As the final larval stage before settlement, barnacle cyprids swim with six pairs of thoracic appendages used to maintain orientation, change direction, and swim against downwelling currents. This thesis examines thoracic appendage morphology and swimming behavior in cyprids of Balanus glandula Darwin, 1854. Cyprid swimming appendages carry arrays of plumose setae, unique among crustaceans in that setules of adjacent setae are permanently fused at their tips, creating a webbed setal array. Cyprids are drag-based swimmers, beating appendages sequentially through metachronal strokes during which interconnected arrays act as paddles. Setal arrays spread apart during metachronal power strokes (increasing surface area and drag force of the appendage) and collapse together during synchronous recovery strokes (decreasing surface area and limiting drag). Cyprids move at an average speed of 1.4 cm/sec (with peak speeds of 6 cm/sec) during a beat cycle, with a frequency of 16 beats/sec. This thesis includes previously unpublished co-authored material. / 2019-01-09