SYNOPSIS. Ascidian larvae settle and metamorphose after only a brief free—swimming period; they are all lecilhotrophic. Extrinsic factors (chemical and physical) may trigger metamorphosis but none are known to be essential. The major larval structures may be classified as transitory larval organs (TLO), prospective juvenile organs (PJO) or larvaljuvenile organs (LJO). TLO are phagocytized or otherwise destroyed at metamorphosis; the PJO and LJO become the functional parts of the juvenile or oozooid. Metamorphosis involves some rapid and some slow morphogenetic movements. Variations in larval morphology are reflected in metamorphosis. Some of the events of metamorphosis are fairly well known in terms of the role of specific cells but we know little about how the various events are initiated and coordinated. The nervous system is more complex than had been previously assumed. The nervous system, neuroid conduction and diffusion of one or more humoral factors are probably all involved in controlling metamorphosis.