  We study the integrable structure of the 2D Laplacian growth problem with
zero surface tension in an infinite channel with periodic boundary conditions
in the transverse direction. Similar to the Laplacian growth in radial
geometry, this problem can be embedded into the 2D Toda lattice hierarchy in
the zero dispersion limit. However, the relevant solution to the hierarchy is
different. We characterize this solution by the string equations and construct
the corresponding dispersionless tau-function. This tau-function is shown to
coincide with the genus-zero part of the generating function for double Hurwitz
numbers.
