  We present Monte Carlo simulation results of the two-dimensional Zwanzig
fluid, which consists of hard line segments which may orient either
horizontally or vertically. At a certain critical fugacity, we observe a phase
transition with a two-dimensional Ising critical point. Above the transition
point, the system is in an ordered state, with the majority of particles being
either horizontally or vertically aligned. In contrast to previous work, we
identify the transition as being of the liquid-gas type, as opposed to
isotropic-to-nematic. This interpretation naturally accounts for the observed
Ising critical behavior. Furthermore, when the Zwanzig fluid is extended to
more allowed particle orientations, we argue that in some cases the symmetry of
a q-state Potts model with q>2 arises. This observation is used to interpret a
number of previous results.
