Practice question

Endostyle is longitudinal ciliated groove on ventral pharyngeal floor of urochordates, cephalochordates and ammocoete larvae of lampreys. It secretes iodine-binding mucoproteins, concentrates environmental iodine, synthesizes monoiodotyrosine and diiodotyrosine precursors of thyroxine, directly homologous to vertebrate thyroid gland follicles ontogenetically. During metamorphosis of lampreys and amphibians endostyle remodels into thyroid tissue, supporting evolutionary theory of thyroid origin. Functionally it traps food particles in mucus net for filter feeding. Similarity to thyroid lies in iodination and hormone synthesis, linking feeding and endocrine evolution in chordate lineage.