These conical groups of long cells, represented in Fig. 87, form on each side of the uterus a gland, which is continuous along its whole length, and thus forms a line of secreting surface on each side, just as in the corresponding arrangement of the glandular structures in the thyroid of Ammocoetes. This uterus and glandular arrangement is found in both sexes; the gland is, however, more developed in the male than in the female scorpion.
[Illustration: FIG. 89.--TRANSVERSE SECTION THROUGH THE BASAL PART OF THE UTERINE GLANDS OF THE SCORPION.]
The resemblance between the structure of the thyroid of Ammocoetes and the uterus of the scorpion is most striking, except in two respects, viz. the nature of the lining of the non-glandular part of the cavity--in the one case ciliated, in the other chitinous--and the place of exit of the cavity, the thyroid of Ammocoetes opening into the respiratory chamber, while the uterus of Scorpio opens direct to the exterior.
[Illustration: FIG. 90.--SECTION OF CENTRAL CHAMBER OF THYROID OF AMMOCOETES AND SECTION OF UTERUS OF SCORPION.]
With respect to the first difference, the same difficulty is met { 206}with in the comparison of the ciliated lining of the tube in the central nervous system of vertebrates with the chitinous lining of the intestine in the arthropod. Such a difference does not seem to me either unlikely or unreasonable, seeing that cilia are found instead of chitin in the intestine of the primitive arthropod Peripatus. Also the worm-like ancestors of the arthropods almost certainly possessed a ciliated intestine. Finally, the researches of Hardy and McDougall on the intestine of Daphnia point directly to the presence of a ciliated rather than a chitinous epithelial lining of the intestine in this animal--all evidence pointing to the probability that in the ancient arthropod forms, derived as they were from the annelids, the intestine was originally ciliated and not chitinous. It is from such forms that I suppose vertebrates to have sprung, and not from forms like the living king-crabs, scorpions, Apus, Branchipus, etc. I only use them as illustrations, because they are the only living representatives of the great archaic group, from which the Crustacea, Arachnida, and Vertebrata all took origin.