  We show that the effective theory describing single component continuous
media with a linear and constant equation of state of the form $p=w\rho$ is
invariant under a 1-parameter family of continuous disformal transformations.
In the special case of $w=1/3$ (ultrarelativistic gas), such a family reduces
to conformal transformations. As examples, perfect fluids, homogeneous and
isotropic solids are discussed.
