The eight pairs of thoracic limbs are referred to as " cirri" which are feathery and very long. Adult barnacles have few appendages on their heads, with only a single, vestigial pair of antennae, attached to the cement gland. Segmentation is usually indistinct, and the body is more or less evenly divided between the head and thorax, with little, if any, abdomen. Inside the carapace, the animal lies on its stomach, projecting its limbs downwards. The plates are held together by various means, depending on species, in some cases being solidly fused. In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. These consist of the rostrum, two lateral plates, two carinolaterals, and a carina. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. These glands secrete a type of natural quick cement able to withstand a pulling strength of 5,000 pounds-force per square inch (30,000 kilopascals 400 kilograms-force per square centimetre) and a sticking strength of 22–60 pounds-force per square inch (200–400 kilopascals 2–4 kilograms-force per square centimetre). In some barnacles, the cement glands are fixed to a long, muscular stalk, but in most they are part of a flat membrane or calcified plate. įree-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. Pedunculate barnacles ( goose barnacles and others) attach themselves by means of a stalk. The most common among them, "acorn barnacles" ( Sessilia), are sessile where they grow their shells directly onto the substrate. Barnacles are encrusters, attaching themselves temporarily to a hard substrate or a symbiont such as a whale ( whale barnacles), a sea snake ( Platylepas ophiophila), or another crustacean, like a crab or a lobster ( Rhizocephala).
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