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.
In zoology, sessility is a characteristic of some animals, such that they are not able to move about. Sessile animals are usually permanently attached to a solid substrate of some kind, such as a part of a plant, a dead tree trunk, or a rock. For example, barnacles attach themselves to the hull of a ship, but corals lay down their own substrate.
Sessile animals typically have a motile phase in their development. Sponges have a motile larval stage, which becomes sessile at maturity. In contrast, many jellyfish develop as sessile polyps early in their life cycle. In the case of the cochineal, it is in the nymph stage (also called the crawler stage) that the cochineal disperses. The juveniles move to a feeding spot and produce long wax filaments. Later they move to the edge of the cactus pad where the wind catches the wax filaments and carries the cochineals to a new host.
Many sessile animals, including sponges, corals and hydra, are capable of asexual reproduction in situ by the process of budding.
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