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 cœlomic 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 Ammocœtes, 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 Ammocœtes 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.
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.
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 Ammocœtes—Its structure.—Its openings.—The nature of the thyroid segment.—The uterus of the scorpion.—Its glands.—Comparison with the thyroid gland of Ammocœtes.—Cephalic genital glands of Limulus.—Interpretation of glandular tissue filling up the brain-case of Ammocœtes.—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 Ammocœtes, and gives the type-form from which to search for other serially homologous, although it may be specially modified, segments.
As long as the branchial unit consisted of the gill-pouch the segments of the head-region were always referred to such units, hence we find Dohrn and Marshall picturing to themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the anterior end of the body. Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the segmentation were sound, his interpretation only was in fault, because he was obliged to express his segments in terms of the gill-pouch unit. Once abandon that point of view and take as the unit a branchial appendage, then immediately we see that in the region in front of the branchiæ we may still have segments homologous to the branchial segments, originally characterized by the presence of appendages, but that such appendages need never have carried branchiæ. The new mouth may have been formed by such appendages, which would express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant in calling it a branchial sense-organ.
The Facial Nerve and the Foremost Respiratory Segment.