Also known as the common acorn barnacle, Balanus glandula is a prevalent animal on Southeast Alaska's rocky shores.  Barnacles, including B. glandula, are sessile crustaceans which, due to their calcareous plates and otherwise cryptic form and life cycle, were placed in the phylum Mollusca until 1830. Once the planktonic larval stages of the barnacle were found and studied, they were moved to the phylum Arthropoda.
    B. glandula is a stalkless barnacle with a symmetrical wall of six plates (carina, rostrum, and lateral wall plates) and two pair of movable plates, called the terga and scuta, which form an operculum. When covered with water, the barnacle extends six pairs of long biramous appendages (ciiri) from the operculum in a sweeping motion to collect food via suspension feeding.
    B. glandula reaches sizes of about 2 cm in diameter and can settle and grow on many surfaces, including rocks, mussels, docks and boats. B. glandula has been known to reach population densities of up to 70,000 individuals in a square meter.