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 Ammocoetes, 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 Ammocoetes. 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 {203}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 Ammocoetes. I then turned my attention to the organs of the scorpion, in which the two ducts have fused to form a single uterus. [Illustration: 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.] {204}[Illustration: FIG. 87.--LONGITUDINAL SECTION THROUGH THREE OF THE CONES OF THE UTERINE GLANDS OF THE SCORPION.] [Illustration: FIG. 88.--SAGITTAL SECTION THROUGH THE UTERINE GLAND OF SCORPION, SHOWING THE INTERNAL CHITINOUS SURFACE (_b_) AND THE GLANDULAR CONES (_a_) CUT THROUGH AT VARIOUS DISTANCES FROM THE INTERNAL SURFACE.]