Young Facelina bostoniensis, length 5.5 mm.
The short cerata at this stage might lead it to be mistaken for F. auriculata but the following features show that it is F. bostoniensis.
1: Broad foot, length 4X width. 2: long, slender propodial tentacles. 3: shield shape subapical white mark. 4: no splashes of white further down the ceras. 5: white pigment between the rhinophores.
Menai Strait, Wales. February 2013.