Class of Crustacea - Cirripedia
These crustaceans are completely marine, and include the barnacles. Moa species are free living, attached to rock, and other objects. Some are commensals living on whales, fish, turtles and other organisms. Some are parasites. Free living forms are either stalked or sessile. The stalked forms (goose barnacles) measure a few mm to 75 cm in length. Sessile types are a few centimeters long. Stalked forms have a muscular, flexible stalk or peduncle which is attached to the substratum at its lower end and has the main parts of the body, the capitulum, at the other end. The peduncle is the preoral end of the animal and bears the vestiges of the larval first antennae and the cement glands. The secretion of the cement gland is adhesive in nature. The capitulum involves the entire body except the preoral parts. The body is covered by a carapace or the mantle. The mantle is covered through five plates: a posterior dorsal plate carina, a pair of terga and a pair of scuta. Sessile barnacles are attached to the substratum by a membranous or calcareous, basis. This forms the preoral region of the animal and consists of cement glands. The animal is surrounded by a wall of plates and within this wall the animal is covered by an operculum formed by movable terga and scuta.