The Endostyl ( gr stylos "pillar" ), also called hypobranchial groove, a ventral in the gills and intestine Flimmerrinne with glandular epithelium of the lancelet, the lamprey larvae ( ammocoetes ) and tunicates (all the chordates, respectively.) The glandular epithelium produces a mucus net which lies over the gill slits and are captured with the filtered in the gills and intestine food particles. This is collected in the Epibranchialrinne and outputs the nutrients to the overlying Dorsalgefäß from.

The cells of the Endostyls accumulate iodine from the environment and build this into molecules of the hormone thyroxine. Because of this ability, the hypobranchial groove is considered as homologue of the thyroid gland of vertebrates, that is, one can derive the thyroid phylogenetically from Endostyl.