  Let G be the separable Galois group of a finite field F of characteristic p,
and X/F an imaginary hyperelliptic curve such that G acts transitively on its
set W(X) of Weierstrass points. The existence of a G-invariant 2-torsion point
on the Jacobian J(X) of X depends only on the parity of |W(X)|, but for large
enough |F|, there exist two such curves X and X' with |W(X)|=|W(X')|, such that
J(X) has (and J(X') does not have) a G-invariant 4-torsion point. The problem
is equivalent to a study of the 2-,4- and 8-rank of the class number of the
maximal order in the function field of such curves, and is investigated via the
2-primary class field tower. Contrary to the case of number fields, the
ambiguous class depends on the discriminant, and a governing field for the
8-rank of such function fields is not known.
