Suppose X is a hyperelliptic curve of genus g defined over an algebraically closed field k of characteristic p = 2. We prove that the de Rham cohomology of X decomposes into pieces indexed by the branch points of the hyperelliptic cover. This allows us to compute the isomorphism class of the 2-torsion group scheme J X [2] of the Jacobian of X in terms of the Ekedahl-Oort type. The interesting feature is that J X [2] depends only on some discrete invariants of X , namely, on the ramification invariants associated with the branch points. We give a complete classification of the group schemes that occur as the 2-torsion group schemes of Jacobians of hyperelliptic k-curves of arbitrary genus, showing that only relatively few of the possible group schemes actually do occur. Theorem 1.3. Let X be a hyperelliptic k-curve with affine equation y 2 − y = f (x) defined over an algebraically closed field of characteristic 2 as described in Notation 1.1. Then the 2-torsion group scheme of the Jacobian variety of X isand the a-number of X is a X = (g + 1 − #{α ∈ B | d α ≡ 1 mod 4})/2.