Sessile describes animals that don't move around, such as barnacles and corals. There may be mobile phases in the life cycle, often in the larval stage, where organisms might actively swim or merely drift about, but they will eventually fix themselves in place and remain there for the rest of their lives. Because sessile animals can't go off in search of food, this is only a practical lifestyle if you live in water, where the currents or tides will carry food particles to you.
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