Walter Holbrook Gaskell

“ To sum up, the evidence is clear and conclusive that the Ammocœtes possesses in its pharyngeal chamber mucus-secreting glands, which take an active part in the digestive process, which do not in the least resemble either in structure or arrangement the remarkable cells of the thyroid gland, and that the experimental evidence that the latter cells either secrete mucus or take any part in digestion is so far absolutely negative. ”

“ Further, and this is a most striking fact, the uterus masculinus does not open direct to the exterior, but into the genital cavity, "which lies above the uterus, so that the latter is situated between the lower wall of the genital cavity and the outer integument." The opening, therefore, of the uterus is not external but internal, into the large internal space known as the genital cavity. The arrangement is shown in Fig. 91, taken from Tarnani's paper, which represents a diagrammatic sagittal section through the exit of the male genital duct. ”

“ 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 Ammocœtes, 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 Ammocœtes. ”

“ The evidence, then, of Ammocœtes points directly to this conclusion: The facial nerves represent the foremost of the mesosomatic group of nerves, and supply two segments, which have amalgamated with each other. The most posterior of these, the hyoid segment, is a branchial segment of the same character as those supplied by the vagus and glossopharyngeal nerves; represents, therefore, the foremost pair of branchial appendages. The anterior or thyroid segment, on the other hand, differs from the rest in that, instead of branchiæ, it carries the thyroid gland with its two ciliated grooves. ”

“ From the generative glands proceed the genital ducts to terminate on the posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic stomach, and carry the fluid by means of which the food is digested, for, in all these animals, the active digesting juices are formed in the so-called liver, and not in the cells of the stomach or intestine. ”

“ Not only is there no evidence of any mucous secretion in the thyroid of the fully developed Ammocœtes, but also no necessity for such secretion from Dohrn's point of view, for so copious a supply of mucus is poured out by the glands of the branchiæ, along the whole pharyngeal tract, especially from the cells of the foremost or hyoid gills, as to mix up with the food as thoroughly as can possibly be needed. ”

“ Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the segmentation were sound, his interpretation only was in fault, because he was obliged to express his segments in terms of the gill-pouch unit. ”

“ Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is composed of two separate glands, the generative glands and the hepatico-pancreatic glands—the so-called liver—and surrounds closely the central nervous system and the alimentary canal. ”

“ In the male, he says that the epithelium of the uterus masculinus and its processes is extraordinarily elongated, the chitin covering being thick. In these animals, then, the common chamber or uterus into which the genital ducts empty, which, like the corresponding chamber in the scorpion, occupies the middle region of the operculum, is a large and conspicuous organ. ”

“ Probably in the Palæostraca the generative mass was situated in the cephalic region as in Limulus, and it is probable that the remnant of it still exists in Ammocœtes in the shape of the peculiar large cells packed together, with pigment masses in between them, which form such a characteristic feature of the glandular-looking material, which fills up the space between the cranial walls and the central nervous system. ”

“ 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 Ammocœtes. This uterus and glandular arrangement is found in both sexes; the gland is, however, more developed in the male than in the female scorpion. ”

“ Just as in Eurypterus the fused segment, carrying the uterus on its internal surface, forms a long median tongue which separates the most anterior branchial segments on each side, so also the fused segment carrying the thyroid forms in Ammocœtes a long median tongue, which separates the most anterior branchial segments on each side. ”

“ On the contrary, the manner in which it is eaten up at transformation and absolutely disappears in its original form is, like the other instances mentioned, strong evidence that we are dealing here with an ancestral organ, which is confined to the larval form, and disappears when the change to the higher adult condition takes place. ”

“ Dohrn rightly separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. 81 (6) ) from the paired pseudo-branchial grooves; the former is a shallow depression which opens into the rim of the thyroid orifice, while the latter has a much more intimate connection with the thyroid gland itself. ”

“ It is a very striking fact that the brain of Ammocœtes is much too small for the brain-case, and that the space between brain and brain-case is filled up with a very peculiar glandular-looking tissue, which is found in Ammocœtes and not elsewhere. ”

“ They are composed of very long, wedge-shaped cells, each of which possesses a large, round nucleus at the basal end of the cell (Fig. 87) . These cells are arranged in bundles of about eight to ten, which are separated from each other by connective tissue, the apex of each conical bundle being directed into the cavity of the uterus ”

“ It might seem at first sight improbable that a closed tube should become an open groove, although the reverse phenomenon is common enough; the difficulty, however, is clearly not considered great, for it is precisely what Dohrn imagines to have taken place in the conversion of the thyroid of Ammocœtes into the endostyle of Amphioxus and the Tunicata ”