Larval behaviour in G. caespitosa is described from trochophore to settled juvenile. The trochophore swims, spiralling counterclockwise, with the apical tuft outstretched. Large cilia on the lips of the mouth assist in food intake. Two sphincter muscles control passage of food along the gut. By 4-5 days; the trochophore has circular, oblique, radial and longitudinal larval muscles. The radial muscles move the apical tuft. The other muscles brace the larval body and assist the passage of food through the gut. At 6-7 days the larva becomes demersal. Feeding and growth continue. At 8-9 days three pairs of setal sacs develop in rapid succession. Metamorphosis now takes place. beginning at 11 days with collapse of the prototroch. Tentacle buds and thoracic membrane rudiments develop even if settlement is not achieved. Other events of metamorphosis (collar evagination, tube secretion. tentacle growth and shrinkage of the head region) require prior settlement. Larval muscles play a part in the shape changes occurring during metamorphosis. Settlement conditions are complex and may include a response to light-coloured surfaces. Development to the three- setiger stage appears to be genetically preprogrammed. Metamorphosis and settlement must involve an interplay of several internal and external causal processes in a sequential manner.