Echinoderm
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Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all depths. This phylum appeared in the early Cambrian Period and contains about 7,000 living species and 13,000 extinct ones. Five or six classes (six counting Concentricycloidea) are alive today:
- Asteroidea (asteroids or sea stars): about 1,500 species that capture prey for their own food.
- Concentricycloidea (sea daisies), notable for their unique water vascular system; two species; recently merged into Asteroidea.
- Crinoidea (crinoids, feather stars or sea lilies): about 600 species that are suspension feeders.
- Echinoidea (echinoids, sea urchins and sand dollars): notable for their movable spines; about 1,000 species.
- Holothuroidea (sea cucumbers): elongated animals resembling slugs; about 1,000 species.
- Ophiuroidea (brittle stars and basket stars), the physically largest of echinoderms; about 1,500 species.
Extinct forms known from fossils include blastoids, edrioasteroids, and several early Cambrian animals such as Helicoplacus, carpoids, Homalozoa, and eocrinoids such as Gogia.
Echinodermata is the largest animal phylum which has no freshwater or terrestrial representatives.
[edit] Physiology
Echinoderms evolved from animals with bilateral symmetry; later forms were lopsided. Echinoderms' larvae are ciliated free-swimming organisms that organize 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, in which the body is arranged in five parts around a central axis.
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 endoskeleton made of tiny calcified plates and spines, that forms a rigid support contained within tissues of the organism; some groups have modified spines called pedicellariae that keep the animal free of debris.
Echinoderms possess a hydraulic water vascular system, a network of fluid-filled canals that function in locomotion, feeding, and gas exchange. They also possess an open and reduced circulatory system, and have a complete digestive tube (tubular gut).
They have a simple radial nervous system that consists of a modified nerve net (interconnected neurons with no central organs); nerve rings with radiating nerves around the mouth extending into each arm; the branches of these nerves coordinate the movements of the animal. Echinoderms have no brain, although some do have ganglia.
The sexes are usually separate. Sexual reproduction typically consists of releasing eggs and sperm into the water, with fertilization taking place externally.
Many echinoderms have remarkable powers of regeneration: a sea star cut radially into a number of parts will, over the course of several months, regenerate into as many separate, viable sea stars. A section as small as a single arm (with the commensurate central-body mass and neural tissue) will, in ideal circumstances, successfully regenerate in this way.
[edit] Bibliography
- Black, R M (1973). The Elements of Palaeontology, 3rd impression. Cambridge University Press, 340pp + xviii, ISBN 0-521-09615-4. (Chapter 9 deals with Echinoids).
- Clark, A M (1968). Starfishes and their relations, 2nd edition. Trustees of the British Museum (Natural History), 120pp + vi.
- Clarkson, E N K (1993). Invertebrate Palaeontology and Evolution, 3rd edition. Chapman & Hall, 434pp + ix, ISBN 0-412-47990-7. (Chapter 9 covers Echinoderms).
- Nichols, D (1969). Echinoderms, 4th (revised) edition. Hutchinson University Library, 192pp, ISBN 0-09-065994-5. (This is the same Nichols who produced the seminal work on the mode of life of the irregular echinoid, Micraster, in the English chalk).
- Shrock R R & Twenhofel W H (1953). Principles of Invertebrate Paleontology, 2nd edition. McGraw Hill International Series on the Earth Sciences, 816pp + xx, LCC 52-5341. (Chapter 14 covers Echinoderma).
[edit] External links
- The Echinoid Directory from the Natural History Museum.
- Echinodermata from the Tree of Life website.
- Berkley taxonomy on the Echinodermata
