Polyplacophora

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Polyplacophorans include about 600 extant species. Entirely marine, they inhabit hard bottoms and rocky coasts in all of the world's oceans. Although commonly intertidal, living chitons have been dredged from waters as deep as 7000 m.

Of the two subclasses, Paleoloricata and Neoloricata, only the latter is represented by living examples. The shell plate of a neoloricatan extends beneath the adjacent plate. Lacking this feature, the paleoloricatates are considered less derived. The polyplacophoran nervous system and configuration of the heart and gonads resemble those of members of the molluscan class Aplacophora; also like an aplacophoran, a chiton has calcareous spicules in the mantle.

Polyplacophorans are well suited for life in the fringe of the ocean where there are the combined dangers of beating waves and exposure to the atmosphere. A chiton is flattened and elongated, and can adhere tightly to hard substrata with its strong foot. It usually attaches by a combination of muscular contraction and adhesive secretions, but may attach more firmly by creating a seal against the surface with its girdle and then raising its inner margin to generate negative pressure.

Adults of modern species range from 8 mm to 33 cm in length. A chiton has eight overlapping shell plates, and can, if dislodged, roll into a rough ball with its plates on the exterior. The shell has three layers, and, at least in young individuals, small tubes called aesthetes that bear photoreceptors pierce the plates. The thickened mantle edge is termed a girdle; it extends onto the dorsal side of the plates, completely covering them in Cryptochiton stelleri. The girdle is reinforced with calcareous spicules.

The head is reduced, and lacks eyes and tentacles. A chiton's subradular chemosensory organ can be extended out of the mouth to sense the substrate. Most chitons feed by rasping algae and other encrusted food off of the rocks on which they crawl. One genus is predatory, trapping small invertebrates under the fringe of the mantle, and then eating the captured prey. In some chitons, the radula has teeth tipped with magnetite, which hardens them.

The digestive tract consists of a mouth, buccal cavity, esophagus, stomach, two-section intestine, and anus. Digestion is extracellular. Polyplacophorans have six to 88 pairs of bipectinate ctenidia, the number of which is typically not species specific, increasing as the individual grows. The ctenidia are arranged in the mantle folds that extend from front to rear along each side of the animal's foot.

The sexes are separate. Fertilization is external in seawater or in the female's mantle cavity; there is no copulation. Chitons disperse in the plankton as trochophore larvae, then settle directly as juveniles.

The class Polyplacophora extends back to the Late Cambrian Period with the early genus Matthevia. Some fossil species had only seven plates. Examples of the order Paleoloricata are found from the Late Cambrian through the Late Cretaceous. The order Neoloricata extends from the present back to the Mississippian Period. The fossil record of chitons is difficult to assess given the rarity of articulated specimens.

Class Polyplacophora is not of significant economic importance to humans in contemporary society, although indigenous people of the Pacific coast of North America are reported to have eaten Cryptochiton stelleri.

References:

Kozloff, E. N. 1990. "Class Polyplacophora," pp. 376-382 in Invertebrates. Philadelphia: Saunders College Publishing.

Pojeta, J. 1987. "Class Polyplacophora," pp. 293-297 in Fossil Invertebrates, ed. R. Boardman, A. Cheetham, and A. Rowell. Palo Alto: Blackwell Scientific Publications.

Ruppert, E. E. and R. D. Barnes. 1994. "Class Polyplacophora," pp. 372-378 in Invertebrate Zoology, 6th ed. Fort Worth: Saunders College Publishing.

Smith, J E., J. D. Carthy, G. Chapman, R. B. Clark, R., and D. Nichols. 1971. The Invertebrate Panorama. New York: Universe Books, 406 pp.

Contributors

Andrew Campbell (author), Daphne G. Fautin (author).

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature