It follows from this that the branchial segments supplied by the vagus and glossopharyngeal nerves ought to have arisen from appendages bearing branchiæ. Although the evidence of such appendages has entirely disappeared in the higher vertebrates, together with the disappearance of branchiæ, and is not strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so great is the difference here from all other fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of these appendages possesses its own mixed nerve, glossopharyngeal or vagus, {184}its own cartilage, its own set of visceral muscles, its own sense-organs, just as do the respiratory appendages of Limulus. The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or appendage between the pouches. Embryology shows how each such appendage grows inwards, how a coelomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in scorpions. We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the present day somewhat in form, but they are in many respects closely allied to Limulus. The present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral somatic muscles, while those of Limulus are the appendage-muscles. The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion, being water-breathers, even although their respiratory appendages were no longer free but sunk in below the surface of the body. The probability that such was the case is increased after consideration of the method of breathing in Ammocoetes, for the respiratory muscles of the latter animal are directly comparable with the muscles of the respiratory appendages of Limulus, and are not somatic. Even the gills themselves of Ammocoetes are built up in the same fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate heart, which is so strikingly different from that of all invertebrate hearts, in that it originates as a branchial and not as a systemic heart, and is formed by the coalescence of two longitudinal veins.