The origin of these two longitudinal veins is immediately apparent if the vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal venous sinuses, situated on each side of the middle line of the animal in an exactly corresponding position to that of the two longitudinal veins, which come together to form the heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of its blood-supply in the vertebrate still further points to the origin of vertebrates from the Palæostraca. {185}CHAPTER V _THE EVIDENCE OF THE THYROID GLAND_ The value of the appendage-unit in non-branchial segments.--The double nature of the hyoid segment.--Its branchial part.--Its thyroid part.--The double nature of the opercular appendage.--Its branchial part.--Its genital part.--Unique character of the thyroid gland of Ammocoetes--Its structure.--Its openings.--The nature of the thyroid segment.--The uterus of the scorpion.--Its glands.--Comparison with the thyroid gland of Ammocoetes.--Cephalic genital glands of Limulus.--Interpretation of glandular tissue filling up the brain-case of Ammocoetes.--Function of thyroid gland.--Relation of thyroid gland to sexual functions.--Summary. I have now given my reasons why I consider that the glossopharyngeal and vagus nerves were originally the nerves belonging to a series of mesosomatic branchial appendages, each of which is still traceable in the respiratory chamber of Ammocoetes, and gives the type-form from which to search for other serially homologous, although it may be specially modified, segments.