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  • Monoplacophora


Monoplacophora is a class of mollusks thought to be extinct until April 1952, when a living animal was dredged up from deep marine sediments in the Middle America Trench off the Pacific coast of Costa Rica, and named Neopilina by its discoverer, Danish biologist Dr. Henning Lemche. Malacology experiences few shattering moments, but in this case "their discovery has been described as 'the most dramatic one in the history of malacology.'" (Clausen). So far, more than a dozen living species of Monoplacofora have been discovered. The name "Monoplacophora" means 'bearing one plate'. All the present species live deep down in ocean trenches.



Little is known about the monoplacophorans. They have a single, flat, rounded bilateral shell that is often thin and fragile. The apex of the shell is forward. The fossil shells resemble chitons (Class Polyplacophora) or more precisely, limpets (Class Gastropoda); however, fossils showed a series of scars on the inner side of the shell that had served as muscle attachments: it was enough to determine that the fossil mollusks were in fact neither chiton-like nor limpet-like in the arrangement of their soft parts.

Now 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, that 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. They seem to feed on microscopic organisms in mud or bottom detritus.

Neopilina galatheae was the first known living specimen. Previous specimens dated from the Paleozoic. These "living fossils" are about 0.5 to 3.0 cm in length.



Order Tryblidioidea


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