  We study the influence of the matter content of extragalactic jets on their
morphology, dynamics and emission properties. For this purpose we consider jets
of extremely different compositions including pure leptonic and baryonic
plasmas. Our work is based on two-dimensional relativistic hydrodynamic
simulations of the long-term evolution of powerful extragalactic jets
propagating into a homogeneous environment. The equation of state used in the
simulations accounts for an arbitrary mixture of electrons, protons and
electron-positron pairs. Using the hydrodynamic models we have also computed
synthetic radio maps and the thermal Bremsstrahlung X-ray emission from their
cavities.
  Although there is a difference of about three orders of magnitude in the
temperatures of the cavities inflated by the simulated jets, we find that both
the morphology and the dynamic behaviour are almost independent on the assumed
composition of the jets. Their evolution proceeds in two distinct epochs.
During the first one multidimensional effects are unimportant and the jets
propagate ballistically. The second epoch starts when the first larger vortices
are produced near the jet head causing the beam cross section to increase and
the jet to decelerate. The evolution of the cocoon and cavity is in agreement
with a simple theoretical model. The beam velocities are relativistic ($\Gamma
\simeq 4$) at kiloparsec scales supporting the idea that the X-ray emission of
several extragalactic jets may be due to relativistically boosted CMB photons.
The radio emission of all models is dominated by the contribution of the hot
spots. All models exhibit a depression in the X-rays surface brightness of the
cavity interior in agreement with recent observations.
