An
endostyle is a longitudinal
ciliated groove on the
ventral wall of the
pharynx which produces
mucus to gather food particles. It is found in
urochordates and
cephalochordates, and in the larvae of
lampreys. 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. 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.
