Fig. 85.—Facial Segment of Ammocœtes marked out by Shading.
VII. 1, thyroid part of segment; VII. 2, hyoid or branchial part; 3-9, succeeding branchial segments belonging to IXth and Xth nerves; V, the velar folds; Ps. br., Dohrn's pseudo-branchial groove; Th. o., thyroid opening; C, curled portion of thyroid.
Moreover, another most instructive piece of evidence pointing in the same direction is afforded by the behaviour of the ventral epithelial pits, as determined by Miss Alcock. Although there is no indication on the ventral surface of the skin of any difference between the anterior and posterior portions of the respiratory region, yet when the ventral rows of the epithelial pits supplied by each branchial nerve are mapped out, we see how the most anterior ones diverge more and more from the mid-ventral line, following out exactly the limits of the underlying muco-cartilaginous thyroid plate (Fig. [84]).
The whole evidence strongly leads to the conclusion that the thyroid portion of the facial segment was inserted as a median tongue between the foremost branchial segments on each side, and that, therefore, the whole facial segment, consisting as it does of a thyroid part and a hyoid or branchial part, may be represented as in Fig. 85, which is obtained by splitting an Ammocœtes longitudinally along the mid-dorsal line, so as to open out the pharyngeal chamber and expose the whole internal surface. The facial segment is marked out by shading lines, the glosso-pharyngeal and vagus segments and the last of the trigeminal segments being indicated faintly. The position of the thyroid gland is indicated by oblique lines, C being the curled portion.
The Uterus of the Scorpion Group.
Seeing how striking is the arrangement and the structure of the glandular tissue of this thyroid, how large the organ is and how absolutely it is confined to Ammocœtes, disappearing entirely as such at transformation, we may feel perfectly certain that a corresponding, probably very similar, organ existed in the invertebrate ancestor of the vertebrate; for the transformation process consists essentially of the discarding of invertebrate characteristics and the putting on of more vertebrate characters; also, so elaborate an organ cannot possibly have been evolved as a larval adaptation during the life of Ammocœtes. We may therefore assert with considerable confidence that the thyroid gland was the palæo-hysteron, and was derived from the uterus of the ancient palæostracan forms. If, then, it be found that a glandular organ of this very peculiar structure and arrangement is characteristic of the uterus of any living member of the scorpion group, then the confidence of this assertion is greatly increased.
In Limulus, as already stated, the genital ducts open separately on each side of the operculum, and do not combine to form a uterus; I have examined them and was unable to find any glandular structure at all resembling that of the thyroid gland of Ammocœtes. I then turned my attention to the organs of the scorpion, in which the two ducts have fused to form a single uterus.
Fig. 86.—Section through the Terminal Chamber or Uterus of the Male Scorpion.
C, cavity of chamber. A portion of the epithelial lining of the channels of emission is drawn above the section of the uterus.