The morphology of the egg and the formation of the larva in Polymastia robusta Bowerbank are described. The larva is a flattened blastula, formed of equal and flagellated blastomeres; it lives on the bottom and glides actively on its flatened side. The metamorphosis and the differentiation of the cells are long delayed. The first differentiatcd cells are those of the basal and marginal
epithelium.