  Recently, the BESIII Collaboration reported a new measurement of the $\eta_c
\rho$ decay mode of $Z_c^{(\prime)}$, which motivated us to study the inner
structure of $Z_c^{(\prime)}$ via investigating the hidden charm decays of
these two $Z_c$ states. We consider the {triangle loop mechanism} contribution
in the hidden charm decays of $Z_c^{(\prime)}$. Our estimations indicate that
the triangle loop mechanism plays an important role in the decays of the
$Z_c^{(\prime)}$, where our results are in agreement with the experimental
observations in a reasonable parameter range. Furthermore, we point out that
the $Z_c^{(\prime)}$ can be interpreted as the hadronic molecules, while the
tetraquark scenario is less favored.
