(Montagu, 1815)
Description (shell):
The shell is internal (covered by the mantle), length up to 50 mm, transparent, fragile, having the aperture so wide that it occupies nearly the whole of the ventral surface.
Animal:
The body may reach 120 mm in length, pale brown, with patches of darker brown on the foot and between the retractile, soft, conical mantle tubercles. The skin can secrete defensive sulphuric acid if attacked. Pale blue-green stellate calcareous spicules are present in the skin. An elongated, bipectinate gill is present under the right side of the mantle skirt; a double row of alternating tubercles is present along its midrib. At the rear of the body, a conspicuous metapodial gland develops at a pre-sexual stage, when the body is about 40 mm long. Anteriorly there are two pairs of head tentacles, the more dorsal of which (the rhinophores) are longitudinally inrolled (P. membranaceus-animal).
Habitat:
P. membranaceus occurs mainly in clean rocky areas, in shallow waters. This species is a good swimmer, using asynchronous strokes of the dilated sides of the foot, so that the whole body flutters in an upside-down posture.
Distribution:
This species has been recorded from the Atlantic and Mediterranean coasts of France, Sicily, Naples and Algeria, to 70 m (Distr. P. membranaceus).