Body (excluding appendages)
Up to 55 mm long. Translucent white showing oesophagus behind rhinophores. Medial line of white surface pigment on the tail  may be discontinuous . Very rarely any iridescence.
Often held to expose dorsum. Linear, often exceeding half body length . Rather untidily arranged groups on each side of the body with the gap between the first and second clumps overhung by cerata of first group. Semi-transparent whitish, revealing light reddish brown to dark chocolate brown internal digestive gland; see Scottish Nudibranchs for colour range. White cnidosacs clearly visible inside the near transparent distal end. Small patch of white surface pigment extends a short way over digestive glands from cnidosacs . No white pigment splashes on the rest of the ceratal surface; rarely a minute white spot or two (ScotNud). Usually no iridescence; occasionally a little on head, all over cerata very rarely.
Translucent rosy fawn , yellowish or whitish (juveniles ) with up to 30 lamellae (all complete disks), distally whitish with small amount of white pigment.
Translucent white, often with some white pigment between rhinophores , occasionally slight iridescence. Oral tentacles half as long as body, translucent white with white surface pigment on distal third. Small internal eye visible at posterior of rhinophore base .
Anterior edge convex with centre of curve indented. Slender sickle shape propodial tentacles. Translucent white sole, much wider than body .
- Cerata long, linear, rather untidy and lack iridescence . (Occasionally, a small amount of iridescence on the head or oral tentacles; very rarely all over cerata).
- No white pigment splashes down cerata, apart from small patch near top . (Sometimes a minute white spot or two on front of cerata.)
- Rhinophore lamellae all complete disks.
- Slender sickle-shape propodial tentacles .
- Usually iridescence on cerata and, sometimes, on head and body, occasionally not.
- Cerata usually neatly swept back, each group only slightly overlaps the next behind.
- Rhinophore lamellae alternate complete/incomplete disk.
- Propodial extensions shorter and stouter than on F. bostoniensis.
- White pigment splashes down front of cerata.
Facelina dubia (EML) Very similar to F. bostoniensis
- Smooth rhinophores lack lamellae.
- White pigment flecks on head and oral tentacles.
- In British & Irish waters, so far only recorded in Lough Hyne in SW Ireland.