A type of sea sponge with a skeleton made of calcium carbonate spicules that gives it a chalky or limey structure.
From Latin 'calx' (lime, limestone) + 'spongia' (sponge). The term combines the Latin word for calcium-based minerals with the sponge classification, emerging in 19th-century marine biology.
These sponges are living fossils—some species have barely changed in 500 million years, and their calcium skeletons can actually help scientists study ancient ocean chemistry!
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