Sessile is a common term in biology; it is the opposite of motile. It means to stay in one place on the ground. It is used mainly for marine and freshwater species. They often have larvae which are motile, float or swim, and adult forms which are stationary. Many sessile forms can move from place if they need to. The key idea is that they choose a suitable place, and stay fixed as long as they can. They usually secrete some substance which holds them on.
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Sessility (zoology) Facts for Kids. Kiddle Encyclopedia.