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Barnacles are encrusters, attaching themselves permanentlyto a hard substrate. The most common, “acorn barnacles” (Sessilia), are sessile, growing their shells directly onto the substrate.
The order Pedunculata ("goose barnacles" and others) attach themselves by means of a stalk.
Free-living barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead.
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