Tryblidiida is a taxon of monoplacophoran molluscans containing the only extant representatives: 37 species are still alive today, inhabiting the ocean at depths of between 175 and 6,400 metres (574 and 21,000 ft).

[2] The first captured living monoplacophoran was Veleropilina zografi in 1896, but at that time it was described as if it were an archaeogastropod, a true limpet, mainly because of its patelliform (limpet-like) shell.

In April 1952, a living specimen was collected from deep depths in the Middle America Trench off Costa Rica's Pacific coast.

[5][6] The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977.

They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres (in recent species).

The mantle cavity forms a horseshoe-shaped groove running around the muscular foot, in a similar fashion to that of the chitons, and contains five or six gills on either side.

The left and right aorta fuse shortly after leaving the heart, and supply blood to the open circulatory system.