been hollowed out by dissociating and ionizing radiation and/or shaped by the action of jet outflows (see Section 1) into bi-cones, rings, tori, etc., which may be oriented at different angles to the line of sight. Red-blue asymmetries in the profiles then arise from back-front asymmetries in the distribution of molecular gas. A second effect is the likely decrease in the opacity of the CO line as the CO is destroyed. A third effect is the size of the PN cavity relative to the telescope beam size. If the third effect were dominant, one would expect that the profile shapes would be correlated with the angular diameter of the PNe. 068.3$-$02.1 is among the smallest PNe (1.3 Aaquist & Kwok 1990), and is the case with the most parabolic profile, consistent with the emission arising in a relatively undisturbed envelope. However, there is no general correlation between the profile shapes and the angular sizes of the ionized nebulae for the sample as a whole. The prevalence of asymmetries, and strongly double-peaked profiles even among small nebulae, suggest that the structural effects in the envelopes are of major importance.

A second, striking feature of the CO spectra is the large widths 