They have thick smooth leathery skin which is strengthened with calcareous spicules; some of these are plates pierced by more than four holes, and others are cup-shaped.

[3][2] Oncus planci is known to show neoteny, which means the juvenile form is retained beyond the usual age of maturation.

The fertilised eggs are retained by the female on her tentacles where they develop directly into the adult form without undergoing metamorphosis.

[5] These sea cucumbers are found in European waters and off the coast of West Africa.

They are epifaunal species living on hard substrates, on rocks, shells, tubeworms and algae.