Authors: Ian Smith & Malcolm Storey. Revised September 2015
Up to 65mm long, exceptionally up to 120mm in Netherlands (P.H. van Bragt). Ample mantle covers whole body and extends well beyond foot [image7]. Often, low profile similar to planarian worm, wide mantle skirt almost horizontal  even when body humped up . Mantle covered with tightly packed rounded spiculose tubercles . Various shades of, usually dull, yellow (SSF) & , brown , orange-brown , pink-brown  or purple-brown  & . Up to about 16 whitish acid producing papillae , usually arranged subdorsally along flanks . Acid-papillae often surrounded by a pale patch which may radiate a short distance . Underside of mantle usually has distinct dark marks of reddish  or purplish brown , but sometimes few  or no marks (SSF).
Stout stem mottled brown and purplish . 12 or 13 yellowish lamellae . Margins of pits slightly raised by tubercles.
About 7 small pale tripinnate  gills in ring, partially separated into left and right groups . Often marked with brown ; retract into cavity .
Small and indistinct , apart from well developed linear oral tentacles. More distinct when mouth protruded .
Rounded anterior bilaminate ; upper lamina cut centrally and extends forward of lower layer. Sole and upper surface of foot yellow  or orange . Upper surface of foot usually also has dark marks  similar to underside of mantle. May protrude slightly when in motion , otherwise much smaller than mantle .
- Up to 65mm long (exceptionally 120mm in Netherlands).
- Dull yellow or shade of brown, generally less colourful and variegated than A. pseudoargus.
- Tubercles  smaller than largest ones of A.pseudoargus, bigger than on Jorunna tomentosa.
- Dark marks on underside of mantle . (Occasionally absent SSF )
- Foot much smaller than mantle .
- Up to 12 whitish papillae on sides of mantle, often in pale patch .
- Head has well developed linear oral tentacles .
- Gill ring partially separated into left and right groups ; not surrounded by raised collar.
- Spawn a broad, strongly undulate ribbon laid in a spiral attached to substrate by one edge. Ova not arranged in discernible lines 
- Up to 120mm long, half grown ones similar size to G. planata.
- Mantle usually yellow or buff-orange, often with large blotches of bright colours [3Ap].
- Variously sized spiculose tubercles give stiff unyielding feel to mantle, but G. planata even stiffer.
- Gills often blotched with colour.
- Gills tilt backwards, not upwards from a raised collar.
- Underside of mantle lacks dark markings (may have slight staining at edge) [4Ap].
- Head small with vestigal oral tentacles.
- Spawn a broad, slightly undulate ribbon laid in a spiral attached to substrate by one edge. Ova arranged in transverse lines EML
- Up to 60mm long, similar size to Geitodoris planata.
- Sandy [1Jt], buff, whitish (Flickr) or bluish white (Flickr), any other marks few, small and dark.
- Very small tubercles; velvety appearance, soft feel.
- Large pale gills around dark brown anus [1Jt] .
- Base of extended gills surrounded by raised collar [2Jt].
- Underside of mantle lacks dark markings.
LWS and sublittorally to 45m. Well camouflaged on stone by dull colour and lack of shadow from flat profile . Feeds on sponges such as Hemimycale columella (EML), Mycale rotalis (EML) and Mycale micracanthoxea (SSF). Exudes defensive acid from large whitish papillae on mantle. When disturbed, tends to remain flattish while it contorts, rather than contracting tightly like Archidoris pseudoargus. Simultaneous hermaphrodite. Little known about breeding season and biology. Spawn seen in September;a broad, strongly undulate ribbon laid in a spiral attached to substrate by one edge. yellow  or white. Ova not arranged in discernible lines .
Britain, Ireland, Netherlands (common in Oosterschelde SSF), Brittany, Spain and New England (USA) (GBIF map). Population explosion around year 2000 in Oosterschelde, Netherlands led to its Dutch name, "Millennium wratslak".
Scattered finds on west and south coasts of Britain and Ireland, but few records despite large size; perhaps overlooked as Archidoris pseudoargus, but probably scarce in Britain, possibly increasing. ( UK interactive distribution map. NBN.)