Echinoderm

Echinoderms (Echinodermata) is a phylum of marine animals that are covered
with spines and plates. This phylum dates back to the lower Cambrian and
represents about 6000 modern species. Modern classes include crinoids, sand
dollars, sea urchins, starfish, and Holothurids (Sea Cucumbers). Fossil
forms included Blastoids, Edrioasteroids and several peculiar Early Cambrian
animals such as Helioplacus, Carpoids, Homalozoa and possibly Machaerids.

They evolved from bilaterally symmetric creatures. Later forms were
lopsided. In development, the first cells organize themselves in a
bilaterally symmetric fashion that makes them look like embryonic chordates.
Later, the left side of the body grows at the expense of the right side,
which is eventually absorbed. The left side then grows in a pentaradially
symmetric fashion.

All echinoderms exhibit fivefold radial symmetry in portions of their body
at some stage of life, even if they have secondary bilateral symmetry. They
also have a mesodermal internal skeleton made of tiny calcified plates. They
have a decentralized nervous system.

Echinoderms, like chordates are deuterostomes and are therefore thought to
be the most closely related of the major phyla to the chordates, being a
sister group to chordates plus hemichordates. (Some believe that acorn worms
are more closely related to echidnoderms than chordates.) Because of a
controversial interpretation of Homalozoa, a minority of classifiers place
the echidnoderms into the Chordata.

Classification

   * Class Asteroidea (asteroids / starfish): 1,500 species
   * Class Echinoidea (echinoids / sea urchins): 1,000 species
   * Class Crinoidea (crinoids / feather stars): 600 species
   * Class Holothuroidea (sea cucumbers): 1,000 species
   * Class Ophuiroidea (brittle stars and basket stars)

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