Berthella plumula

(Montagu, 1803)

The shell is internal (covered by the mantle), length up to 30 mm, thin, transparent, colourless or pale amber, the aperture occupying nearly the whole of the ventral surface. Stellate calcareous spicules are present in the skin.
The body may reach 60 mm in overall length, pale lemon-yellow to orange, often with net-like markings in the middle of the smooth dorsal mantle. The skin can secrete defensive sulphuric acid if attacked. The bipectinate gill can be seen under the right side of the mantle skirt; its midrib lacks the prominent alternating tubercles characteristic of Pleurobranchus membranaceus. At the rear of the body, a conspicuous metapodial gland develops at a presexual stage when the body is 17.5-30 mm long. Anteriorly there is a pair of inrolled rhinophoral tentacles above the flattened oral veil (B. plumula-drawing).

It is an epifaunal carnivore, specialised for feeding upon compound ascidians such as Botryllus. It is a common species, found under stones and in clean pools on the lower shore.

From Norway to the Mediterranean Sea (Naples), always in shallow water, to 10 m (Distr. B. plumula).