monoplacophoran, (class Tryblidia), any of a group of primitive marine mollusks characterized by a single, cap-shaped shell and bilateral symmetry. The term Tryblidia is preferred over Monoplacophoran and Galeroconcha, because both latter terms are taken to include several fossil groups of uncertain relationships.

In 1952 several live monoplacophorans were dredged from a depth of 3,570 m (about 11,700 feet) off the coast of Costa Rica. Until then it was thought that they had become extinct 400,000,000 years ago. Existing monoplacophorans are represented by fewer than 10 species, including Neopilina galatheae, N. ewingi, and N. valeronis. They have been found to depths of about 5,800 m off the coasts of Central and South America.

Monoplacophorans are unusual because of the combination of primitive characteristics that they possess. In addition to the single, cap-shaped shell, they have paired multiple organs, reflecting at least partial segmentation (metamerism). The gill structure and the type of metamerism suggest a much closer affinity of the mollusks with the annelid worms than was previously believed to exist; these features may also associate the mollusks more closely with the arthropods (e.g., insects, crustaceans).

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