  We present optical morphologies obtained from deep HST and ground-based
images for galaxies selected from the first sub-millimeter survey of the
distant Universe. Our sample comprises galaxies detected in deep 850-micron
continuum maps of seven massive clusters, obtained using SCUBA, the new
bolometer camera on the JCMT. The survey covers a total area of 0.01 square
degrees to 1-sigma noise levels of about 2 mJy/beam. We detect a total of 25
sources at 850 microns, of which 17 and 10 are brighter than the respective 50%
and 80% completeness limits. Optical counterparts are identified for 14 of the
16 sources in the f(50%) sample and for 9 of the 10 sources in the f(80%)
sample that lie within our optical fields. The morphologies of those galaxies
for which we have HST imaging fall into three broad categories: faint disturbed
galaxies and interactions; faint galaxies too compact to classify reliably; and
dusty, star-forming galaxies at intermediate redshifts. The disturbed and
interacting galaxies constitute the largest class, which suggests that
interactions remain an important mechanism for triggering star formation and
the formation of ultraluminous galaxies in the distant Universe. The faint,
compact galaxies may represent a later evolutionary stage in these mergers, or
more centrally-concentrated starbursts. It is likely that some of these will
host AGN. Analysis of the colors of our sample allow us to estimate a crude
redshift distribution: >75% have z<5.5 whilst >50% lie at z<4.5, suggesting
that the luminous sub-mm population is coeval with the more modestly
star-forming galaxies selected by UV/optical surveys of the distant Universe.
This imposes important constraints on models of galaxy formation and evolution.
