Monoplacophora

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Monoplacophora

[‚män·ō·plə′käf·ə·rə]
(invertebrate zoology)
A group of shell-bearing mollusks represented by few living forms; considered to be a sixth class of mollusks.

Monoplacophora

 

a class of marine mollusks. Until 1952 they were known only from fossil shells from deposits of the Lower Paleozoic. In 1952, Danish zoologists discovered the first modern representative of Monoplacophora—Neopilinagalatheae. The body consists of a head, a foot, and a visceral hump covered with a mitriform or flat-spiral shell lined with a mantle. The head and foot can retract into the shell by means of one to eight pairs of muscles. Between the mantle and the foot is a mantle groove, in which there are five or six pairs of feathery gills; into the groove open the ducts of six pairs of kidneys and the anal opening. The digestive system includes an esophagus with a radula, a loop-shaped gut with a stomach, and a paired liver. The heart consists of two ventricles and four auricles. The nervous system consists of four longitudinal cords united by an esophageal ring and transverse commissures. There is an organ of equilibrium—the statocyst. There are two pairs of sex glands; the sexes are separate.

Of the approximately 60 original species of Monoplacophora, six are extant. They are united in the genus Neopilina. Extinct forms from the Cambrian to the Devonian are grouped in three orders: Tryblidiida (miter-shaped forms with many body retractors; Neopilina are included here), Cyrtonellida (flat-spiral or miter-shaped forms with two pairs of retractors), and Sinuitopsida (flat-spiral forms with one pair of retractors).

REFERENCES

Beklemishev, V. N. “K voprosu o rannei evoliutsii molliuskov.” Zoologicheskii zhurnal, 1958, vol. 37, issue 4.
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.

IA. I. STAROBOGATOV

References in periodicals archive ?
Revised classification, nomenclator and typification of gastropod and monoplacophoran families.
A new Silurian retractile monoplacophoran and the origin of the gastropods.
Sheet nacre, with tablets in vertical sections in a stair-step or brick-wall pattern, is the common fabric of bivalves and monoplacophorans and occurs less commonly in cephalopods and only rarely in gastropods (Wise, 1970a, b; Carter and Clark, 1985).
Crystal growth in the foliated aragonite of monoplacophorans (Mollusca).
In contrast, no such anterior complex centers have been reported in polyplacophorans and conchiferans such as tryblidiids (monoplacophorans) and basal gastropods (Fig.
Aplacophorans, monoplacophorans, polyplacophorans, scaphopods: the lesser classes.
Apart from monoplacophorans and limpets, the Conchifera show no evidence of mineralized compounds in the radulae (Jones et al., 1935; Lowenstam, 1962a; Haszprunar, 1988).
The two are presumed to be descendant remnants of serially duplicated, symmetrical pairs of dorso-ventral shell muscles as retained by extant monoplacophorans and chitons (Knight, 1952; Stasek, 1972; Wingstrand, 1985).