Members of the first two groups are rarely seen outside of museum collections, the aplacophorans because they are shell-less and wormlike, monoplacophorans because they live in great depths. They have no shell, but have calcareous spicules in the body surface. The foot is restricted to an anterior pedal shield or to a narrow groove running the length of the body.
Aplacophorans have a radula and a posterior mantle cavity.
Some are detritus feeders, others are predators. Previously the group had been known only as fossils dating from 50 to 370 million years ago. They were once thought to represent a link to the annelid worms, in having several pairs of foot retractor muscles, gills, and hearts, but unlike annelids, their bodies are not truly segmented. The animal is bilaterally symmetrical, with a well-developed foot surrounded by a groove in which there are 6 to 88 pairs of gills. The radular teeth contain the mineral magnetite, which hardens them.