The Morphological and Structural Classification of Planetary Nebulae
  We present a statistical analysis of a complete sample (255) of northern planetary nebulae (PNe).   Our analysis is based on morphology as a main parameter.   The major morphological classes are: round (26 % of the sample), elliptical (61 %), and bipolar (13 %) PNe.   About a half of the round and 30 % of the elliptical PNe present multiple shells.   Round PNe have higher galactic latitude (| b| = 12) and galactic height   (<z> = 753 pc), than the elliptical (|b| = 7,<z> = 308 pc) and bipolar (|b| = 3, <z>=179 pc). This possibly implies a different progenitor mass range across morphology, as a different stellar population would suggest. 
A. Manchado (1), E. Villaver (1), L. Stanghellini (2), M. A. Guerrero   (3) ((1)Instituto de Astrofisica de Canarias - Spain...