. Among the most common predators on barnacles are whelks. They're able to grind through the calcareous exoskeletons of barnacles and feed on the softer inside parts. Mussels also prey on them. Adult Anatomy Free-living barnacles are attached to the substratum by cement glands that form from the base of the first pair of antennae; in effect, the animal's fixed upside down by means of its forehead. In some barnacles, the cement glands are fixed to a long muscular stalk, but in most they're part… (More on Barnacles)