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 Ammocctes—Its structure.— 
Its openings.—The nature of the thyroid segment.—The uterus of the 
scorpion.—Its glands.—Comparison with the thyroid gland of Ammoceetes. 
—Cephalic genital glands of Limulus.—Interpretation of glandular tissue 
filling up the brain-case of Ammoccetes.—Function of thyroid gland.— 
Relation of thyroid gland to sexual functions —Summary. 
I HAVE now given my reasons why I consider that the glosso- 
pharyngeal 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 Ammoccetes, 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 branchie we may still have segments 
