Ultrastructural characterization of a rostral secretory gland present in larval Branchiostoma lanceolatum

A conspicuous epidermal thickening at the right ventral side of the anterior end of early larvae of Branchiostoma lanceolatum is described from serial TEM sections. This enigmatic structure is documented in several older studies concerned with other larval structures, but has not been described recently. Its cells show the typical characteristics of mucus-producing cells. Specializations of the basal extracellular matrix and the underlying mesothelium suggest that transport processes into the glandular cells occur. The function of this secretory organ in an ontogenetic stage that is thought to be planktonic is enigmatic. Suggestions of homologies from the literature are briefly discussed.