  The Born--Infeld-like effective world-volume theory of a single 3-brane is
deduced from a manifestly space-time supersymmetric description of the
corresponding $D$-brane.
  This is shown to be invariant under $SL(2,R)$ transformations that act on the
abelian gauge field as well as the bulk fields. The effective theory of two
nearby parallel three-branes involves massive world-volume supermultiplets
which transform under $SL(2,Z)$ into the dyonic solitons of four-dimensional
$N=4$ spontaneously broken $SU(2)$ Yang--Mills theory.
