Echinoderms

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Glossary

          Members of the phylum Echinodermata, meaning “spiny-skinned,” display many unique traits for invertebrates.  There are over 6,000 species of echinoderms, all of which are found only in the marine environment and the majority of which are benthic organisms, meaning they live on the seafloor.  Most echinoderms show pentamerous radial symmetry, five equal segments radiating from a central body.  Unique to their phylum, echinoderms possess a water vascular system, a network of water-filled canals that along with muscular extensions called podia (tube feet) allow these organisms to move around the seafloor.  By expanding or contracting various chambers in the water vascular system, water is forced into the tube feet, extending or retracting them and allowing echinoderms to walk.  Connecting the water vascular system to the exterior is a porous plate called the madreporite. 

            There are five extant classes of echinoderms:

1)      Asteroidea (true seastars)   

 (picture: http://www.seasky.org/reeflife/sea2d.html)

 2)      Ophiuroidea (brittle stars)

(picture: http://www.starfish.ch/reef/echinoderms.html)

3)      Echinoidea (sea urchins and sand dollars)

  (picture: http://mbgnet.mobot.org/salt/animals/echinod.htm)

 
4)      Holothuroidea (sea cucumbers)

  (picture: http://www.starfish.ch/reef/echinoderms.html)

5)      Crinoidea (feather stars and sea lilies)

(picture: http://animaldiversity.ummz.umich.edu/site/accounts/classification/Echinodermata.html)