Due to their spiny skin, it is named as (Greek "echinos" meaning "spiny" and echinoderms. This name is derived from Greek where "echinos" means "spiny and "dermos" means "skin. This phylum is a collection of some 7,000 living species described here. Echinodermata are exclusively marine species. Echinoderms are all examples of marine stars, sea cucumbers, sea urchins, sand dollars, and brittle stars. 

Echinoderms Characteristics

  • An adult echinoderm body is radially symmetrical. 

  • echinoderm larva is bilaterally symmetrical 

  • An echinoderm normally has 5 parts which make them pentamerous

  • The mouth is surrounded by a central disk leading to grooves with podia. These grooves are called ambulacral grooves. 

  • An echinoderm's endoskeleton is composed of individual parts, called ossicles. The ossicles are protected with skin or epidermis. 

  • The ossicles form a rigid shell known as a test in some echinoderms, such as sand dollars and sea urchins. 

  • In Sea cucumbers, these ossicles may also fuse to form various structures, such as the brittle spines of the sea urchin.

  • The water vascular system is a primary part of echinoderm biology. 

  • The system consists of a series of fluid-bearing tubes that attach all over the organism in a ring-like structure. The system attaches to the podia and can be used to fill it with fluid that extends the podium and stiffens it.

  • A few echinoderms are sessile filter feeders while others are actively hunting their prey. Though some filter feeder 's food is direct to their mouth

  • Echinodermata circulatory system and nervous system is a basic system that circulates around its body. Their hemal system is environmentally open and allows for the exchange of gas through a series of channels throughout the body.

Classification of Echinodermata

The phylum echinoderms are classified into five classes: They are-

  • Asteroidea (sea stars)

  • Ophiuroidea (brittle stars)

  • Echinoidea (sea urchins and sand dollars)

  • Crinoidea (sea lilies or feather stars)

  • Holothuroidea (sea cucumbers).

In this article, Echinodermata classes are discussed below in detail


  • Over all 1,800 species are well known

  • Come in a large variety of shapes, colors, and sizes

  • The key characteristic is it includes thick arms that extend from a central disk where organs penetrate into the arms.

  • They use their tube feet for gripping surfaces and to grasp prey

  • They have two stomachs, one of which can protrude into or on prey through their mouths and secrete digestive juices even before ingestion;

  • Prey is liquified to make the digestion process easier.


  • They have long, thin arms that are sharply demarcated from the central disk 

  • They have the strongest tendency toward 5-segment radial (pentaradial) symmetry

  • They are scavengers or detritivores

  • Regrowth or regeneration of lost arms and disk is seen


  • They do not have arms.

  • Tube feet that are present are extruded through pores of a continuous internal shell called a test.

  • They have roughly spherical bodies, with five equally-sized parts radiating out from their central axes.

  • They are oval in shape.

  • The body is slightly domed on the upper surface of, but the underside is flat.


  • These species are suspension feeders.

  • They exist in shallow water and in depths as far as 6,000 meters.

  • The crinoids are characterized by a mouth surrounded by feeding arms on the top surface.

  • It consists of U-shaped gut and their anus is located next to the mouth

  • They have more than five arms.


  • They are extended in the oral-aboral axis and have five rows of tube feet.

  • It has bilateral symmetry.

  • reduced to isolated microscopic ossicles joined by connective tissue.

Echinodermata Examples

Unlike other phyla invertebrate which contains hundreds or thousands of different animal types, Echinodermata contains only five different animal classes:

Starfish, or sea stars: These are the classic echinoderms that can come in a range of colors and textures and have five arms of varying thickness.

Brittle stars: These are also called serpent stars. It looks like typical starfish except that their legs are much skinnier and more fragile (thus the 'brittle' in their name), and they have more prominent spines than the sea stars.

Echinoid (sand dollars, sea urchins, and heart urchins): Echinoids contain no arm-like appendages. At the top, they are rounded and flat on the bottom and have spines of varying length. Sea urchins having longer spines and sand dollars and heart urchins having shorter spines.

Sea cucumbers: They have long, cucumber-like bodies with bumps on the surface.

Sea lilies: These are the most ancient echinoderms. They are lone sessile, immobile, organisms in the phylum. They have at least ten appendages that extend from their body's main stalk, which they use to feed on detritus or organic matter floating through the ocean.

Key Points

  • Echinoderms exist in aquatic environments only; they are widely divergent, with more than 7,000 recognized species in the phylum.

  • Echinoderms have pentaradial symmetry and a calcareous endoskeleton and may have pigment cells and give them a wide variety of colors and toxin-possessing cells.

  • Echinoderms have a water vascular system consisting of a central canal ring that extends along each arm, through which water circulates for exchange and nutrition of gasses.

  • The Echinodermata nervous system is very simple. It consists of a concentrated nerve ring and five radial nerves running outwards along the arms; no structure resembles a brain.

  • In echinoderms, there are two sexes, each releasing their eggs and sperm into the water; here, the sperm will fertilize the eggs.

  • Through regeneration echinoderms can reproduce asexually.

FAQ (Frequently Asked Questions)

1. Write any five Features of Echinodermata?

• These are aquatic only species.

• Bilateral symmetry of the larval forms, and radial symmetry of the adult forms.

• Is triploblastic.

• It displays the organizational grade of the organ system.

• They've got real coelom.

• Uniquely shaped body.

2. Do echinoderms have blood?

Alternatively, an echinoderm uses a water vascular system to bring oxygen to its vital organs, without blood or pulse. The tubes then squeeze oxygenated water into the rest of its body after drawing seawater through its tubular feet.

3. Why are echinoderms only Marine?

Echinoderms are nothing more than a skeleton of water and tiny plates. Echinoderms don't have large muscles operating on body parts like many other species. Instead they use a special water-vascular system to pass, feed and breathe. They are purely marine animals.