The histology of the nectochaeta larva of the polychaete Sabellaria cementarium Moore and its changes during metamorphosis are described. The epidermis of the nectochaeta consists of five types of cells: locomotory, sensory, pigment, gland, and nonciliated. Setal sacs, containing provisional setae and settling paleae, are located on either side of the body. The muscle system consists of circular and longitudinal trunk muscles and a setal sac – esophageal muscle complex. The alimentary tract consists of an esophagus possessing three types of gland cells, a stomach composed of large vacuolated cells anteriorly and squamous cells containing lipid droplets posteriorly, and an intestine. The nervous system is composed of a cerebral ganglion, circumesophageal connectives, subesophageal ganglia, and paired ventral nerve cords. The blood vascular system consists of supraesophageal, lateral esophageal, dorsal and ventral blood vessels, and dorsal and ventral blood sinuses. Mucous glands are present in the episphere and pygidium and five types of gland cells are found in the parathoracic region. Metamorphosis of the nectochaeta into a sedentary, benthic juvenile involves the following morphological changes: atrophy of prototrochal cells, loss of provisional setae and regional histolysis within the setal sacs, loss of epispheral and pygidial mucous glands, discharge of parathoracic glands that form the mucoid tube, formation of a head coelom, enlargement of cerebral ganglion, histolysis of the setal sac – esophageal muscle complex, and hypertrophy and dissociation of the vacuolated stomach cells.