  Passive systems $\tau={T,M,N,H}$ with $M$ and $N$ as an input and output
space and $H$ as a state space are considered in the case that the main
operator on the state space is normal. Basic properties are given and a general
unitary similarity result involving some spectral theoretic conditions on the
main operator is established. A passive system $\tau$ with $M=N$ is said to be
quasi-selfadjoint if $ran(T-T^*)\subset N$. The subclass $S^{qs}$ of the Schur
class $S$ is the class formed by all transfer functions of quasi-selfadjoint
passive systems. The subclass $S^{qs}$ is characterized and minimal passive
quasi-selfadjoint realizations are studied. The connection between the transfer
function belonging to the subclass $S^{qs}$ and the $Q$-function of $T$ is
given.
