The shell of sea urchins prevent cracking and breaking via oblate shape.
"Spheres are also distorted by gravity. If a drop of liquid, held together by surface tension, is placed on a surface and therefore subjected to the force of gravity, it tends to become a more flattened shape, called 'oblate'. The shell of a sea urchin, stripped of its spines, is oblate. This shape distributes stress evenly over the surface and therefore reduces the likelihood of cracking or breaking. The guiding principle of economy is always apparent: a shape is most efficient when it reduces its work to a minimum." (Foy and Oxford Scientific Films 1982:20)
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