Why Round shape?

Viewing the world through the eyes of his Pythagorean forebears, Archytas could not avoid pondering the possible hidden, underlying numbers and geometry. "Why are the parts of plants and animals (except for the organs) all round?" he asked, "of plants, the stems and branches, of animals, the legs, thighs, arms, thorax? Neither the whole animal nor any part is triangular or polygonal." He suspected there was a "proportion of equality in natural motion, since all things move proportionately, and this is the only motion that returns back to itself, so that when it occurs it produces circles and rounded curves." ~ page 121 (The Music of Pythagoras)