The eight plates of which the shell is composed are smooth and glossy, reddish-brown, marbled or patterned with intricate pale brown or white zigzag lines.

Round its margin are small, flattened spines in red, purple or green, sometimes with paler bands.

[3] Tonicella marmorea has a strong muscular foot that enables it to grip the substrate firmly; the plates have individual bundles of muscle fibres and this allows it to conform to the irregularities of the rock surface.

However, it differs from a gastropod in having a long, coiled gut, and forming faecal pellets, which pass out with the water current.

The larvae are planktonic and when sufficiently developed, settle on the seabed and undergo metamorphosis into juveniles without passing through an intervening veliger stage.