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Proposed phylogenetic relationships of extant sponge classes, Eumetazoa, and Eiffelia globosa. Eiffelia is shown as a stem hexactinellid (Silicispongea) and a derived member of the “Heteractinida” (stem Calcarea). Eiffelia and early hexactinellids share tetraradial hexactines and a quadruled spicule geometry, but the spicules of the latter are entirely siliceous.
