This is a class of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world.

The mantle cavity is reduced into a simple cloaca, into which the anus and excretory organs empty, and is located at the posterior of the animal.

[6] They probably start life as amorphous calcium carbonate, which the organic matrix coaxes into an aragonitic habit as the spines mature.

[6] Sclerites can be shaped as simple spines, straight, curved, keeled, striated or hooked; or as cupped blades; more complex arrangements are common in copulatory spicules.

[10] The relationship with other molluscs, however, is apparent from some features of the digestive system; aplacophorans possess both a radula and a style.

During development, the mantle cavity of the larva curls up and closes, creating the worm-like form of the adult.

Molecular and fossil evidence seemed to put Aplacophorans in the clade Aculifera, as a sister group to Polyplacophora.