Endostyle

The endostyle is an organ in the pharynx of chordate animals that assists in filter-feeding, and is found in adult cephalochordates (lancelets) and urochordates (sea squirts, salps and larvaceans), as well as in the larvae of some basal vertebrates (specifically the lampreys),^[1] but has been transformed through evolution into the thyroid in other vertebrates. It is one of the five common anatomical features (synapomorphies) found in all chordates, along with the notochord, dorsal nerve cord, pharyngeal slits and a post-anal tail.

Since the endostyle is found in all three chordate lineages, it is presumed to have arisen in the common ancestor of these taxa, along with a shift to internal feeding for extracting suspended food particles from the water.^[2] When feeding, food particles suspended in the water adhere to the mucus the endostyle produces.^[3] The filtered water is then expelled through the gill slits, while the food and mucus are swept into the esophagus by movements of the cilia that coat the endostyle.^[3][4]

The endostyle in larval lampreys (ammocoetes) metamorphoses into the thyroid gland in adults, and is regarded as being homologous to the thyroid in all other vertebrates due to its iodine-concentrating activity.^[5]

Early beliefs of the endostyle included the idea by Muller in 1873 stating that the endostyle that reside in the ammocoete has extremely similar functions as the protochordate hypobranchial groove. Thus, numerous investigations into the endostyle ensued, only for the theory to be denied by future researchers. However, during this research, it was found that ammocoete endostyles include the capability and functionality to accumulate radioactive iodine isotopes. This once again made the endostyle a very pertinent topic of research by a multitude of scientists. Recent research has concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research.^[4]