A conjugating pair of Acanthostomella norvegica from the Chukchi Sea in 2012. Z stack of images made with a 60x objective, DIC optics, lugol's preserved specimens.
Drawing from Claparde & Lachmann 1858 (Plate 8, figure 5) of the species now known as Steenstrupiella steenstrupii as "Tintinnus steenstrupii". The scale bar reflects the text description (pg 200) as about 0.2 mm in overall length. The species (and now genus) was presumably named after the Danish biologist Japetus Steenstrup (1813-1897).