In the other animal (Ammocoetes) the foremost segments of the mesosomatic or respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal, for the facial supplies two segments fused together; the anterior one, the thyroid segment, carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of branchiæ. Just as in Eurypterus the fused segment, carrying the uterus on its internal surface, forms a long median tongue which separates the most anterior branchial segments on each side, so also the fused segment carrying the thyroid forms in Ammocoetes a long median tongue, which separates the most anterior branchial segments on each side. Finally, and this is the most conclusive evidence of all, this thyroid gland of Ammocoetes is totally unlike that of any of the higher vertebrates, and, indeed, of the adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a comparison valid with respect to its shape, but also with respect to its structure, which is absolutely unique among vertebrates, and very different to that of any other vertebrate gland, but resembles in a striking manner a glandular structure found in the uterus, both of male and female scorpions. The generative glands in Limulus, together with the liver-glands, form a large glandular mass, situated in the head-region closely surrounding the central nervous system, so that the genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in the abdominal region, so that their ducts pass headwards to the operculum. Probably in the Palæostraca the generative mass was situated in the cephalic region as in Limulus, and it is probable that the remnant of it still exists in {217}Ammocoetes in the shape of the peculiar large cells packed together, with pigment masses in between them, which form such a characteristic feature of the glandular-looking material, which fills up the space between the cranial walls and the central nervous system. Finally, the relationship which has been known from time immemorial to exist between the sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without any explanation, may possibly be the last reminiscence of a time when the thyroid glands were the uterine glands of the palæostracan ancestor. The consideration of the facial nerve, and the segments it supplies, still further points to the origin of the Vertebrata from the Palæostraca. {218}CHAPTER VI _THE EVIDENCE OF THE OLFACTORY APPARATUS_