CHAPTEE 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 Ammoccetes — Its structure. — 

 Its openings. — The nature of the thyroid segment. — The uterus of the 

 scorpion. — Its glands. — Comparison with the thyroid gland of Ammoccetes. 

 — 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 mesosoinatic branchial appendages, each of which is still 

 traceable in the respiratory chamber of Ammoco^tes, 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 branchiae we may still have segments 



