The body is prismatic; that is to say, lengthened with ridges or angles running along it; it is often thick and firm. There are two tentacles, consisting of several branched filaments arranged in the form of a bundle or brush, and capable of being withdrawn into tubular sheaths. The head is protected by a veil sometimes cleft into filaments. The gills are small branching plumes, arranged in a single series along a ridge bordering each side of the back.
The largest British species of the whole Order is a member of this genus, Tritonia Hombergi, which grows to six inches or more in length. It varies considerably in colour, being generally of a pink or purple hue, often varied with blue, but sometimes the entire animal is of an amber yellow.This very fine species occurs all round our coast,