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Characteristics
Echinoderms are among the most distinctive of all animal phyla. Inclusion in the phylum is readily diagnosable on basis of the four synapomorphies below. |
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Classes of Echinoderms
Sea stars typically have five arms (though they may have more) which merge gradually with the central disc. On the aboral surface can be found the madreporite and many spines, with pedicellariae at the base of the spines. |
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Crinoids
The crinoids are the most ancient and, in some respects the most primitive, of echinoderms. Attached stalked crinoids, called sea-lilies, flourished during the Palaezoic era, and some 80 species still exist today. |
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Cushion Stars
Asterina gibbosa is found from the lower shore down to 30 metres. It prefers the underside of stones, in fairly exposed locations, where there is some sand and gravel and abundant organic matter on which it feeds. |
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Discussion of Phylogenetic Relationships
With the possible exception of two species, all extant echinoderms fall into five well-defined clades, traditionally ranked as Classes: the Crinoidea, Asteroidea, Ophiuroidea, Echinoidea, and Holothuroidea. |
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Holothuroidea
The Holothuroidea, or sea cucumbers, are an abundant and diverse group of worm-like and usually soft-bodied echinoderms. They are found in nearly every marine environment, but are most diverse on tropical shallow-water coral reefs. |
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Introduction
Echinoderms form a well-defined and highly-derived clade of metazoans. They have attracted much attention due to their extensive fossil record, ecological importance in the marine realm, intriguing adult morphology, unusual biomechanical properties, and experimentally manipulable embryos. |
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Ophiuroidea
Ophiuroids are a large group (over 1600 species) of echinoderms that includes the brittle stars (Ophiurida) and basket stars (Euryalida). |
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Stelleroidea
The class Stelleroidea contains two subclasses i.e. the Asteroidea (starfish) and the Ophiuroidea (brittle stars). |
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