Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world. All known modern forms are shell-less: only some extinct primitive forms possessed valves. The group comprises the two clades Solenogastres (Neomeniomorpha) and Caudofoveata (Chaetodermomorpha), which between them contain 28 families and about 320 species. The aplacophorans are traditionally considered ancestral to the other mollusc classes. However, the relationship between the two aplacophoran groups and to the other molluscan classes and to each other is as yet unclear.