The evidence which I have just given of the intimate connection of the two pseudo-branchial grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that morphologically these two grooves and the thyroid must be considered together. His explanation is that the whole system represents a modified pair of branchial segments distinct from those belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in various papers, is, I think, right in so far as the separateness of the thyroid segment is concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid gland is a modified pair of gills. We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial segments, without any necessity of deriving the thyroid gland from a pair of gills. {200}The evidence that such a median segment has been interpolated ventrally between the foremost pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial segments are marked out on each side by the ventral border of the cartilaginous basket-work; and it is well known, as seen in Fig. 80, that whereas this cartilaginous framework on the two sides meets together in the middle ventral line in the posterior branchial region, it diverges in the anterior region so as to form a tongue-shaped space between the branchial segments on the two sides. This space is covered over with a plate of muco-cartilage which bears on its inner surface the thyroid gland. [Illustration: FIG. 84.--DIAGRAM OF (A) VENTRAL SURFACE AND (B) LATERAL SURFACE OF AMMOCOETES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PITS ON THE BRANCHIAL REGION, AND THEIR INNERVATION BY _VII._, THE FACIAL, _IX._, THE GLOSSOPHARYNGEAL, AND _X^1_-_X^6_, THE VAGUS NERVES.]