Monoplacophora

Monoplacophora, meaning 'bearing one plate', is a polyphyletic superclass of molluscs with a cap-like shell now living at the bottom of the deep sea. Extant representatives were not recognized as such until 1952; previously they were known only from the fossil record. Although the shell of many monoplacophorans is limpet-like in shape, they are not gastropods, nor do they have any close relation to gastropods. Discussion about monoplacophorans is made difficult by the slippery definition of the taxon; some authors take it to refer to all non-gastropod molluscs with a single shell, or all single-shelled molluscs with serially repeated units; whereas other workers restrict the definition to cap-shaped forms, excluding spiral and other shapes of shell. The inclusion of the gastropod-like Bellerophontoidea within the group is also contentious. One attempt to resolve this confusion was to separate out the predominantly coiled helcionelloids from the traditional, cap-like tergomyans, this latter group containing extant Tryblidiids. Monoplacophorans are univalved (though not gastropodal), limpet-shaped, and are untorted. They have a pseudometamerism of bilaterally symmetrical repeated organs and muscles. The extant members of the class live only in the deep ocean (the abyssal zone, the continental shelf, and the continental slope) at depths below 180 metres (590 ft). Cambrian forms predominately lived in shallow seas, whereas later Paleozoic forms are more commonly found in deeper waters with soft, muddy sea floors. The repeated organs include from three to six pair of 'gills' (actually ctenidia) located in a curved line along each side of the foot (though the number is not always considered definitive of a given species), and as many as six 'kidneys' (actually nephridia). The tip or point of their low shells points forward rather than towards the back. The shell ranges from 3mm to 37mm in diameter depending on species. The mouth is located within the animal's undeveloped head in front of its single large foot and contains a radula, a defining characteristic of the mollusca. They also have a cone-shaped stomach with a single crystalline style though no gastric shield. The intestines are long and make between four and six loops before reaching the posteriorly-positioned anus. The sexes are separate with any given animal having two pair of either ovaries or testes connected to either the third or fourth pair of kidneys. One genus, Micropilina, has apparently been recorded as brooding young in the distal oviduct and pallial groove, releasing the young when approximately 300 micrometers in diameter. In 2006 a molecular study on Laevipilina antarctica suggested that extant Monoplacophora and Polyplacophora form a well-supported clade with the researched Neopilina closest to the chitons. The two classes in this new clade, with the proposed name Serialia, all show a variable number of serially repeated gills and eight sets of dorsoventral pedal retractor muscles.