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There are dozens or hundreds of mythical creature shapes.
Such a shape cannot exist in ordinary 3 dimensional space.
Cyanobacteria, like other bacteria, exist in many shapes and sizes. However, three common shapes are observed most frequently in cyanobacteria: spherical, rod and spiral.
None. A line of symmetry, if it exists, is infinitely long and so a third of the line cannot exist as a separate entity.
There is no limit to the number of sides that a shape can have. It is not possible to tell what the largest number is unless you know all the shapes that exist in the universe.