The Five Classes of Echinoderms

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Echinodermata refers to a phylum of marine life that share a few commonalities. The name for this phylum comes from a Greek term meaning "spiny skin." Echinoderms feature radial symmetry, meaning their appendages radiate outward from a central location on the body. In many echinoderms, the appendages occur in multiples of five. Echinoderms have a water vascular system and skin gills that handle the waste. While echinoderms have a mouth, digestive tract and anus, they do not have heads or brains. Echinoderms are divided into five different classes.

Asteroidea

  • The asteroidea class of echinoderms is made up of starfish, also known as sea stars. Starfish live among coral reefs, as well as on sand and near rocks. When injured, starfish can regenerate the injured limbs. This regeneration occurs slowly, sometimes taking up to a year for the limb to fully return, according to the Starfish.ch website. This regeneration can sometimes cause a starfish to have a number of limbs that aren't a multiple of five.

    Starfish have a mouth and anus on their underside, with the top side usually displaying bright colors. Starfish eat by projecting their stomach outward over their prey, then liquefying the prey with digestive juices before sucking their stomach back in. Some starfish are carnivorous, and their typical prey includes sponges and mollusks. Other starfish are scavengers or specialized feeders. Some starfish, such as the crown-of-thorns starfish, are venomous and can injure people when touched or stepped on.

Ophiuroidea

  • Brittle stars fall into the class of echinoderms known as ophiuroidea. These marine invertebrates include basket stars and serpent stars. Basket stars have an almost puffball look, with multiple branches on the arms that catch plankton. Serpent stars coil up to hang on branches underwater. Brittle stars have radial symmetry and five arms.

    Instead of having an anus and mouth, brittle stars have only a mouth. The mouth, located on the underside, excretes waste produced by the brittle star. The body disk of brittle stars features several slits at the base of each side of each arm, which are used for breathing and reproduction.

Echinoidea

  • Echinoidea, or sea urchins, have radial symmetry, an external chitinous skeleton and a central jaw with teeth, known as Aristotle's lantern, according to Starfish.ch. The anus of sea urchins is located on the upper surface. Some sea urchins have a cloaca that extends out of the anus to excrete waste. Sea urchins move by using a tube foot that extends to pull the urchin along. Some urchins also use their spines to help with movement.

Holothuroidea

  • Holothuroidea, known as sea cucumbers, lack the radial symmetry of other echinoderms. These creatures are long and tube-shaped, with a mouth at one end and an anus at the other end. The mouth of sea cucumbers features tentacles that help them retrieve food. Sea cucumbers regenerate missing body parts quickly, according to Starfish.ch.

Crinoidea

  • Feather stars make up the class of echinoderms known as crinoidea. Feather stars can have as few as five arms, known as pinnules, or upwards of 200. These creatures differ from starfish in that their pinnules are feather-like while their bodies are cup-shaped. The pinnules of feather stars feature a sticky coating, which they use to catch prey.

    On the underside of feather stars are appendages known as cirri. Cirri help the feather stars stick to coral reef and sponges. Feather stars are mostly nocturnal. During the day, they hang on rocks with their pinnules curled.

References

  • Photo Credit starfish image by Christopher Meder from Fotolia.com Coral and Brittle Stars image by Peter Hedges from Fotolia.com sea urchin image by Miroslava Holasová from Fotolia.com sea cucumber image by Christian Schoettler from Fotolia.com feather star image by Amjad Shihab from Fotolia.com
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