We discuss our analysis of long-slit optical spectra of the emission-line nebulae associated with 21 low-redshift (z < 0.2) radio galaxies. We complement our data with similar data from the literature on 19 other low-redshift radio galaxies. We find that the nebulae can be classified kinematically into three types: rotators, calm nonrotators, and violent nonrotators. The kinematic classification correlates with the relative emission-line strengths in the nebulae and with the optical and radio properties of the galaxies. The nebulae in the rotator class have [O I] 6300 angstrom, [N II] 6584 angstrom, and [S II] 6717 angstrom emission lines that are weak relative to H-alpha. The rotators are nearly always associated with galaxies having luminous "high-ionization" nuclear emission-line regions and powerful edge-brightened (Fanaroff and Riley Class II) radio sources. The ionization of the nebulae in these sources is most likely dominated by photoionization from the nuclear continuum source. These galaxies usually inhabit regions of only modest galaxy density. Their radio jet and gas rotation axes are typically aligned to within 20-degrees. The calm nonrotator nebulae have [O I], [N II], and [S II] lines that are strong relative to H-alpha. They are preferentially associated with galaxies having "low-ionization" nuclear emission-line regions and less powerful, edge-dimmed (Fanaroff & Riley Class I) or amorphous radio sources. Heating and photoionization by the surrounding hot (10(7-8) K) ISM/ICM is likely to provide an important component of the ionization of the nebulae in these sources. These galaxies generally inhabit regions of high galaxy density. The violent nonrotators seem to be more similar to the rotators in their optical/radio properties, We propose that the rotators have dynamically young disks of gas recently acquired by the radio galaxy in an interaction or merger with a gas-rich galaxy. This is consistent with the data on the morphologies, colors, and stellar dynamics of radio galaxies with strong emission lines. This gas may fuel the nuclear activity and appears to be influential in the good collimation of these classical double (FRII class) radio sources. The calm nonrotators include seven radio galaxies with well-known "cooling flows." We suggest that these nebulae are in general the optical signature of a cooling flow which may fuel the active nucleus. The violent nonrotators may be related to the rotators, but either tidal forces or interactions with outflowing radio plasma have induced large noncircular motions in the ionized gas. The association of the large-scale gas kinematics with the radio and optical properties of the active galaxy then suggests that the angular momentum of the gas which fuels the AGN may be an important parameter in the determinant of how activity is manifest in an AGN.
