 | Echinoderm: Encyclopedia II - Echinoderm - Physiology
Echinoderm - Physiology
Echinoderms evolved from bilaterally symmetric creatures. 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 exoskeleton 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. Most echinoderms, with the exception of the sea cucumber, have a brain, although it is very small.
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 starfish cut radially into a number of parts will, over the course of several months, regenerate into as many separate, viable starfish. 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.
Other related archivesAsteroidea, Blastoidea, Blastoids, Cambrian, Cenozoic, Concentricycloidea, Crinoidea, Echinoidea, Helicoplacus, Holothuroidea, Machaerids, Ophiuroidea, Ophuiroidea, Sexual reproduction, acorn worms, animals, bilaterally symmetric, brain, brittle stars, calcified, chordates, ciliated, circulatory system, classes, deuterostomes, fossils, geological era, hemichordates, hydraulic, mesodermal, nervous system, phylum, regeneration, sand dollars, sea cucumber, sea cucumbers, sea daisies, sea urchins, species, starfish, water vascular system
 Adapted from the Wikipedia article "Physiology", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
