Echinoderms Study Guide


Echinoderm is any group of marine invertebrates with a hard, spiny covering or skin belonging to the phylum Echinodermata. Echinoderms have a rich fossil history dating back to the Cambrian Period (542 million to 488 million years ago).

They are well represented by numerous strange taxa, which are now extinct. Crinoidea (sea lilies and feather stars), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), Asteroidea (starfishes or sea stars), Ophiuroidea (basket stars and snake stars, or brittle stars), and Concentricycloidea are living examples (sea daisies; discovered in the 1980s).

Echinoderms are classified into 21 groups based mostly on skeletal structural distinctions. There are around 6,500 living species, and over 13,000 fossil species have been described.

General features of echinoderms

Common examples of echinodermataSource

Structure variety and size range

Although most echinoderms are tiny, measuring up to 10 cm (four inches) in length or diameter, some can grow to be quite enormous; for example, certain sea cucumbers can grow to be two meters (6.6 feet) long, and a few starfishes can grow to be one meter in diameter. Some of the largest echinoderms include crinoids (sea lilies) with stems as long as 20 meters (66 feet). However, crinoids are extinct now.

  • Echinoderms have a wide range of body shapes, especially among extinct taxa.
  • Although all living echinoderms have pentamerous (five-part) radial symmetry, an internal skeleton, and a water-vascular system derived from the coelom (central cavity).
  • Their general appearance varies from stemmed, flowerlike sea lilies to wormlike, burrowing sea cucumbers to heavily armored intertidal starfish or sea urchins.
  • The echinoderm’s typical form might be that of a star, with arms extending from a central disc, or it can be spherical to cylindrical, with branching and feathery arms extending from a body frequently attached to a stalk.
  • Internal skeleton plates can articulate (like in sea stars) or be sutured together to produce a rigid test (sea urchins).
  • The phylum gets its name from skeletal projections that resemble spikes and are characteristic of echinoderms (from Greek echinos, “spiny,” and derma, “skin”).

The surface of holothurians, on the other hand, is just warty. Echinoderms have vibrant colors, like red, orange, green, and purple. Many tropical species are dark brown or black, although brighter colors, especially yellows, are widespread among species not generally exposed to direct sunshine.

Nature’s important role

  • Echinoderms are excellent scavengers of decaying debris on the bottom, and they feed on a range of microscopic species, helping to keep their numbers in check. When in huge numbers, sea urchins may ruin tropical seagrass meadows, harming the creatures that live there.
  • Sea urchins that dig into rocks and along shorelines can hasten shoreline erosion.
  • On the other hand, other tropical sea urchins limit the growth of seaweeds on coral reefs, allowing the corals to thrive. The removal of sea urchins causes algae overgrowth and damages coral reef habitats.


  • Echinoderms can change the structure of seabed sediments in several ways.
  • Many sea cucumbers ingest enormous amounts of sediment, remove organic stuff as it goes through the gut, and eject the rest.
  • Large populations of sea cucumbers in a region can flip over huge amounts of surface sediments, changing the physical and chemical content of the sediments significantly.
  • Burrowing starfish, sand dollars, and heart urchins disrupt surface and underlying strata to depths of up to 30 cm (12 inches).
  • Furthermore, echinoderms generate many larvae that serve as food for other planktonic species.


1. What are the 5 characteristics of echinoderms?

  • They have a star-like appearance and are spherical or elongated.
  • They are exclusively marine animals.
  • The organisms are spiny-skinned.
  • They exhibit organ system level of organization; most members have a circulatory system as well as a digestive system.
  • They are triploblastic and have a coelomic cavity.

2. Which aanimals belong to echinoderms?

Echinoderms are marine ‘invertebrates’ belonging to the phylum Echinodermata. These include starfish, brittle stars, sea cucumbers, sea urchins, sand dollars, and crinoids.

3. Give two examples of echinoderms.

Echinodermata are exclusively marine organisms. Sea stars, sea cucumbers, sea urchins, sand dollars, and brittle stars are all examples of echinoderms.

4. State two facts about echinoderms.

  • Echinoderms don’t have blood
  • Some echinoderms eat by ejecting their stomach
  • An Echinoderm can regenerate its body parts

5. What is unique about echinoderms?

Echinoderms have a distinct ambulacral or water vascular system that consists of a central ring canal and radial canals that run the length of each arm. Water travels through these systems, facilitating gas exchange, feeding, predation, and mobility.

6. What kind of food do echinoderms eat?

They eat a variety of invertebrates, including bivalves, snails, crustaceans, marine worms, other echinoderms, and even fish. They are drawn to the carcasses of deceased creatures on the seafloor.

7. Why are echinoderms weird?

Echinoderms are very sophisticated creatures with organ systems unlike anything seen in other animals. They often lack a front end, a rear end, a head, and a brain.

8. How are echinoderms important to humans?

They are a source of food and medicine for humans. Echinoderms play numerous ecological roles.

9. What do echinoderms have in common?

One feature that all echinoderms have is radial symmetry. This signifies that the organisms have appendages (or body architecture) that protrude from the body’s center, similar to the spokes on a bicycle wheel.

10. What characteristics do echinoderms have?

  • These are marine creatures.
  • The larval forms have bilateral symmetry, whereas the adult forms have radial symmetry.
  • They have three lobes.
  • It demonstrates organ system grade organisms.
  • They have a genuine coelom.
  • The body is a one-of-a-kind form and, and it can be star-shaped, elongated, or spherical.
  • The body is unsegmented and lacks ahead.

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