  We consider fermionic states bound on domain walls in a Weyl superfluid
$^3$He-A and on interfaces between $^3$He-A and a fully gapped topological
superfluid $^3$He-B. We demonstrate that in both cases fermionic spectrum
contains Fermi arcs which are continuous nodal lines of energy spectrum
terminating at the projections of two Weyl points to the plane of surface
states in momentum space. The number of Fermi arcs is determined by the index
theorem which relates bulk values of topological invariant to the number of
zero energy surface states. The index theorem is consistent with an exact
spectrum of Bogolubov- de Gennes equation obtained numerically meanwhile the
quasiclassical approximation fails to reproduce the correct number of zero
modes. Thus we demonstrate that topology describes the properties of exact
spectrum beyond quasiclassical approximation.
