The placenta of the salp (Tunicata: Thaliacea)

The morphology of the mature 'placenta' of the pelagic tunicate Salpa fusiformis is described, and it is shown that two syncytial layers, intimately connected by interdigitating microvilli, separate maternal and embryonic circulations. The central placental layer facing the maternal circulation is bordered by membrane infoldings; the cortical layer facing the embryonic circulation is bordered by extensively branching microvilli. Both layers are of maternal origin, although embryonic leucocytes pass into, and add to, the cortical layer.