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( Monoplacophora)
Monoplacophora is a class or, more likely, polyphyletic group of shelled mollusks (the name "Monoplacophora" means 'bearing one plate'). Previously, these organisms were known only from the fossil record, and thought to have become extinct in mid-Devonian times, until in April 1952 a living specimen was collected from deep marine sediments in the Middle America Trench off the Pacific coast of Costa Rica. In 1957 it was described and named Neopilina galatheae by its discoverer, Danish biologist Dr. Henning M. Lemche (1904-1977) [1]. So far, more than two dozen living species of Monoplacophora have been discovered; the first to be photographed live was Vema hyalina, at a depth of 400 m off Catalina Island, California, in 1977.[2] All the present species live deep down in the abyssal depths of ocean trenches. An attempt at a common name, gastroverm, has proved unsuccessful.[3] Little is known about the monoplacophora. They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres. The apex of the shell is forward. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism; indeed, with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation, which suggested a "missing link" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary. All known mollusks are thus non-segmented, and a derivation from annelids, which are always segmented, is very unlikely. The ancestors of mollusks were maybe flatworms. Monoplacophorans move on a rounded foot. Respiration is through five or six pairs of gills on either side of the body. Their reduced head lacks eyes or tentacles. It is presumed that they graze on microscopic organisms in mud or bottom detritus. They are apparently a widespread component of the benthos, having been dredged from depths of between 2000 and 6500 meters in the South Atlantic, the Gulf of Aden, the East Pacific, and the Southern Ocean off Antarctica. In 2006 a new molecular study [4] on Laevipilina antarctica revealed that 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. This study goes against previous cladistic hypotheses that the Monoplacophora are the sister group to the remainder of the conchiferans [5], [6], [7]
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