THE EVIDENCE OF THE THYROID GLAND 197 
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 (Zh. 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 paleo-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 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. 
