ABSTRACT
Barnacles possess paired excretory organs, each consisting of an end sac, efferent duct and terminal duct. Maxillary glands alone form the adult and cyprid excretory organs, while antennal glands are present in the nauplius. The adult end sac epithelium consists of podocyte-like cells, and is the site of primary urine production. The end sac is linked to the efferent duct via a valve formed of four cells. The efferent duct is a relatively simple structure composed of a flattened epithelium and whose primary function is probably urine storage. The terminal duct is a short cuticle-lined tube. The adult excretory organs are surrounded by a network of elastic fibres and supported in the body by a series of ligaments and muscles which may also aid urine flow. The nauplius antennal gland has a similar structure to the adult maxillary gland, although the efferent duct has an intracellular canal. Throughout its life cycle, Semibalanus balanoides excretes between 73% and 94% of the nitrogen in the form of ammonia. Not all the products of nitrogen metabolism are necessarily excreted, for guanine and tyrosine crystals are present in the tissues of several cirripedes. Marked seasonal fluctuations in ammonia release occur in S. balanoides probably as a result of metabolic changes accompanying variations in feeding and reproduction. The ratio of oxygen consumed to nitrogen released indicates some utilization of protein reserves in starved of S. balanoides nauplii but not in the non-feeding cyprid.
