The endostyle is an organ which assists lower-chordates (urochordates and cephalochordates, as well as the larvae of lampreys) in filter-feeding. It is a longitudinal ciliated fold located in the ventral pharyngeal wall, which secretes mucus to accumulate food particles and pass them along to the digestive tract. It aids in transporting food to the esophagus. The endostyle in larval lampreys (ammocetes) metamorphoses into the thyroid gland in adults, and is regarded as being homologous to the thyroid gland in vertebrates due to its iodine-concentrating activity. Since the endostyle is found in the three branches of chordates, it is presumed to have arisen in the common ancestor of these taxa, along with a shift to internal feeding for extracting suspended food from the water.