Polyplacophora

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 

also known as Loricata or, more commonly, chitons, a class of invertebrates of the type Mollusca. Chitons are marine bottom-dwelling animals. The body is elongate-oval or wormlike and measures from 0.5 to 33 cm long. It is divided into a head, trunk, and foot (by which the animal is attached to the substrate). The dorsal surface is covered with a shell consisting of eight imbricate plates. The plates do not cover the entire body, but along the periphery, the mantle forms a leathery girdle with spicules. Between the edge of the mantle and the foot lies the mantle groove, or cavity, which usually has numerous feathery gills. The paired kidney ducts, the anal aperture, and the ducts of the unpaired gonad open into the mantle cavity.

The digestive system consists of radula, the pharynx, the stomach, the intestine, the liver, and two digestive glands. The heart is composed of two auricles and one ventricle. The nervous system consists of four cords and the circumpharyngeal ring. Sensory organs include aesthetes, which seem to determine the water pressure and are located in canals penetrating the shell; numerous shell eyes; and tactile organs on the girdle. The sexes are usually separate, and fertilization is external. Development involves metamorphosis.

Chitons are found mostly in shallow water and dwell on stones and rocks. They feed on algae and detritus. There are approximately 1,000 species, distributed in almost all seas. About 40 species are encountered in the USSR. Some species, such as Cryptochiton stelleri, are eaten as food by the inhabitants of Pacific coastal regions.

IA. I. STAROBOGATOV