What is "aplacophora"

Aplacophora \Ap`la*coph"o*ra\, n. pl. [NL., fr. Gr. 'a priv. + ? a flat cake + fe`rein to bear.] (Zo["o]l.) A division of Amphineura in which the body is naked or covered with slender spines or set[ae], but is without shelly plates.

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.

Aplacophorans are cylindrical and worm-like in form, and most very small, being no longer than ; some species, however, can reach a length of .