  The relativistic four-quark equations are found in the framework of
coupled-channel formalism. The dynamical mixing of the meson-meson states with
the four-quark states is considered. The approximate solutions of these
equations using the method based on the extraction of leading singularities of
the amplitudes are obtained. The four-quark amplitudes of cryptoexotic mesons
including the quarks of three flavours (u, d, s) and the charmed quark are
constructed. The poles of these amplitudes determine the masses of tetraquarks.
The mass values of low-lying tetraquarks are calculated.
