  We are building a theory of simple Hurwitz numbers for the reflection groups
B and D parallel to the classical theory for the symmetric group. We also study
analogs of the cut-and-join operators. An algebraic description of Hurwitz
numbers and an explicit formula for them in terms of Schur polynomials are
provided. We also relate Hurwitz numbers for B and D to ribbon decomposition of
surfaces with boundary -- a similar result for the symmetric group was proved
earlier by Yu.Burman and the author. Finally, the generating function of
B-Hurwitz numbers is shown to give rise to two independent tau-function of the
KP hierarchy.
