198 THE ORIGIN OF VERTEBRATES 
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 direc- 
tion 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 Ammoccetes 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 some- 
where in the region of the auditory capsule. 
Dohrn has pointed out that a somewhat similar structure and 
topographical arrangement is found in Amphioxus and the Tunicata, 
the gland-cells being here arranged along the hypobranchial groove 
to form the endostyle and not shut off to form a closed organ, as in 
the thyroid of Ammoccetes. Dohrn concludes, in my opinion rightly, 
that the endostyle in the Tunicata and in Amphioxus represents the 
remnants of the more elaborate organ in Ammoccetes, and that, 
therefore, in order to explain the meaning of these organs in the 
former animals, we must first find out their meaning in Ammoccetes. 
Dohrn, however, goes further than this; for just as he considers 
Amphioxus and the Tunicata to have arisen by degeneration from an 
Ammoccetes-like form, so he considers Ammoccetes to have arisen 
