polyplacophora
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Polyplacophora
(′päl·i·pla′käf·ə·rə)
(invertebrate zoology) The chitons, an order of mollusks in the class Amphineura distinguished by an elliptical body with a dorsal shell that comprises eight calcareous plates overlapping posteriorly.
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Polyplacophora
A class within the phylum Mollusca whose members are popularly called chitons, coat-of-mail shells, or sea cradles. All chitons are marine, and they typically live in the intertidal zone, although some live in deeper waters. They are found from subarctic to tropical latitudes but are most abundant in warmer waters. There are roughly 750 chiton species living today.
Polyplacophorans exhibit bilateral symmetry and are oval or elongate in outline. Chitons vary in length from a few millimeters (&frac116; in.) to over 30 cm (12 in.), though most are a few centimeters (about 1 in.) long. They are flattened dorsoventrally and bear eight shell plates on their back. The plates, termed valves, are formed from crystals of the mineral aragonite, and the upper layer of the valves contains canals filled with sensory organs called esthetes. The larger of these structures serve as photoreceptors and hence act as eyes. See also Chiton.
Chitons possess three valve types—head, intermediate, and tail. The valves are laterally embedded in the fleshy but tough girdle, which typically bears small aragonite spicules or scales of varying shapes. Ventrally, polyplacophorans possess a large foot which they use for moving about and for attaching to rocks in a manner similar to a limpet gastropod. The gills hang along both sides of the animal, in a deep groove between the foot and the shell. The anus is located near the posterior end of the foot. At the front of the foot is the head which bears the mouth with its associated rasping structure (the radula, which is characteristic of most mollusks). A unique feature of the chiton radula is that the largest teeth are hardened with magnetite, an iron mineral. Characters of the valves, the gills, and the radula typically have been used to differentiate major groups within the Polyplacophora. See also Limpet; Mollusca.
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Polyplacophora
IN BRIEF: n. - Small class of marine mollusks comprising the chitons.
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McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.
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McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
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