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This thesis mainly concerns the cohomology of the moduli spaces ℳ₃[2] and ℳ_3,1[2] of genus 3 curves with level 2 structure without respectively with a marked point and some of their natural subspaces. A genus 3 curve which is not hyperelliptic can be realized as a plane quartic and the moduli spaces 𝒬[2] and 𝒬₁[2] of plane quartics without respectively with a marked point are given special attention. The spaces considered come with a natural action of the symplectic group Sp(6,𝔽2) and their cohomology groups thus become Sp(6,𝔽₂)-representations. All computations are therefore Sp(6,𝔽₂)-equivariant. We also study the mixed Hodge structures of these cohomology groups.

The computations for ℳ₃[2] are mainly via point counts over finite fields while the computations for ℳ_3,1[2] primarily uses a description due to Looijenga in terms of arrangements associated to root systems. This leads us to the computation of the cohomology of complements of toric arrangements associated to root systems. These varieties come with an action of the corresponding Weyl group and the computations are equivariant with respect to this action.