Archisotoma pulchella is a moderately common littoral species that is probably present around the entire coast of Britain and Ireland. The body is blue/black to greyish in colour, up to 1.8mm long, a PAO is present and the eyes each have eight ocelli. The definitive feature for identification is the structure of the maxilla (Fig. 1 - compare this photo with Fig. 346 in Gisin (1960)). There are several specimens of Archisotoma pulchella in the NHML slide collection, many of them under other names.
Steve Hopkin suggested that the first Archisotoma species Bagnall found was Archisotoma pulchella, which he assumed to be Archisotoma besselsi. Thus when he found the true Archisotoma besselsi he described it as the new species Archisotoma nigricans. All of Bagnall's specimens labelled Archisotoma besselsi are Archisotoma pulchella.
This coastal / intertidal species can be found far inland, eg under Kew Bridge in London, in the intertidal zone of the Thames.