  We develop further our extension of the Ellis-Bruni covariant and
gauge-invariant formalism to the general relativistic treatment of density
perturbations in the presence of cosmological magnetic fields. We present
detailed analysis of the kinematical and dynamical behaviour of perturbed
magnetized FRW cosmologies containing fluid with non-zero pressure. We study
the magnetohydrodynamical effects on the growth of density irregularities
during the radiation era. Solutions are found for the evolution of density
inhomogeneities on small and large scales in the presence of pressure, and some
new physical effects are identified.
