CONDENSING LENSES AND SHELL MICROSTRUCTURE IN CORCULUM (MOLLUSCA, BIVALVIA)

The microstructure of the non-window portions of the shell of Corculum cardissa resembles other Fraginae, with predominantly fibrous prismat ic outer, branching crossed lamellar middle, and complex crossed lamel lar inner layers. Both the anterior and posterior windows in its shell reflect reduced pigmentation and incursion of the outer shell layer, but the posterior windows involve deeper incursion plus reduction of t he outer and middle sublayers of the outer shell layer and microstruct ural modification of the middle shell layer to enhance light transmiss ion. The planoconvex shape of the posterior windows has more likely ev olved to direct and focus light toward the deeper, zooxanthellae-rich gills and anterior mantle, than to merely disperse light.