The opercular segment is marked out by the thick black line. The segments _II.-VI._ bear branchiæ, and segment _I._ is supposed in the male to carry the uterus masculinus (_Ut. Masc._) and the genital ducts.] It is possible that in Holm's representation of Eurypterus, Fig. 104, the genital duct on each side is indicated. THE THYROID GLAND OF AMMOCOETES. If we compare this mesosomatic region of Eurypterus with that of Ammocoetes, the resemblance is most striking, and gives a meaning to the facial nerve which is in absolute accordance with the interpretation already given of the glossopharyngeal and vagus nerves. In both cases the foremost respiratory or mesosomatic segment is double, the posterior lateral part alone bearing the branchiæ, while the median and anterior part bore in the one animal the uterus and genital ducts, in the other the thyroid gland and ciliated grooves. We are driven, therefore, to the conclusion that this extraordinary and unique organ, the so-called thyroid gland of Ammocoetes, which exists only in the larval condition and is got rid of as soon as the adult sexual organs are formed, shows the very form and position of the uterus of this invertebrate ancestor of Ammocoetes. What, then, is the nature of the thyroid gland in Ammocoetes? {193}Throughout the vertebrate kingdom it is possible to compare the thyroid gland of one group of animals with that of another without coming across any very marked difference of structure right down to and including Petromyzon. When, however, we examine Ammocoetes, we find that the thyroid has suddenly become an organ of much more complicated structure, covering a much larger space, and bearing no resemblance to the thyroid glands of the higher forms. At transformation the thyroid of Ammocoetes is largely destroyed, and what remains of the gland in Petromyzon becomes limited to a few follicles resembling those of other fishes. The structure and position of this gland in Ammocoetes is so well known that it is unnecessary to describe it in detail. For the purpose, however, of making my points clear, I give in Fig. 80 the position and appearance of the thyroid gland (_Th._) when the skin and underlying laminated layer has been removed by the action of hypochlorite of soda. On the one side the ventral somatic muscles have been removed to show the branchial cartilaginous basket-work. [Illustration: FIG. 80.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES. _Th._, thyroid gland; _M._, lower lip, with its muscles.] The series of transverse sections in Fig. 81 represents the nature of the organ at different levels in front of and behind the opening into the respiratory chamber; and in Fig. 82 I have sketched the appearance of the whole gland, viewed so as to show its opening into the respiratory chamber and its posterior curled-up termination. {194}[Illustration: FIG. 81.--SAMPLES FROM A COMPLETE SERIES OF TRANSVERSE SECTIONS THROUGH THE THYROID GLAND OF AMMOCOETES.