THE EVIDENCE OF THE THYROID GLAND 1 97 



take into account the clear evidence that it is composed of two 

 tubes, which have in part fused together to form an elongated central 

 chamber, in part remain as horns to that chamber, and that in its 

 walls there exist lines of gland-cells of a striking and characteristic 

 nature. 



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. 0.). In the second place, 

 there exist two ciliated grooves (Ps. br., Ps. br'.), 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 pre- 

 viously 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 paleeo-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 Ammoccetes 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 head wards 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. 



