cannot tolerate changes in salinity, temperature, and light intensity, and they tend to move away from areas where these factors are not optimal. Echinoderms occupy a variety of habitats. Along rocky shores, starfishes and sea urchins cling to rocks, while sea cucumbers and brittle stars conceal themselves underneath. Large populations of all living groups of echinoderms can be found in mud and ooze offshore.

What unique features do echinoderms have?

Echinoderms, belonging to the phylum Echinodermata, possess unique features such as a hard, spiny covering or skin. Their internal skeletons are made of porous blocks of calcium carbonate, and they have muscles to work their skeleton. A key characteristic is their water-vascular system, a hydraulic system of canals and reservoirs that functions in the movement of tube feet. These tube feet are movable projections used for locomotion, feeding, respiration, and sensory perception. Living echinoderms exhibit a conspicuous five-rayed, or pentamerous, radial symmetry.

How do echinoderms move and eat?

Echinoderms use a unique water-vascular system for movement. This internal hydraulic system consists of canals, reservoirs, and tube feet. Water enters through the madreporite, then flows to the ring vessel and radial canals, finally reaching the tube feet. Contractions in the radial canals or ampullae extend the tube feet, enabling movement and attachment via suction and mucus. Echinoderms have diverse feeding habits, ranging from predation to scavenging and suspension feeding. Crinoids capture plankton with mucus-covered tube feet. Asteroids may extend their stomachs to digest prey externally or swallow it whole. Sea urchins scrape algae with their teeth or consume seaweed, while sea cucumbers ingest mud and sand, extracting organic material.

Why are echinoderms important to ocean ecosystems?

Echinoderms play several important roles in marine ecosystems. As scavengers and predators, they regulate populations of smaller organisms and keep the seafloor clean. Sea urchins control seaweed growth on coral reefs, which allows corals to thrive, but can also devastate sea-grass beds if their populations grow too large. Additionally, many echinoderms consume sediment, which alters its composition as it passes through their bodies, while burrowing species disturb surface and subsurface sediments. Echinoderms also serve as a food source for other animals and their larvae feed planktonic organisms.

echinoderm

References & Edit History Quick Facts & Related Topics

Images & Videos

A common comet star and a cushion sea star rest on colorful sponges and seagrass in a vibrant Caribbean underwater scene.

examine a variety of echinoderm species such as starfish, basket star, sand dollar, and sea cucumber

Observe sea star reproduction, from egg and sperm release and fertilization to development of zooplankton embryos and larvae

watch a starfish's tube feet pry open a mussel's shell and extrude its stomach onto the mussel

Two orange, spiny California sea cucumbers (Parastichopus californicus) intertwined on a rocky surface against a dark underwater background.

At a Glance

echinoderm summary

echinoderm

animal phylum

Also known as: Echinodermata

Written by

David Leo PawsonAll

Fact-checked by

Britannica Editors

Last updated

Oct. 25, 2026 •History

Top Questions

What is an echinoderm?
Echinoderms are a phylum of marine animals characterized by a hard, spiny covering or skin. Beginning in the Cambrian Period, they have a rich fossil history. Living echinoderms include sea lilies and feather stars, sea urchins, sea cucumbers, starfish, brittle stars, and sea daisies.

They have a skeleton of mineral calcium carbonate. Part of their body cavity is a water-vascular system with fluid-filled vessels that are pushed out from the body surface as tube feet and other structures used in locomotion, feeding, respiration, and sensory perception. Though many of the earliest echinoderms lacked symmetry or were bilaterally symmetrical, living echinoderms exhibit radial pentamerous symmetry.
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1. echinoderm: General features
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What are some examples of echinoderms?
Echinoderms are a diverse phylum of marine animals characterized by their hard, spiny skin. This group includes familiar creatures such as sea stars (also known as starfishes), sea urchins, sea cucumbers, sea lilies, and brittle stars.

Echinoderms are exclusively marine animals that live in a variety of habitats, from rocky shores to deep-sea trenches. They play important roles in marine ecosystems, including scavenging decaying matter and preying on small organisms. They are found in the warmest and coldest seas. Most echinoderms are small, but some sea cucumbers can be as long as two meters, and some sea stars can be up to one meter in diameter.
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1. muscle: Echinoderms
2. echinoderm
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Where do echinoderms live?
Echinoderms are exclusively marine animals that live in a variety of habitats, from rocky shores to sandy areas, and can be found in the warmest and coldest seas. Most cannot tolerate changes in salinity, temperature, and light intensity, and they tend to move away from areas where these factors are not optimal.

Echinoderms occupy a variety of habitats. Along rocky shores, starfishes and sea urchins cling to rocks, while sea cucumbers and brittle stars conceal themselves underneath. Large populations of all living groups of echinoderms can be found in mud and ooze offshore.
2 Britannica Sources
1. echinoderm: Habitats
2. list of echinoderms
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What unique features do echinoderms have?
Echinoderms, belonging to the phylum Echinodermata, possess unique features such as a hard, spiny covering or skin. Their internal skeletons are made of porous blocks of calcium carbonate, and they have muscles to work their skeleton. A key characteristic is their water-vascular system, a hydraulic system of canals and reservoirs that functions in the movement of tube feet. These tube feet are movable projections used for locomotion, feeding, respiration, and sensory perception. Living echinoderms exhibit a conspicuous five-rayed, or pentamerous, radial symmetry.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do echinoderms move and eat?
Echinoderms use a unique water-vascular system for movement. This internal hydraulic system consists of canals, reservoirs, and tube feet. Water enters through the madreporite, then flows to the ring vessel and radial canals, finally reaching the tube feet. Contractions in the radial canals or ampullae extend the tube feet, enabling movement and attachment via suction and mucus.

Echinoderms have diverse feeding habits, ranging from predation to scavenging and suspension feeding. Crinoids capture plankton with mucus-covered tube feet. Asteroids may extend their stomachs to digest prey externally or swallow it whole. Sea urchins scrape algae with their teeth or consume seaweed, while sea cucumbers ingest mud and sand, extracting organic material.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why are echinoderms important to ocean ecosystems?
Echinoderms play several important roles in marine ecosystems. As scavengers and predators, they regulate populations of smaller organisms and keep the seafloor clean. Sea urchins control seaweed growth on coral reefs, which allows corals to thrive, but can also devastate sea-grass beds if their populations grow too large.

Additionally, many echinoderms consume sediment, which alters its composition as it passes through their bodies, while burrowing species disturb surface and subsurface sediments. Echinoderms also serve as a food source for other animals and their larvae feed planktonic organisms.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

echinoderm, any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin. Beginning with the dawn of the Cambrian Period (542 million to 488 million years ago), echinoderms have a rich fossil history and are well represented by many bizarre groups, most of which are now extinct. Living representatives include the classes Crinoidea (sea lilies and feather stars), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), Asteroidea (starfishes, or sea stars), Ophiuroidea (basket stars and serpent stars, or brittle stars), and Concentricycloidea (sea daisies; discovered in the 1980s).

Echinoderms have been recognized since ancient times; echinoids, for example, were used extensively by Greeks and Romans for medicinal purposes and as food. During the Middle Ages, fossil echinoids and parts of fossil crinoids were objects of superstition. In the early part of the 19th century, Echinodermata was recognized as a distinct group of animals and was occasionally associated with the cnidarians and selected other phyla in a division of the animal kingdom known as the Radiata; the concept of a superphylum called Radiata is no longer valid.

Echinoderms are separated into 21 classes, based mainly on differences in skeletal structures. The number of extant species exceeds 6,500, and approximately 13,000 fossil species have been described.

General features

Size range and diversity of structure

sea cucumberSea cucumber (*Kolga hyalina*) from the deep seafloor of the Arctic Ocean's Canadian Basin.

Although most echinoderms are of small size, ranging up to 10 cm (four inches) in length or diameter, some reach relatively large sizes; e.g., some sea cucumbers are as long as two metres (about 6.6 feet), and a few starfishes have a diameter of up to one metre. Among the largest echinoderms were some extinct (fossil) crinoids (sea lilies), whose stems exceeded 20 metres (66 feet) in length.

Echinoderms exhibit a great diversity of body forms, especially among the extinct groups. Although all living echinoderms have a pentamerous (five-part) radial symmetry, an internal skeleton, and a water-vascular system derived from the coelom (central cavity), their general appearance ranges from that of the stemmed, flowerlike sea lilies, to the wormlike, burrowing sea cucumbers, to the heavily armoured intertidal starfish or sea urchin. The general shape of the echinoderm may be that of a star with arms extended from a central disk or with branched and feathery arms extended from a body often attached to a stalk, or it may be round to cylindrical. Plates of the internal skeleton may articulate with each other (as in sea stars) or be sutured together to form a rigid test (sea urchins). Projections from the skeleton, sometimes resembling spikes, which are typical of echinoderms, give the phylum its name (from Greek echinos, “spiny,” and derma, “skin”). The surface of holothurians, however, is merely warty.

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

Britannica Quiz

Animal Group Names

Echinoderms also exhibit especially brilliant colours such as reds, oranges, greens, and purples. Many tropical species are dark brown to black, but lighter colours, particularly yellows, are common among species not normally exposed to strong sunlight.

echinoderm

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General features

Size range and diversity of structure