  The $\Lambda$ baryon spectrum is studied within the SU(3) flavor symmetry in
a constituent quark model. We found that it is rather difficult to accommodate
some negative-parity $\Lambda$ resonances as single $q^2s$ ($q = u,\,d$ quarks)
states in the conventional three-quark picture. The ground $q^3s\bar q$
pentaquark mass spectrum is evaluated and a possible interpretation is proposed
in the work: the observed $\Lambda(1405)1/2^{-}$, $\Lambda(1670)1/2^{-}$ and
$\Lambda(1800)1/2^{-}$ are three-state mixtures of two $p$-wave $q^2s$ states
and one ground $q^3s\bar q$ pentaquark state, so are the
$\Lambda(1520)3/2^{-}$, $\Lambda(1690)3/2^{-}$ and $\Lambda(2050)3/2^{-}$
resonances.
