The outer sheath or protective covering of a coral or similar organism.
From Greek exō (outside) + thēkē (case or sheath). The term combines the prefix meaning 'external' with the botanical/biological term for a container-like structure.
Corals build these intricate external skeletons called exothecas—basically they're architects creating limestone walls on the ocean floor, and these structures are so durable they become entire reef ecosystems!
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