Are fibres in molecular cloud filaments real objects?
  We analyse the morphology and kinematics of dense filamentary structures
produced in a numerical simulation of a star--forming cloud of $1.4 \times 10^4
\, \Msun$ evolving under their own self--gravity in a magnetized medium. This
study is motivated by recent observations of velocity--coherent substructures
("fibres") in star-forming filaments. We find such "fibres" ubiquitously in our
simulated filament. We found that a fibre in one projection is not necessarily
a fibre in another projection, and thus, caution should be taken into account
when considering them as real objects. We found that only the densest parts of
the filament ($\sim$30\% of the densest volume, which contains $\sim$70\% of
the mass) belong to fibres in 2 projections. Moreover, it is quite common that
they are formed by separated density enhancements superposed along the line of
sight. Observations of fibres can yield insight into the level of turbulent
substructure driven by gravity, but care should be taken in interpreting the
results given the problem of line of sight superposition. We also studied the
morphology and kinematics of the 3D skeleton (spine), finding that subfilaments
accrete structured material mainly along the magnetic field lines, which are
preferentially perpendicular to the skeleton. The magnetic field is at the same
time dragged by the velocity field due to the gravitational collapse.
