In an earlier paper (Boney, 1973) the skeletal variations, and their quantitative significance, were described for autumn collections of Dictyocha speculum obtained with a 600 m.p.i. tow-net from surface waters in the Fairlie Channel, Firth of Clyde, in 1970. It was shown that skeletal abnormalities constituted a very small number (1·78%) of the 6025 skeletons examined. In these same samples numerous double skeletons were observed. These are regarded as pre-division stages (Gemeinhardt, 1930; Hovasse, 1932; Marshall, 1934), in which the replica of the first skeleton usually has thinner tubular skeletal rods (Marshall, 1934). A detailed examination of these double skeletons was undertaken to determine whether true mirror-imaging occurred, and the extent to which a ‘normal’ skeleton was joined to a partner of aberrant form. The methods used to obtain slide preparations in glycerine jelly were described in the previous paper (Boney, 1973) and the terminology of the components of the skeletal unit are also the same (Fig. 1A–C).