  We analyze the canonical structure of a continuum model of ferromagnets and
clarify known difficulties in defining a momentum density. The moments of the
momentum density corresponding to volume-preserving coordinate transformations
can be defined, but a nonsingular definition of the other moments requires an
enlargement of the phase space which illuminates a close relation to fluid
mechanics. We also discuss the nontrivial connectivity of the phase space for
two and three dimensions and show how this feature can be incorporated in the
quantum theory, working out the two-dimensional case in some detail.
