The outer protective layer or casing of a bryozoan (a tiny colonial marine animal), formed by the organism itself.
From Greek 'ekto-' (outer) + 'kystis' (bladder, pouch). The term emerged in 19th-century zoology to describe the external skeletal structure of bryozoans, which are microscopic creatures that build intricate colonies.
Bryozoans are like underwater apartment complexes where each tiny resident builds and lives in its own ectocyst—scientists studying fossils can still see these ancient 'homes' preserved in rocks millions of years old!
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