  We present a survey of mid-infrared gas-phase lines toward a sample of 76
circumstellar disks around low mass pre-main sequence stars from the Spitzer
"Cores to Disks" legacy program. We report the first detections of [Ne II] and
[Fe I] toward classical T Tauri stars in ~20% respectively ~9% of our sources.
The observed [Ne II] line fluxes and upper limits are consistent with [Ne II]
excitation in an X-ray irradiated disk around stars with X-ray luminosities
L_X=10^{29}-10^{31} erg s^{-1}. [Fe I] is detected at ~10^-5-10^-4 L_Sun, but
no [S I] or [Fe II] is detected down to ~10^{-6} L_Sun. The [Fe I] detections
indicate the presence of gas-rich disks with masses of >~0.1 M_J. No H_2 0-0
S(0) and S(1) disk emission is detected, except for S(1) toward one source.
These data give upper limits on the warm (T~100-200K) gas mass of a few Jovian
masses, consistent with recent T Tauri disk models which include gas heating by
stellar radiation. Compact disk emission of hot (T>~500K) gas is observed
through the H_2 0-0 S(2) and/or S(3) lines toward ~8% of our sources. The line
fluxes are, however, higher by more than an order of magnitude than those
predicted by recent disk models, even when X-ray and excess UV radiation are
included. Similarly the [Ne II]/H_2 0-0 S(2) ratios for these sources are lower
than predicted, consistent with the presence of an additional hot molecular gas
component not included in current disk models. Oblique shocks of stellar winds
interacting with the disk can explain many aspects of the hot gas emission, but
are inconsistent with the non-detection of [S I] and [Fe II] lines.
