An axisymmetric density distribution likely to be representative for many Planetary Nebulae (PN). We suggest that the axial symmetry of PN results from a predominant ejection of matter in the equatorial direction due to the duplicity of the central star (Livio et al.,1979, Ap. J.,188,1). We present an illustrative example of the formation of the bipolar PN Soutern Crab (He 2–104). In this model high velocity matter ejected by hot central star interacts with an outer oblate envelope located around a symbiotic binary star. The binary consists of a Mira variable, ejected matter forming a thick disk, and a hot component (Lutz et al.,1989, PASP 101,966). Accretion of some disk matter onto the hot component (dwarf) may lead to recurrent thermal shell flashes (Igumenshchev et al., 1990, Astrofizika,30,282). which result in the double shell nebula, observed in the Southern Crab (Burgarella et al., 1991, A&A,249,199). The phase of forming a single shell stucture of the Southern Crab was simulated numerically. The results of a 2D hydrodynamical nonadiabatic calculation are illustrated in the Figure. The density contours with logarithmic spacing of 0.25 and the velocity field (arrows) of the model are shown at time t ≃ 360 yrs after the central star outburst with energy 1044erg. The dense shell may be observed as a Crab-like nebula (“hour-glass” type). In this model the mass of the envelope is 7 · 10−4 M ⊙ and the maximal velocity is over of 200 km/s.