brightness correspond to locations where there is an enhancement in the total volume density of matter, and not in the fractional abundance of a particular species. Such a finding is consistent with some mechanism that has shaped the envelope by driving the mass loss process out of the strictly uniform isotropic character.

The brightness distributions of different species reveal subtle differences. Even if there is a good match between the arcs and shells seen for different species, when one moves radially from the star some arcs or thin shells that are observed in some species are not seen in others, which provides information about the chemical particularities of each species. This point is illustrated in Fig. \[fig:map\_superimposed\], where we superimpose the emission distribution of the radical C$_3$N, shown as contours, on the brightness distribution of the small radical CN and of the cyanopolyyne HC$_3$N. Referring first to the left panel of Fig. \[fig:map\_superimposed\], the shells and arcs in C$_3$N are well matched by CN, although it is also true that CN shows shells beyond $\sim$20$''$ which are not seen in C$_3$N. That is, the CN radical extends farther from the star than the radical C$_3$N. Referring now to the right 