Endostyle

The endostyle is an organ found in invertebrate chordate species of tunicates, lancelets, and in the larval stage of vertebrate lampreys. It assists in filter-feeding. It has evolved into the thyroid in vertebrate chordates.

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. When feeding, food particles suspended in the water adhere to the mucus the endostyle produces. 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.

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

One early hypothesis for the function of the endostyle, developed in 1873 by Muller, proposed that the ammocoete endostyle has extremely similar functions as the tunicate hypobranchial groove. 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 can accumulate radioactive iodine isotopes. This revived academic interest in the endostyle. Already in 1963, research had concluded that cephalochordate and tunicate endostyles have the ability to capture iodine, thus further perpetuating new research. A half century later, the homology between the thyroid in vertebrates and the endostyle in amphioxi and in ascidian larvae was further supported by showing that their development involved fairly homologous transcription factors. Similar genetic studies on a hemichordate tentatively indicate that the endostyle also might share an origin with the stomochord.