  We develop Fenchel-Nielsen coordinates for representations of surface groups
into Sp(2n,R) with maximal Toledo invariant. Analogous to classical
Fenchel-Nielsen coordinates on the Teichm\"uller space they consist of a
parametrization of representations of the fundamental group of a pair of pants
and a careful investigation of the gluing.
  As applications we obtain results for non-closed surfaces, which have been
known only for closed surfaces before: we count the number of connected
components of their representation space and prove continuity for the limit
curve for a certain type of representations.
