  An eutactic star, in a n-dimensional space, is a set of N vectors which can
be viewed as the projection of N orthogonal vectors in a N-dimensional space.
By adequately associating a star of vectors to a particular sea urchin we
propose that a measure of the eutacticity of the star constitutes a measure of
the regularity of the sea urchin. Then we study changes of regularity
(eutacticity) in a macroevolutive and taxonomic level of sea urchins belonging
to the Echinoidea Class. An analysis considering changes through geological
time suggests a high degree of regularity in the shape of these organisms
through their evolution. Rare deviations from regularity measured in
Holasteroida order are discussed.
