Further, this central chamber, with its horns, is not a closed chamber, but is in communication with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber, as is well known, opens into the respiratory chamber by a funnel-shaped opening--the so-called thyroid duct (_Th. o._). In the second place, there exist two ciliated grooves (_Ps. br._, _Ps. br[prime]._), the pseudo-branchial grooves of Dohrn, which have direct communication with the thyroid chamber. The manner in which these grooves communicate with the thyroid chamber has never, to my knowledge, been described previously to my description in the _Journal of Physiology and Anatomy_; it is very instructive, for, as I have there shown, each groove enters into the corresponding lateral horn, so that, in reality, there are three openings into the thyroid chamber or palæo-hysteron--a median opening into the central chamber, and a separate opening into each lateral horn. The system of ciliated grooves on the inner ventral surface of the respiratory chamber of Ammocoetes was originally described by Schneider as consisting of a single median groove, which extends from the opening of the thyroid to the posterior extremity of the branchial chamber, and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run headwards and diverge from each other, becoming more and more lateral, and more and more dorsal, till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon the whole of this system as a single system, for he speaks of "a ciliated groove, which extends from the orifice of the stomach (_i.e._ anterior intestine) to the orifice of the thyroid, then divides into two, and runs forward right and left of the median ridge, etc." Dohrn rightly separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. 81 (6)) from the paired pseudo-branchial grooves; the former is a shallow depression which opens into the rim of the thyroid orifice, while the latter has a much more intimate connection with the thyroid gland itself.