The name Aplacophora translates as "bearer of no shells". These molluscs, ranging in size from 5 to 300 mm in length, look very like worms and belong to the least-diverse and least-researched class in the phylum.
They were once thought to be sea cucumbers, but were reclassified as molluscs due to their digestive system.
Instead of shells they are protected by tiny calcareous needle-like spicules that protrude from the skin. Many species lack gills and/or a radula (a rasp-like ribbon used to tear up food and draw it into the mouth that is common to most molluscs).
All aplacophorans are marine and many live in deep-water habitats.