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 



