196 THE ORIGIN OF VERTEBRATES 
cavity of the portion A or A’ is formed by the bending over of the tube 
or horn, B or B’, with the partial absorption of the septum so formed 
between the tube and its bent-over part. If, then, we uncoil the 
curled-up part of C,and separate the portion, B, on each side from the 
chamber, C, we see that the so-called thyroid of Ammoccetes may be 
represented as in Fig. 83, 7.e. it consists of a long, common chamber, C, 
Ps br. 
B 
Fic. 82.—D1aGRAMMATIC REPRESENTATION OF THE SO-CALLED THYROID GLAND OF 
AMMOC@TES. 
C, central chamber; A, A’, anterior extremity; B, B’, posterior extremity; Th. o., 
thyroid opening into respiratory chamber; Ps. br., Ps. br’., ciliated grooves, 
Dohrn’s pseudo-branchial grooves. 
Fic. 83.—THYROID GLAND AS IT WOULD APPEAR IF THE CENTRAL CHAMBER WERE 
UNCURLED AND THE ‘Two Horns, B, B’, SEPARATED FROM THE CENTRAL 
CHAMBER. 
which, for reasons apparent afterwards, I will call the palco-hysteron, 
which opens, by means of a large orifice, into the respiratory or 
pharyngeal chamber. The anterior end of this chamber terminates in 
two tubes, or horns, B, B’, the structure of which shows that the median 
chamber, C, is the result of the amalgamation of two such tubes, and 
consequently in this chamber, or paleo-hysteron, the glandular lines 
are symmetrically situated on each side. 
Any explanation, then, of the thyroid gland of Ammoccetes, must 
