  Speculating that the $ud$ diquark with spin 0 has a similar mass to the
constituent $s$ quark, we introduce a symmetry between the $s$ quark and the
$\overline{ud}$ diquark. Constructing an algebra for this symmetry, we regard a
triplet of the $s$ quarks with spin up and down and the $\overline{ud}$ diquark
with spin 0 as a fundamental representation of this algebra. We further build
higher representations constructed by direct products of the fundamental
representations. We propose assignments of hadrons to the multiples of this
algebra. We find in particular that $\{D_{s}, D_{s}^{*}, \Lambda_{c}\}$,
$\{\eta_{s}, \phi, \Lambda, f_{0}(1370)\}$ and $\{\Omega_{c}, T_{sc}\}$ form a
triplet, a nonet and a quintet, respectively, where $T_{sc}$ is a genuine
tetraquark meson composed of $\overline{ud}sc$. We also find a mass relation
between them by introducing the symmetry breaking due to the mass difference
between the $s$ quark and the $\overline{ud}$ diquark. The masses of possible
tetraquarks $\overline{ud}sc$ and $\overline{ud}sb$ are estimated from the
symmetry breaking and the masses of $\Omega_{c}$ and $\Omega_{b}$ to be 2.942
GeV and 6.261 GeV, respectively.
