Invertebrate
Zoology Echinodermata
The approximately 6,000 living species of sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers and sea lilies are all marine and mostly bottom dwellers, as were the approximately 13,000 extinct species that flourished as long ago as the early Cambrian (545 mya). Echinoderms fossilize well because their endoskeleton is composed of calcareous ossicles. The ossicles may articulate with one another, as in the flexible sea stars and brittle stars, or may be fused together to form a rigid skeletal test, as in sea urchins and sand dollars. Although echinoderm adults are radially symmetrical, echinoderms evolved from bilaterally symmetrical ancestors; evidence for this relationship includes bilaterally symmetrical echinoderm larval stages and extinct forms that were not radially symmetrical.
Most
zoologists believe that echinoderms have a common ancestry with hemichordates and
chordates because
of the deuterostome characteristics that they share: an anus that develops
in the region of the blastopore, a coelom that forms from outpockets of the
embryonic gut tract (vertebrate chordates are an exception), and radial, indeterminate
cleavage.
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Phylum
Echinodermata
1.
Calcareous endoskeleton in the form
of ossicles that arise from mesodermal tissue
2.
Adults with pentaradial symmetry and
larvae with bilateral symmetry
3.
Water-vascular system comprised of
water-filled canals used in locomotion, attachment, and/or feeding
4.
Complete digestive tract that may
be secondarily reduced
5.
Hemal system derived from coelomic
cavities
6.
Nervous system consisting of a nerve
net, nerve ring, and radial nerves
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