ECHINODERMATA (Echinoderms)
The name phylum Echinodermata is derived from the Greek word for "spiny skin", although not all of the approximately 6000 species have skin that is spiny! Echinoderms are simple animals that live in the sea - none are found in freshwater or on land.
All (mature) echinoderms have an obvious central mouth, and radial symmetry (the body has a central point with body parts extending outwards from it).
Another characteristic shared by all echinoderms - although less obvious - is their water-vascular system used for "walking". Most mature echinoderms live on the bottom of the ocean and have hundreds of tiny tube feet, called podia, filled with seawater. Certain podia can be extended by the echinoderm when it expands and contracts chambers within its water-vascular system. Muscles in the podia which are used to retract them. The expansion and retraction of the appropriate podia in the correct order allows the echinoderm to walk. Many echinoderms have suckers on the ends of their podia which they use to capture food or to anchor themselves to rocks.
There are five different classes of echinoderms:
Asteroidea (Seastars)
Asteroids lack a distinctive central body (disc); they have arms that connect together at the base in such a way that it is difficult to tell where the arms end and the disc begins. Although there are a few exceptions, arms of seastars usually occur in multiples of five. Once called starfish, their more appropriate common name is seastar as they are not fish. |
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Ophiuroidea (Brittlestars & Basketstars)
The ophiuroids are distinguished from the asteroids in having a distinctive central body (disc) with arms that radiate out from the body; each arm is separate and does not connect with the arm beside it at the disc |
Crinoidea (Featherstars & Sealilies)
The crinoids are stalked echinoderms with a cup-like body that usually havs five branching arms, generally feather-like in appearance. They are passive suspension feeders, which means they rely on the ambient movement of water and produce no current of their own. |
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Echinoidea (Sea Urchins, Heart Urchins & Sand Dollars) The echinoids are encased by a rounded skeleton called a test. Their bodies have an enormous number of sharp spines protruding in all directions, providing protection from would-be predators. They have five paired rows of podia which, when extended, are long enough to reach beyond their spines. Their mouth (containing five teeth that are pointed towards the centre) is on the underside of the animal. Herbivorous urchins use their podia to pull themselves against the ocean floor and gnaw at algae with their mouth. |
Holothuroidea (Sea Cucumbers) The holothurians are distinct within the echinoderms in having a circlet of flexable tentacles around their mouths called oral tentacles. These may be simple, digitate (finger-like); pinnate (feather-like), or peltate (flattened and shield-like). They also have a calcareous (made of calcium) ring that encircles their throat, serving as an attachment point for muscles operating these oral tentacles. |
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