{198}A series of sections, such as is given in Fig. 81, shows the relation of this pair of ciliated grooves to the thyroid better than any elaborate description. In the first place, it is clear that they remain separate up to their termination--they do not join in the middle line to open into the thyroid duct; in the second place, they are separate from the thyroid orifice--they do not terminate at the rim of the orifice, as is the case with the median groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig. 81 (2)), gradually moving further and further away from the actual opening of the duct into the pharyngeal chamber. During the whole of their course on the wall of the funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of the duct. The direction of the groove (_Ps. br._) shifts as it passes deeper and deeper towards the thyroid, until at last, as seen in Fig. 81 (3 and 4), it is continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or turned-down horn, as I have called it. In other words, the median chamber opens into the pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the central chamber, owing to their being semi-canals, and not complete canals. If they were originally canals, and not grooves, then the thyroid of Ammocoetes would be derived from an organ composed of a large, common glandular chamber, which opened into the respiratory chamber by means of an extensive median orifice, and possessed anteriorly two horns, from each of which a canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.