Serotonin-containing cells in the ascidian endostyle

Summary In the ascidian endostyle, all fluorescence due to serotonin is localized in the peripheral, iodine-binding area of the endostylar epithelium, which is homologous to the vertebrate thyroid. The amine appears to be stored in granule-containing cells which may correspond to the vertebrate calcitocytes.We are grateful to Dr M.P. Gorbunova for helpful advise and diseussion.