The evidence that such a median segment has been interpolated ventrally between the foremost pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial segments are marked out on each side by the ventral border of the cartilaginous basket-work; and it is well known, as seen in Fig.80, that whereas this cartilaginous framework on the two sides meets together in the middle ventral line in the posterior branchial region, it diverges in the anterior region so as to form a tongue-shaped space between the branchial segments on the two sides. This space is covered over with a plate of muco-cartilage which bears on its inner surface the thyroid gland.
Fig. 84.—Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammocœtes, showing the arrangement of the Epithelial Pits on the Branchial Region, and their innervation byVII., the Facial,IX., the Glossopharyngeal, andX1-X6, the Vagus Nerves.
Fig. 84.—Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammocœtes, showing the arrangement of the Epithelial Pits on the Branchial Region, and their innervation byVII., the Facial,IX., the Glossopharyngeal, andX1-X6, the Vagus Nerves.
Fig. 84.—Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammocœtes, showing the arrangement of the Epithelial Pits on the Branchial Region, and their innervation byVII., the Facial,IX., the Glossopharyngeal, andX1-X6, the Vagus Nerves.
In addition to this evidence that we are dealing here with a ventral tongue-like segment belonging to the facial nerve which is interpolated between the foremost branchial segments, we find the most striking fact that at transformation the whole of this muco-cartilaginous plate disappears, the remarkable thyroid gland of theAmmocœtes is eaten up, and nothing is left except a small, totally different glandular mass; and now the cartilaginous basket-work meets together in the middle line in this region as well as in the more posterior region. In other words, the striking characteristic of transformation here is the destruction of this interpolated segment, and the resulting necessary drawing together ventrally of the branchial segments on each side.
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.
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.
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 epithelialpits, 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 thepalæo-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 separatelyon 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.
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.
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.
Fig. 87.—Longitudinal Section through three of the Cones of the Uterine Glands of the Scorpion.
Fig. 87.—Longitudinal Section through three of the Cones of the Uterine Glands of the Scorpion.
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.
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.
I there found that both in the male and in the female the genital ducts on each side terminate in a common chamber or uterus, which underlies the whole length of the operculum, and opens to the exterior in the middle line, as shown in Fig.76. In transverse section, this uterus has the appearance shown in Fig.86,i.e.it is a large tube, evidently expansible, lined with a chitinous layer and epithelial cells belonging to the chitinogenous layer, except in two symmetrical places, where the uniformity of the uterine wall is interrupted by two large, remarkable glandular structures. The structure of these glands is better shown by means of sagittal sections. 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; where this brush-like termination of the cells reaches the surface, the chitinous layer is absent, so that this layer is, on surface view, seen (Fig.88(b)) to be pitted with round holes over that part of the internal surface of the uterus where these glands are situated. Each of these holes represents the termination of one of these cone-shaped wedges of cells. If the section is cut across at right angles to the axis of these cones, then its appearance is represented in Fig.88(a), and shows well the arrangement of the blocks of cells, separated from each other by connective tissue. When the section passes through the basal part of the cones, and only in that case, then the nuclei of the cells appear, often in considerable numbers in one section, asis seen in Fig.89. In Fig.88the section shows atbthe holes in the chitin in which the cones terminate, and then a series of layers of sections through the cones further and further away from their apices.
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.
Fig. 89.—Transverse Section through the Basal Part of the Uterine Glands of the Scorpion.
Fig. 89.—Transverse Section through the Basal Part of the Uterine Glands of the Scorpion.
Fig. 89.—Transverse Section through the Basal Part of the Uterine Glands of the Scorpion.
The resemblance between the structure of the thyroid of Ammocœtes 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 Ammocœtes opening into the respiratory chamber, while the uterus of Scorpio opens direct to the exterior.
Fig. 90.—Section of Central Chamber of Thyroid of Ammocœtes and Section of Uterus of Scorpion.
Fig. 90.—Section of Central Chamber of Thyroid of Ammocœtes and Section of Uterus of Scorpion.
Fig. 90.—Section of Central Chamber of Thyroid of Ammocœtes and Section of Uterus of Scorpion.
With respect to the first difference, the same difficulty is metwith 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.
The second difference is more important, and is at first sight fatal to any comparison between the two organs. How is it possible to compare the uterus of the scorpion, which opens on the surface by anexternalgenital opening, with the thyroid of Ammocœtes, which opens by aninternalopening into the respiratory chamber? However close may be the histological resemblance of structure in the two cases, surely such a difference is too great to be accounted for.
It is, however, to be remembered that the operculum of Scorpio covers only the terminal genital apparatus, and does not, therefore, resemble the operculum of the presumed ancestor of Ammocœtes, which, as already argued, must have resembled the operculum of Thelyphonus with its conjoint branchial and genital apparatus, rather than that of Scorpio. Before, therefore, making too sure of the insuperable character of this difficulty, we must examine the uterus of the Pedipalpi, and see the nature of its opening.
The nature of the terminal genital organs in Thelyphonus has been described to some extent by Blanchard, and more recently by Tarnani. The ducts of the generative organs terminate, according to the latter observer, in the large uterus, which is found both in the male and female; he describes the walls of the uterus in the female as formed of elongated glandular epithelium, with a strongly-developed porous, chitinized intima. In the male, he says that theepithelium 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. Further, and this is a most striking fact, theuterus masculinusdoes 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 butinternal, 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. Yet another most striking fact is described by Tarnani. This genital cavity is continuous with the pulmonary or gill cavities on each side, so that instead of a single opening for the genital products and one on each side for each gill-pouch, as would be the case if the arrangement was of the same kind as in the scorpion, there is a single large chamber, the genital chamber, common to both respiratory and genital organs.
Fig. 91.—Sagittal Median Diagrammatic Section through the Operculum of the Male Thelyphonus.(FromTarnani.)The thick line is the operculum, composed of two segments,I.andII.Ut. Masc., uterus masculinus;Gen. Ch., genital chamber;Int. Op., internal opening;Ext. Op., external opening common to the genital and respiratory organs.
Fig. 91.—Sagittal Median Diagrammatic Section through the Operculum of the Male Thelyphonus.(FromTarnani.)The thick line is the operculum, composed of two segments,I.andII.Ut. Masc., uterus masculinus;Gen. Ch., genital chamber;Int. Op., internal opening;Ext. Op., external opening common to the genital and respiratory organs.
Fig. 91.—Sagittal Median Diagrammatic Section through the Operculum of the Male Thelyphonus.(FromTarnani.)
The thick line is the operculum, composed of two segments,I.andII.Ut. Masc., uterus masculinus;Gen. Ch., genital chamber;Int. Op., internal opening;Ext. Op., external opening common to the genital and respiratory organs.
This genital chamber, according to Tarnani, opens to the exterior by a single median opening between the operculum and the succeeding segment; similarly, a communication from side to side exists between the second pair of gill-pouches. I have been able to examineHypoctonus formosusandThelyphonus caudatus, and in both cases, in both male and female, the opening to the exterior of the common chamber for respiration and for the genital products wasnot a single opening, as described by Tarnani inThelyphonus asperatus, but on each side of the middle line, a round orifice closed by a lid, like the nest of the trapdoor spider, led into the common genital chamber (Gen. Ch.) into which both uterus and gills opened. In Fig.77I have endeavoured to represent the arrangement of the genital and respiratory organs in the male Thelyphonus according to Tarnani's and my own observations.
If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the operculum was fused with the first branchial appendage, among which must be included the old sea-scorpions, then it is most significant that their uterus should open internally into a cavity which was continuous with the respiratory cavity. Thus not only the structure of the gland, but also the arrangement of the internal opening into the respiratory, or, as it became later, the pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammocœtes represents the uterus of the extinct Eurypterus-like ancestor.
Into this uterus the products of the generative organs were poured by means of thevasa deferentia, so that there was not a single median opening or duct in connection with it, but also two side openings, the terminations of thevasa deferentia. These are described by Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral character, although the body of the organ is median and single; these ducts then pass within the body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs which are situated in these animals, as in other scorpions, in the abdomen, so that the direction of the ducts from the generative glands to the uterus is headwards. If, however, we examine the condition of affairs in Limulus, we find that the main mass of the generative material is cephalic, forming with the liver that dense glandular mass which is packed round the supra-œsophageal and prosomatic ganglia, and round the stomach and muscles of the head-region. From this cephalic region the duct passes out on each side at the junction of the prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum, near the middle line, as is indicated in Fig.75.
We have, therefore, two distinct possible positions for the genital ducts among the group of extinct scorpion-like animals, the one from the cephalic region to the operculum, and the other from the abdominal region to the operculum.
The Generative Glands of Limulus and its Allies.
The whole argument, so far, has in every case ended with the conclusion that the original scorpion-like form with which I have been comparing Ammocœtes resembled in many respects Limulus rather than the present-day scorpions, and therefore in the case also of the generative organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that they were cephalic in position rather than abdominal. If this were so, then the duct on each side, starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge mass of generative gland lying within the prosomatic carapace, just as I have represented in the figure of Eurypterus (Fig.79), a course which would take much the same direction as the ciliated groove in Ammocœtes.
We ought, therefore, on this supposition, to expect to find the remains of the invertebrate generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in the abdomen.
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. 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.
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. Further, it is also striking that in the brain of Ammocœtes there should still exist the remains of a tube extending from the IVth ventricle to the surface at theconus post-commissuralis, which can actually be traced right into this tissue on the outside of the brain (see Fig.13,a-e, Pl. XXVI., in my paper in theQuarterly Journal of Microscopical Science).This, in my opinion, is the last remnant of one of the old liver-ducts which extended from the original stomach and intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the pineal eye and its nerve, in Fig.31, also in Fig.22, and separately in Fig.92. It is composed of large cells, with a badly staining nucleus, closely packed together with lines of pigment here and there between the cells; this pigment is especially congregated at the spot where the so-called liver-duct loses itself in this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is not true; peculiar it certainly is, but fatty it is not.
Fig. 92.—Drawing of the Tissue which surrounds the Brain of Ammocœtes.
Fig. 92.—Drawing of the Tissue which surrounds the Brain of Ammocœtes.
Fig. 92.—Drawing of the Tissue which surrounds the Brain of Ammocœtes.
This tissue has been largely described as a peculiar kind of connective tissue, which is there as packing material, for the purpose of steadying a brain too small for its case. On the face of it such an explanation is unscientific; certainly for all those who really believe in evolution, it is out of the question to suppose that a brain-case has been laid down in the first instance too large for the brain, in order to provide room for a subsequent increase of brain; just as it is out of the question to suppose that the nervous system was laid down originally as an epithelial tube in order to provide for the further development of the nervous system by the conversion of more and more of that tube into nervous matter. Yet this latter proposition has been seriously put forward by professed believers in evolution and in natural selection.
This tissue bears no resemblance whatever to any form of connective tissue, either fatty or otherwise. By every test this tissue tells as plainly as possible that it is a vestige of some former organ, presumably glandular, which existed in that position; that it is not there as packing material because the brain happened to be too small for its case, but that, on the contrary, the brain is too small for its case, because the case, when it was formed, included this organ as well as the brain; in other words, this tissueis there because it is the remnant of the great glandular mass which so closely surrounds the brain and alimentary canal in animals such as Limulus. In my paper in theQuarterly Journal of Microscopical Science, in which I was comparing the tube of the vertebrate nervous system with the alimentary canal of the invertebrate, I spoke of this tissue as being the remnant of the invertebrate liver. At the same time the whole point of my argument was that the glandular material surrounding the brain of Limulus was made up of two glands—liver and generative gland—so that this tissue might be the remnant of either one or the other, or both. All I desired, at that time, was to point out the glandular appearance of this so-called packing tissue, which surrounded the brain-region of Ammocœtes, in connection with the fact that the brain and alimentary canal of Limulus were closely surrounded with a glandular mass composed partly of liver, partly of the generative gland. At present, I think these large cells found round the brain in Ammocœtes are much more likely to be the remnant of the generative gland than of the liver; the size of the cells and their arrangement recalls Owen's picture of the generative gland in Limulus, and seeing how important all generative glands are in their capacity of internal secreting glands, apart entirely from the extrusion of the ripe generative products, and how unimportant is an hepato-pancreas when the alimentary canal is closed, it is much more likely that of the two glands the former would persist longer than the latter. It may be that all that is left of the old hepato-pancreas consists of the pigment so markedly found in between these cells, especially at the place where the old liver-duct reaches the surface of the brain; just as the only remnant of the two pineal eyes in the higher vertebrates is the remains of the pigment, known as brain-sand, which still exists in the pineal gland of even the highest vertebrate. This, however, is a mere speculation of no importance. What is important is the recognition of this tissue round the brain as the remnant of the glandular mass round the brain of animals such as Limulus. Still further confirmation of the truth of this comparison will be given when the origin of the auditory organ comes up for discussion.
I conclude, therefore, from the evidence of Ammocœtes, that the generative glands in the ancestral form were situated largely in the cephalic region, and suggest that the course and direction of the ciliated pseudo-branchial grooves on each side indicate the direction of theoriginal opercular ducts by which the generative products were conveyed to the uterine chamber, i.e. to the chamber of the thyroid gland, and thence to the common genital and respiratory cavity, and so to the exterior.
It is easy to picture the sequence of events. First, the generative glands, chiefly confined to the cephalic region, communicating with the exterior by separate ducts on the inner surface of the operculum as in Limulus. Then, in connection with the viviparous habit, these two oviducts fused together to form a single chamber, covered by the operculum, which opened out to the exterior by a single opening as in Scorpio: or, in forms such as Eurypterus, in which the operculum had amalgamated with the first branchial appendage and possessed a long, tongue-like ventral projection, the amalgamated ducts formed a long uterine chamber which opened internally into the genital chamber—a chamber which, as in Thelyphonus, was common with that of the two gill-chambers, while at the same time the genital ducts from the cephalic generative material opened into two uterine horns which arose from the anterior part of the uterus, as in Thelyphonus.
Such an arrangement would lead directly to the condition found in Ammocœtes, if the generative material around the brain lost its function, owing to a new exit for generative products being formed in the posterior part of the body. The connection of the genital duct with this cephalic gland being then closed and cut off by the brain-case, the position of the oviducts would still be shown by the ciliated grooves opening into the folded-down thyroid tube,i.e.the folded-down horns of the uterus; the uterus itself would remain as the main body of the thyroid and still open by a conspicuous orifice into the common respiratory chamber. Next, in the degeneration process, we may suppose that not only the oviducts opened out to form the ciliated groove, but that the uterine chamber itself also opened out, and thus formed the endostyle of Amphioxus and of the Tunicata.
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; it is only carrying on the same idea a stage further to see in the open, ciliated groove of Ammocœtes the remains of the closed genital duct of Limulus and its allies.
Such is the conclusion to which the study of the thyroid gland in Ammocœtes seems to me to lead, and one cannot help wondering why such an unused and rudimentary organ should have remained after its original function had gone. Is it possible to find out its function in Ammocœtes?
The Function of the Thyroid Gland in Ammocœtes.
The thyroid gland has been supposed to secrete mucus into the respiratory chamber for the purpose of entangling the particles of food, and so aiding in digestion. I see no sign of any such function; neither by the thionin method, nor by any other test, have Miss Alcock and myself ever been able to see any trace of mucous secretion in the thyroid, and, indeed, the thyroid duct is always remarkably free from any sign of any secretion whatever. 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. Further, too, the ciliated pharyngeal bands described by Schneider are amply sufficient to move this mixed mass along in the way required by Dohrn. Finally, the evidence given by Miss Alcock is absolutely against the view that the thyroid takes any part in the process of digestion, while, on the other hand, her evidence directly favours the view that these glandularbranchialmucus-secreting cells play a most important part in the digestive process.
In Fig.93, A is a representation of the respiratory tissue of a normal gill; B is the corresponding portion of the first or hyoid gill, in which, as is seen, the whole of the respiratory epithelium is converted into gland-tissue of the nature of mucous cells.
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. It is, of course, possible, that theymay contain mucin in the younger developmental stages, and therefore possible that they might at that stage secrete it; they certainly, however, show no sign of doing so in their more adult condition, and cannot be compared in the very faintest degree to the glandular cells of the pharyngeal region. It is also perfectly possible for gland-cells belonging to a retrograde organ to become mucus-secreting, and so to give rise to the cells of Amphioxus and the Tunicata.
Fig. 93.—A, Portion of a Gill of Ammocœtes with ordinary Respiratory Epithelium; B, Corresponding Portion of the First or Hyoid Gill.
Fig. 93.—A, Portion of a Gill of Ammocœtes with ordinary Respiratory Epithelium; B, Corresponding Portion of the First or Hyoid Gill.
Fig. 93.—A, Portion of a Gill of Ammocœtes with ordinary Respiratory Epithelium; B, Corresponding Portion of the First or Hyoid Gill.
If, then, these cells were not retained for digestive purposes, what was their function? To answer this question we must first know the function of the corresponding gland-cells in the uterus of the scorpion, which undoubtedly secreted into the cavity of the uterus and took some part in connection with the generative act, and certainly not with digestion. What the function of these cells is or in what way they act I am unable at present to say. I can only suppose that the reason why the thyroid gland has persisted throughout the vertebrate kingdom, after the generative tissues had found a new outlet for their products in the body-cavity of the posterior region, is because it possessed some important function in addition to that connected with the exit of the products of the generative organs; a function which was essential to the well-being, or even to the life of the animal. We do not know its function in the scorpion, or the nature of its secretion in that animal. We know only that physiology at the present day has demonstrated clearly that the actual external secretion of a gland may be by no means its most important function; in addition, glands possess what is called an internal secretion, viz. asecretion into the blood and lymph, and this latter secretion may be of the most vital importance. Now, the striking fact forces itself prominently forward, that the thyroid gland of the higher vertebrates is the most conspicuous example of the importance of such internal secretion. Here, although ductless, we have a gland which cannot be removed without fatal consequences. Here, in the importance of its internal secretion, we have a reason for the continued existence of this organ; an organ which remains much the same throughout the Vertebrata down to and including Petromyzon, but, as is seen at transformation, is all that remains of the more elaborate, more extensive organ of Ammocœtes. Surely we may argue that it is this second function which has led to the persistence of the thyroid, and that its original form, without its original function, is seen in Ammocœtes, because that is a larval form, and not a fully-developed animal. As soon as the generative organs of Petromyzon are developed at transformation, all trace of its connection with a genital duct vanishes, and presumably its internal secretory function alone remains.
Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it is in the neck, should have any sympathy with sexual functions if it was originally a gland concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary, likely enough if it originated from a glandular organ in connection with the sexual organs of the palæostracan ancestor of the vertebrate.
Freund has shown, and shown conclusively, that there is an intimate connection between the condition of the thyroid gland and the state of the sexual organs, not only in human beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty, may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may become so extreme as to threaten suffocation, or even cause death; that the period of puberty and the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic goitre especially is associated with a special disease connected with the uterus.
Summary.
Step by step in the preceding chapters the evidence is accumulating in favour of the origin of vertebrates from a member of the palæostracan group. In a continuously complete and harmonious manner the evidence has throughout been most convincing when the vertebrate chosen for the purpose of my arguments has been Ammocœtes.So many fixed points have been firmly established as to enable us to proceed further with very great confidence, in the full expectation of being able ultimately to homologize the Vertebrata with the Palæostraca even to minute details.Perhaps the most striking and unexpected result of such a comparison is the discovery that the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the evidence that it is difficult to see how the homology can be denied.In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the operculum, which always bears the terminal generative organs and is fused in the middle line. In many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed of two segments fused together: an anterior one which carried the uterus, and a posterior one which carried the first pair of branchiæ.In the other animal (Ammocœtes) the foremost segments of the mesosomatic or respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal, for the facial supplies two segments fused together; the anterior one, the thyroid segment, carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of branchiæ.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.Finally, and this is the most conclusive evidence of all, this thyroid gland of Ammocœtes is totally unlike that of any of the higher vertebrates, and, indeed, of the adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a comparison valid with respect to its shape, but also with respect to its structure, which is absolutely unique among vertebrates, and very different to that of any other vertebrate gland, but resembles in a striking manner a glandular structure found in the uterus, both of male and female scorpions.The generative glands in Limulus, together with the liver-glands, form a large glandular mass, situated in the head-region closely surrounding the central nervous system, so that the genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in the abdominal region, so that their ducts pass headwards to the operculum.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 inAmmocœtes 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.Finally, the relationship which has been known from time immemorial to exist between the sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without any explanation, may possibly be the last reminiscence of a time when the thyroid glands were the uterine glands of the palæostracan ancestor.The consideration of the facial nerve, and the segments it supplies, still further points to the origin of the Vertebrata from the Palæostraca.
Step by step in the preceding chapters the evidence is accumulating in favour of the origin of vertebrates from a member of the palæostracan group. In a continuously complete and harmonious manner the evidence has throughout been most convincing when the vertebrate chosen for the purpose of my arguments has been Ammocœtes.
So many fixed points have been firmly established as to enable us to proceed further with very great confidence, in the full expectation of being able ultimately to homologize the Vertebrata with the Palæostraca even to minute details.
Perhaps the most striking and unexpected result of such a comparison is the discovery that the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the evidence that it is difficult to see how the homology can be denied.
In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the operculum, which always bears the terminal generative organs and is fused in the middle line. In many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed of two segments fused together: an anterior one which carried the uterus, and a posterior one which carried the first pair of branchiæ.
In the other animal (Ammocœtes) the foremost segments of the mesosomatic or respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal, for the facial supplies two segments fused together; the anterior one, the thyroid segment, carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of branchiæ.
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.
Finally, and this is the most conclusive evidence of all, this thyroid gland of Ammocœtes is totally unlike that of any of the higher vertebrates, and, indeed, of the adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a comparison valid with respect to its shape, but also with respect to its structure, which is absolutely unique among vertebrates, and very different to that of any other vertebrate gland, but resembles in a striking manner a glandular structure found in the uterus, both of male and female scorpions.
The generative glands in Limulus, together with the liver-glands, form a large glandular mass, situated in the head-region closely surrounding the central nervous system, so that the genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in the abdominal region, so that their ducts pass headwards to the operculum.
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 inAmmocœtes 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.
Finally, the relationship which has been known from time immemorial to exist between the sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without any explanation, may possibly be the last reminiscence of a time when the thyroid glands were the uterine glands of the palæostracan ancestor.
The consideration of the facial nerve, and the segments it supplies, still further points to the origin of the Vertebrata from the Palæostraca.
CHAPTER VI
THE EVIDENCE OF THE OLFACTORY APPARATUS
Fishes divided into Amphirhinæ and Monorhinæ.—Nasal tube of the lamprey.—Its termination at the infundibulum.—The olfactory organs of the scorpion group.—The camerostome.—Its formation as a tube.—Its derivation from a pair of antennæ.—Its termination at the true mouth.—Comparison with the olfactory tube of Ammocœtes.—Origin of the nasal tube of Ammocœtes from the tube of the hypophysis.—Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group—Summary.
Fishes divided into Amphirhinæ and Monorhinæ.—Nasal tube of the lamprey.—Its termination at the infundibulum.—The olfactory organs of the scorpion group.—The camerostome.—Its formation as a tube.—Its derivation from a pair of antennæ.—Its termination at the true mouth.—Comparison with the olfactory tube of Ammocœtes.—Origin of the nasal tube of Ammocœtes from the tube of the hypophysis.—Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group—Summary.
In the last chapter I finished the evidence given by the consideration of the mesosomatic or opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with that of previous chapters, and not only confirms the conclusion that vertebrates arose from some member of the Palæostraca, but helps still further to delimit the nature of that member. It is almost startling to see how the hypothesis put forward in the second chapter, suggested by the consideration of the nature of the vertebrate central nervous system and of the geological record, has received stronger and stronger confirmation from the consideration of the vertebrate optic apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland. All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony will be found upon consideration of the rest of the vertebrate organs.
Following naturally upon the segments supplied by the opisthotic (mesosomatic) cranial nerves, we ought to consider now the segments supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments belonging to the trigeminal nerve-group in the vertebrate, and in the invertebrate the segments of the prosoma with their characteristic appendages. There are, however, in all vertebrates in this foremost cranial region, in addition to the optic nerves, two other well-marked nerves of special sense, the olfactory and the auditory. Of these, the former are in the same class as the optic nerves, for they arisein the vertebrate from the supra-infundibular nerve-mass, and in the invertebrate from the supra-œsophageal ganglia. The latter arise in the vertebrate from the infra-infundibular nerve-mass, and, as the name implies, are situated in the region where the pro-otic nerves are contiguous to the opisthotic,i.e.at the junction of the prosomatic and mesosomatic nerve-regions.
The chapter dealing with the evidence given by the olfactory nerves and the olfactory apparatus ought logically to have followed immediately upon the one dealing with the optic apparatus, seeing that both these special sense-nerves belong to the supra-infundibular segments in the vertebrate and to the supra-œsophageal in the invertebrate.
I did not deal with them in that logical sequence because it was necessary for their understanding to introduce first the conception of modified appendages as important factors in any consideration of vertebrate segments; a conception which followed naturally after the evidence afforded by the skeleton in Chapter III., and by the branchial segments in Chapter IV. So, too, now, although the discussion of the prosomatic segmentation ought logically to follow immediately on that of the mesosomatic segmentation, I have determined to devote this chapter to the evidence of the olfactory organs, because the arguments as to the segments belonging to the trigeminal nerve-group are so much easier to understand if the position of the olfactory apparatus is first made clear.
In all vertebrates the nose is double and opens into the pharynx, until we descend to the fishes, where the whole group Pisces has been divided into two subsidiary groups, Monorhinæ and Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening. The Monorhinæ include only the Cyclostomata—the lampreys and hag-fishes.
In the lampreys the single olfactory tube ends blindly, while in the hag-fishes it opens into the pharynx. In the lamprey, both in Petromyzon and Ammocœtes, the opening of this nasal tube is a conspicuous object on the dorsal surface of the head in front of the transparent spot which indicates the position of the right median eye. It is especially significant, as showing the primitive nature of this median olfactory passage, that a perfectly similar opening in thesame position is always found in the dorsal head-shields of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X.
All the evidence points to the conclusion that the olfactory apparatus of the vertebrate originated as a single median tube, containing the special olfactory sense-epithelium, which, although median and single, was innervated by the olfactory nerve of each side. The external opening of this tube in the lamprey is dorsal. How does it terminate ventrally?
The ventral termination of this tube is most instructive and suggestive. It terminates blindly at the very spot where the infundibular tube terminates blindly and the notochord ends. After transformation, when the Ammocœte becomes the Petromyzon, the tube still ends blindly, and does not open into the pharynx as in Myxine; it, however, no longer terminates at the infundibulum, but extends beyond it towards the pharynx.
This position of the nasal tube suggests that it may originally have opened into the tube of the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in value by the fact that in the larval Amphioxus the tube of the central nervous system is open to the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is situated at the base of a pit, known as the olfactory pit because it is supposed to represent the olfactory organ of other fishes.
Following the same lines of argument as in previous chapters, this suggestion indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates consisted of a single median olfactory tube or passage, which led directly into the œsophagus and was innervated, though single and median, by a pair of olfactory nerves which arose from the supra-œsophageal ganglia. Let us see what is the nature of the olfactory organs among arthropods, and whether such a suggestion possesses any probability.
The Olfactory Organs of the Scorpion Group.
At first sight the answer appears to be distinctly adverse, for it is well known that in all the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often called the antennules, are olfactory in function, and these are free-moving, bilaterallysituated, independent appendages. Still, even here there is the striking fact that the nerves of these olfactory organs always arise from the supra-œsophageal ganglia, although those to the second pair of antennæ arise from the infra-œsophageal ganglia, just as the olfactory nerves of the vertebrate arise from the supra-infundibular brain-mass. Not only is there this similarity of position, but also a similarity of structure in the olfactive lobes of the brain itself of so striking a character as to cause Bellonci to sum up his investigations as follows:—
"The structure and connections of the olfactive lobes present the same fundamental plan in the higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does of separate groups, may each one be called an olfactory glomerulus."
He attributes this remarkable resemblance to a physiological necessity that similarity of function necessitates similarity of structure, for he considers it out of the question to suppose any near relationship between arthropods and vertebrates.
Truly an interesting remark, with the one fallacy that relationship is out of the question.
The evidence so far has consistently pointed to some member of the palæostracan group as the ancestor of the vertebrates—a group which had affinities both to the crustaceans and the arachnids; indeed, many of its members resembled scorpions much more than they resemble crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely than any others to give a clue to the position of the desired olfactory organs. In these animals and their allies paired olfactory antennæ are not present, either in the living land-forms or the extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in Pterygotus, etc. (Fig.8), represent the cheliceræ, and correspond, therefore, to the second pair of antennæ in the crustaceans.
What, then, represents the olfactory antennæ in the scorpions? The answer to this question has been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have combined together to form a hollow tube at the base of which the mouth of the animal is situated, so that the food passes along this olfactory passage before it reaches the mouth. This organ is often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in position and appears, therefore, to be an unpaired organ; its pairedcharacter is, of course, evident enough, for it is innervated by a pair of nerves, and these nerves, as ought to be the case, arise from the supra-œsophageal ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig.94).
Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of appendages similar to the other appendages. This last observation of Croneberg has been confirmed by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar terms, without, however, referring to Croneberg's paper. Croneberg further shows that this foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower lip, for from the basal part of the camerostome there projects on each side of the pharynx a dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were, a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral entosclerite of Lankester and Miss Beck.