Barnacles Arthropod 491

A barnacle is a type of arthropod constituting the infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles arthropod 491 are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile (nonmobile) and most are suspension feeders, but those in superorder Rhizocephala are parasitic. They have four nektonic (active swimming) larval stages. Around 1,000 barnacle species are currently known.^[1] The name “Cirripedia” is Latin, meaning “curl-footed”.^[2] The study of barnacles is called cirripedology.  Barnacles arthropod 491 are encrusters, attaching themselves temporarily to a hard substrate. The most common, “acorn barnacles” (Sessilia), are sessile, growing their shells directly onto the substrate.^[3] The order Pedunculata (goose barnacles and others) attach themselves by means of a stalk.^[3]  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. 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. A ring of plates surrounds the body, homologous with the carapace of other crustaceans. These consist of the rostrum, two lateral plates, two carinolaterals, and a carina.^[4] In sessile barnacles, the apex of the ring of plates is covered by an operculum, which may be recessed into the carapace. The plates are held together by various means, depending on species, in some cases being solidly fused.  The main sense of barnacles appears to be touch, with the hairs on the limbs being especially sensitive. The adult also has three photoreceptors (ocelli), one median and two lateral. These photoreceptors record the stimulus for the barnacle shadow reflex, where a sudden decrease in light causes cessation of the fishing rhythm and closing of the opercular plates.^[5] The photoreceptors are likely only capable of sensing the difference between light and dark.^[6] This eye is derived from the primary naupliar eye.