  An explicit expression of the canonical 8-form on a Riemannian manifold with
a Spin(9)-structure, in terms of the nine local symmetric involutions involved,
is given. The list of explicit expressions of all the canonical forms related
to Berger's list of holonomy groups is thus completed. Moreover, some results
on Spin(9)-structures as G-structures defined by a tensor and on the curvature
tensor of the Cayley planes, are obtained.
