Fig. 71.—Diagram constructed from a series of Transverse Sections through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, Muscles, Nerves, and Blood-Vessels.Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system.Br. cart., branchial cartilage;M. con. str., striated constrictor muscles;M. con. tub., tubular constrictor muscles;M. add., adductor muscle;D.A., dorsal aorta;V.A., ventral aorta;S., sense-organs on diaphragm;n. Lat., lateral line nerve;X., epibranchial ganglia of vagus;R. br. prof. VII.,ramus branchialis profundusof facial;J.v., jugular vein;Ep. pit., epithelial pit.
Fig. 71.—Diagram constructed from a series of Transverse Sections through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, Muscles, Nerves, and Blood-Vessels.Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system.Br. cart., branchial cartilage;M. con. str., striated constrictor muscles;M. con. tub., tubular constrictor muscles;M. add., adductor muscle;D.A., dorsal aorta;V.A., ventral aorta;S., sense-organs on diaphragm;n. Lat., lateral line nerve;X., epibranchial ganglia of vagus;R. br. prof. VII.,ramus branchialis profundusof facial;J.v., jugular vein;Ep. pit., epithelial pit.
Fig. 71.—Diagram constructed from a series of Transverse Sections through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, Muscles, Nerves, and Blood-Vessels.
Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system.Br. cart., branchial cartilage;M. con. str., striated constrictor muscles;M. con. tub., tubular constrictor muscles;M. add., adductor muscle;D.A., dorsal aorta;V.A., ventral aorta;S., sense-organs on diaphragm;n. Lat., lateral line nerve;X., epibranchial ganglia of vagus;R. br. prof. VII.,ramus branchialis profundusof facial;J.v., jugular vein;Ep. pit., epithelial pit.
From this description it is clear that the vascular supply of the branchial segment of Ammocœtes would resemble most closely the vascular supply of the Limulus branchial appendage, if the ventral aorta of the former was derived from two longitudinal veins, homologous with the paired longitudinal venous sinuses of the latter.
A priori, such a derivation seems highly improbable; and yet it is precisely the manner in which embryology teaches us that the heart and ventral aorta of the vertebrate have arisen.
The Origin of the Invertebrate Heart and the Origin of the Vertebrate Heart.
Not only does the vertebrate heart differ from that of the invertebrate, in that it is branchial while the latter is systemic, but also it is unique in its mode of formation in the embryo. In the Appendiculata the heart is formed as a single organ in the mid-dorsal line by the growth of the two lateral plates of mesoblast dorsalwards, the heart being formed where they meet. In Mammalia and Aves, the heart and ventral aorta commence as a pair of longitudinal veins, one on each side of the commencing notochord.
If the embryo be removed from the yolk, the surface of the embryo covering these two venous trunks can be spoken of as the ventral surface of the embryo at that stage, and indeed we find that in the present day there is an increasing tendency to speak of this surface as the ventral surface of the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays great stress on the importance of surface views and when the embryo has been removed from the yolk, figures and speaks of its ventral surface. So, also, Locy and Neal find that the best method of seeing the early segments of the embryo is to remove the embryo from the yolk, and examine what they speak of as a ventral view. At the period, then, before the formation of the throat, we may say that on the ventral surface of the embryo a pair of longitudinal venous sinuses are found, one on each side of the mid-ventral line, which are in the same position with respect to the mid-axis of the embryo as are the longitudinal venous sinuses in Limulus.
The next step is the formation of the throat by the extension of the layers of the embryo laterally to meet in the mid-line and so form the pharynx, with the consequence that a new ventral surface is formed; these two veins, as is well known, travel round also, and, meeting together in the new mid-ventral line, form the subintestinal vein, the heart, and the ventral aorta.
What is true of Mammalia and Aves, has been shown by P. Mayer to be true universally among vertebrates, so that in all cases the heart and ventral aorta have arisen by the coalescence in the new mid-ventralline of two longitudinal venous channels, which were originally situated one on each side of the notochord, in what was then the ventral surface of this part of the embryo. This history is especially instructive in showing how the pharyngeal region is formed by the growing round of the lateral mesoblast,i.e.the muscular and other mesoblastic tissues of the branchial segments, and how the two longitudinal veins take part in this process. The phylogenetic interpretation of this embryological fact seems to be, that the new ventral surface of the vertebrate in this region is formed, not only by the branchial appendages, but also by the growth ventrally of that part of the original ventral surface which covered each longitudinal venous sinus.
The following out of the consecutive clues, which one after the other arise in harmonious succession as the necessary sequence of the original working hypothesis, brings even now into view the manner in which the respiratory portion of the alimentary canal arose, and gives strong hints as to the position of that part of the arthropod which gave origin to the notochord. Here I will say no more at present, for the origin of the new alimentary canal of the vertebrate and of the notochord will be more fittingly discussed as a whole, after all the other organs of the vertebrate have been compared with the corresponding organs of the arthropod.
Fig. 72.—Diagram (Upper Half of Figure) of the Original Position of Veins (H) which come together to form the Heart of a Vertebrate.C.N.S., central nervous system;nc., notochord;m., myotome.The lower half of figure shows comparative position of the longitudinal venous sinus (L.V.S.) in Limulus.C.N.S., central nervous system;Al., alimentary canal;H., heart;m., body-muscles.
Fig. 72.—Diagram (Upper Half of Figure) of the Original Position of Veins (H) which come together to form the Heart of a Vertebrate.C.N.S., central nervous system;nc., notochord;m., myotome.The lower half of figure shows comparative position of the longitudinal venous sinus (L.V.S.) in Limulus.C.N.S., central nervous system;Al., alimentary canal;H., heart;m., body-muscles.
Fig. 72.—Diagram (Upper Half of Figure) of the Original Position of Veins (H) which come together to form the Heart of a Vertebrate.
C.N.S., central nervous system;nc., notochord;m., myotome.
The lower half of figure shows comparative position of the longitudinal venous sinus (L.V.S.) in Limulus.C.N.S., central nervous system;Al., alimentary canal;H., heart;m., body-muscles.
The strong evidence that the vertebrate heart was formed from a pair of longitudinal venous sinuses on the ventral side of the central canal, carries with it the conclusion that the original single median dorsal heart of the arthropod is not represented in the vertebrate,for the dorsal aorta cannot by any possibility represent that heart.
Although it is not now functional the original existence of so important an organ as a dorsal heart may have left traces of its former presence; if so, such traces would be most likely to be visible in the lowest vertebrates, just as the median eyes are much more evident in them than in the higher forms. In Fig.58the position of the dorsal heart is shown in Limulus, and in Fig.70the shape and extent of this dorsal heart is shown. It extends slightly into the prosomatic region, and thins down to a point there, runs along the length of the animal and finally thins down to a point at the caudal end.
The heart is surrounded by a pericardium, from which at regular intervals a number of dorso-ventral muscles pass, to be inserted into the longitudinal venous sinus on each side. These veno-pericardial muscles are absolutely segmental with the mesosomatic segments, and are confined to that region, with the exception of two pairs in the prosomatic region. Their homologies will be discussed later.
Any trace of a heart such as we have just described must be sought for in Ammocœtes between the central nervous system and the mid-line dorsally. Now, in this very position a large striking mass of tissue is found, represented in section in Fig.73,f. It forms a column of similar tissue along the whole mid-dorsal region, except at the two extremities; it tapers away in the caudal region, and headwards grows thinner and thinner, so that no trace of it is seen anterior to the commencement of the branchial region. It resembles in its dorsal position, in its shape, and in its size a dorsal heart-tube such as is seen in Limulus and elsewhere, but it differs from such a tube in its extension headwards. The heart-tube of Limulus ceases at the anterior end of the mesosomatic region, this fat-column of Ammocœtes at the posterior end. In its structure there is not the slightest sign of anything of the nature of a heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat, staining intensely black with osmic acid. Nowhere else in the whole body of Ammocœtes is such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply fat-cells, such as might easily have taken the place of some other previously existing organ.
I do not know how to decide the question which thus arises. Supposing, for the sake of argument, that this column of fat-cells has really taken the place of the original dorsal heart, what criterion would there be as to this? The heartex hypothesihaving ceased to function, the muscular tissue would not remain, and the space would be filled up, presumably with some form of connective tissue. As likely as not, the connective tissue might take the form of fatty tissue, the storage of fat being a physiological necessity to an animal, while at the same time no special organ has been developed for such a purpose, but fat is being laid down in all manner of places in the body.
This dorsal fat-column, as it is seen in Ammocœtes, is not found in the higher vertebrates, so that it possesses, at all events, the significance of being a peculiarity of ancient times before the vertebrate skeletal column was formed.
I mention it here in connection with my view as to the origin of vertebrates, because there it is, in the very place where the dorsal heart ought to have been. For my own part, I should not have expected that a muscular organ such as the heart would leave any trace of itself if it disappeared, so that its absence in the dorsal region of the vertebrate does not seem to me in the slightest degree to invalidate my theory.
Fig. 73.—Section through the Notochord (nc.), the Spinal Canal and the Fat-column (f.), of Ammocœtes, drawn from an Osmic Preparation.sp. c., spinal cord;gl., glandular tissue filling the spinal canal;sk., Gegenbaur's skeletogenous cells;p., pigment.
Fig. 73.—Section through the Notochord (nc.), the Spinal Canal and the Fat-column (f.), of Ammocœtes, drawn from an Osmic Preparation.sp. c., spinal cord;gl., glandular tissue filling the spinal canal;sk., Gegenbaur's skeletogenous cells;p., pigment.
Fig. 73.—Section through the Notochord (nc.), the Spinal Canal and the Fat-column (f.), of Ammocœtes, drawn from an Osmic Preparation.
sp. c., spinal cord;gl., glandular tissue filling the spinal canal;sk., Gegenbaur's skeletogenous cells;p., pigment.
Summary.
From the close similarity of structure and position between the branchial skeleton of Limulus and of Ammocœtes, as given in the preceding chapter, it logically follows that the branchiæ of Ammocœtes must be homologous with the branchiæ of Limulus. But the respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that the branchiæ of Ammocœtes, and consequently of the vertebrates, must have been derived from branchial appendages, and as they are internal, not external, such branchial appendages must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no external respiratory appendages have been discovered in any of the sea-scorpions.In the vertebrates—and it is especially well shown in Ammocœtes—a double segmentation exists in the head-region, a body or somatic segmentation, and a branchial or splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric segmentations. The nerves which supply the latter segments form a very well-marked group (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves from the very beginning.The system of cranial segmental nerves is older than the spinal system, and cannot, therefore, be derived from it, but can be arranged as a system supplying two segments, somatic and splanchnic, which differ in the following way: Each somatic segment is supplied by two roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic segment possesses only one root, which is mixed in function.The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been unexplained, immediately receive a straightforward and satisfactory explanation if the splanchnic or branchiomeric segments owe their origin to a system of appendages after the style of those of Limulus.In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, the nerves which supply the somatic segments arise from separate sensory and motor roots, while the single nerve which supplies each appendage contains all the fibres for the appendage, both motor and sensory.It follows from this that the branchial segments supplied by the vagus and glossopharyngeal nerves ought to have arisen from appendages bearing branchiæ.Although the evidence of such appendages has entirely disappeared in the higher vertebrates, together with the disappearance of branchiæ, and is not strikingly apparent in the higher gill-bearing fishes, yet in Ammocœtes, so great is the difference here from all other fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of these appendages possesses its own mixed nerve, glossopharyngeal or vagus,its own cartilage, its own set of visceral muscles, its own sense-organs, just as do the respiratory appendages of Limulus.The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or appendage between the pouches. Embryology shows how each such appendage grows inwards, how a cœlomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in scorpions.We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the present day somewhat in form, but they are in many respects closely allied to Limulus. The present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral somatic muscles, while those of Limulus are the appendage-muscles.The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion, being water-breathers, even although their respiratory appendages were no longer free but sunk in below the surface of the body. The probability that such was the case is increased after consideration of the method of breathing in Ammocœtes, for the respiratory muscles of the latter animal are directly comparable with the muscles of the respiratory appendages of Limulus, and are not somatic. Even the gills themselves of Ammocœtes are built up in the same fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate heart, which is so strikingly different from that of all invertebrate hearts, in that it originates as a branchial and not as a systemic heart, and is formed by the coalescence of two longitudinal veins.The origin of these two longitudinal veins is immediately apparent if the vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal venous sinuses, situated on each side of the middle line of the animal in an exactly corresponding position to that of the two longitudinal veins, which come together to form the heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of its blood-supply in the vertebrate still further points to the origin of vertebrates from the Palæostraca.
From the close similarity of structure and position between the branchial skeleton of Limulus and of Ammocœtes, as given in the preceding chapter, it logically follows that the branchiæ of Ammocœtes must be homologous with the branchiæ of Limulus. But the respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that the branchiæ of Ammocœtes, and consequently of the vertebrates, must have been derived from branchial appendages, and as they are internal, not external, such branchial appendages must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no external respiratory appendages have been discovered in any of the sea-scorpions.
In the vertebrates—and it is especially well shown in Ammocœtes—a double segmentation exists in the head-region, a body or somatic segmentation, and a branchial or splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric segmentations. The nerves which supply the latter segments form a very well-marked group (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves from the very beginning.
The system of cranial segmental nerves is older than the spinal system, and cannot, therefore, be derived from it, but can be arranged as a system supplying two segments, somatic and splanchnic, which differ in the following way: Each somatic segment is supplied by two roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic segment possesses only one root, which is mixed in function.
The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been unexplained, immediately receive a straightforward and satisfactory explanation if the splanchnic or branchiomeric segments owe their origin to a system of appendages after the style of those of Limulus.
In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, the nerves which supply the somatic segments arise from separate sensory and motor roots, while the single nerve which supplies each appendage contains all the fibres for the appendage, both motor and sensory.
It follows from this that the branchial segments supplied by the vagus and glossopharyngeal nerves ought to have arisen from appendages bearing branchiæ.
Although the evidence of such appendages has entirely disappeared in the higher vertebrates, together with the disappearance of branchiæ, and is not strikingly apparent in the higher gill-bearing fishes, yet in Ammocœtes, so great is the difference here from all other fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of these appendages possesses its own mixed nerve, glossopharyngeal or vagus,its own cartilage, its own set of visceral muscles, its own sense-organs, just as do the respiratory appendages of Limulus.
The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or appendage between the pouches. Embryology shows how each such appendage grows inwards, how a cœlomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in scorpions.
We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the present day somewhat in form, but they are in many respects closely allied to Limulus. The present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral somatic muscles, while those of Limulus are the appendage-muscles.
The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion, being water-breathers, even although their respiratory appendages were no longer free but sunk in below the surface of the body. The probability that such was the case is increased after consideration of the method of breathing in Ammocœtes, for the respiratory muscles of the latter animal are directly comparable with the muscles of the respiratory appendages of Limulus, and are not somatic. Even the gills themselves of Ammocœtes are built up in the same fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate heart, which is so strikingly different from that of all invertebrate hearts, in that it originates as a branchial and not as a systemic heart, and is formed by the coalescence of two longitudinal veins.
The origin of these two longitudinal veins is immediately apparent if the vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal venous sinuses, situated on each side of the middle line of the animal in an exactly corresponding position to that of the two longitudinal veins, which come together to form the heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of its blood-supply in the vertebrate still further points to the origin of vertebrates from the Palæostraca.
CHAPTER V
THE EVIDENCE OF THE THYROID GLAND
The value of the appendage-unit in non-branchial segments.—The double nature of the hyoid segment.—Its branchial part.—Its thyroid part.—The double nature of the opercular appendage.—Its branchial part.—Its genital part.—Unique character of the thyroid gland of Ammocœtes—Its structure.—Its openings.—The nature of the thyroid segment.—The uterus of the scorpion.—Its glands.—Comparison with the thyroid gland of Ammocœtes.—Cephalic genital glands of Limulus.—Interpretation of glandular tissue filling up the brain-case of Ammocœtes.—Function of thyroid gland.—Relation of thyroid gland to sexual functions.—Summary.
The value of the appendage-unit in non-branchial segments.—The double nature of the hyoid segment.—Its branchial part.—Its thyroid part.—The double nature of the opercular appendage.—Its branchial part.—Its genital part.—Unique character of the thyroid gland of Ammocœtes—Its structure.—Its openings.—The nature of the thyroid segment.—The uterus of the scorpion.—Its glands.—Comparison with the thyroid gland of Ammocœtes.—Cephalic genital glands of Limulus.—Interpretation of glandular tissue filling up the brain-case of Ammocœtes.—Function of thyroid gland.—Relation of thyroid gland to sexual functions.—Summary.
I have now given my reasons why I consider that the glossopharyngeal and vagus nerves were originally the nerves belonging to a series of mesosomatic branchial appendages, each of which is still traceable in the respiratory chamber of Ammocœtes, and gives the type-form from which to search for other serially homologous, although it may be specially modified, segments.
As long as the branchial unit consisted of the gill-pouch the segments of the head-region were always referred to such units, hence we find Dohrn and Marshall picturing to themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the anterior end of the body. 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. Once abandon that point of view and take as the unit a branchial appendage, then immediately we see that in the region in front of the branchiæ we may still have segmentshomologous to the branchial segments, originally characterized by the presence of appendages, but that such appendages need never have carried branchiæ. The new mouth may have been formed by such appendages, which would express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant in calling it a branchial sense-organ.
The Facial Nerve and the Foremost Respiratory Segment.
This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid segments, for embryologically there is evidence of two segments. As already mentioned, the facial nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the pro-otic group of nerves forms one of the main reasons why this group has been supposed to have originally supplied gill-pouch segments, for the hyoid segment is clearly associated with branchiæ.
When, however, we examine Ammocœtes (cf.Figs. 63 and 64) it is clear that the foremost of the segments forming the respiratory chamber, which must be classed with the rest of the mesosomatic or opisthotic segments, is that supplied by the facial nerves.
An examination of this respiratory chamber shows clearly that there are six pairs of branchial appendages or diaphragms, which are all exactly similar to each other. These are those already considered, the foremost of which are supplied by the IXth or glossopharyngeal nerves. Immediately anterior to this glossopharyngeal segment is seen in the figures the segment supplied by the VIIth or facial nerves. It is so much like the segments belonging to the glossopharyngeal and vagus nerves as to make it certain that we are dealing here with a branchial segment, composed of a pair of branchial appendages similar to those in the other cases, except that the cartilaginous bar is here replaced by a bar of muco-cartilage and the branchiæ are confined to the posterior part of each appendage. The anterior portion is, as is seen in Fig.74, largely occupied by blood-spaces, but in addition carries the ciliated groove (ps. br.) called by Dohrn 'pseudo-branchiale Rinne.' This groove leads directly into the thyroid gland, which is a large bilateral organ situated in the middle line, as seen in Fig.80and Fig.85. As shown by Miss Alcock, the facial nerve supplies this thyroid gland, as well as the posterior hyoid branchial segment, and, as pointed out by Dohrn, there is every reason to consider this thyroid gland as indicative of a separate segment, especially when van Wijhe's statement that the hyoid segment is in reality double is taken into account.
Fig. 74.—Ventral half of Head-region of Ammocœtes.Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching.Tent., tentacles;Tent. m.c., muco-cartilage of tentacles;Vel. m.c., muco-cartilage of the velum;Hy. m.c., muco-cartilage of the hyoid segment;Ps. br., pseudo-branchial groove;Br. cart., branchial cartilages;Sp., space between somatic and splanchnic muscles;Th. op., orifice of thyroid;H., heart.
Fig. 74.—Ventral half of Head-region of Ammocœtes.Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching.Tent., tentacles;Tent. m.c., muco-cartilage of tentacles;Vel. m.c., muco-cartilage of the velum;Hy. m.c., muco-cartilage of the hyoid segment;Ps. br., pseudo-branchial groove;Br. cart., branchial cartilages;Sp., space between somatic and splanchnic muscles;Th. op., orifice of thyroid;H., heart.
Fig. 74.—Ventral half of Head-region of Ammocœtes.
Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching.Tent., tentacles;Tent. m.c., muco-cartilage of tentacles;Vel. m.c., muco-cartilage of the velum;Hy. m.c., muco-cartilage of the hyoid segment;Ps. br., pseudo-branchial groove;Br. cart., branchial cartilages;Sp., space between somatic and splanchnic muscles;Th. op., orifice of thyroid;H., heart.
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. If this segment, which is the foremost of the mesosomatic segments, also indicates a pair of appendages which carry the thyroid gland instead of branchiæ, then it follows that this pair of appendages has joined together in the mid-line ventrally and thus formed a single median organ—the thyroid gland. If, then, we find that the foremost of the mesosomatic appendages in the Palæostraca was really composed of two pairs of appendages, of which the most posterior carried branchiæ, while the anterior pair had amalgamated in the mid-line ventrally, and carried some special organ instead of branchiæ, then the accumulation of coincidences is becoming so strong as to amount to proof of the correctness of our line of investigation.
The First Mesosomatic Segment in Limulus and its Allies.
What, then, is the nature of the foremost pair of mesosomatic appendages in Limulus. They differ from the rest of the mesosomatic appendages in that they do not carry branchiæ, and instead of beingseparate are joined together in the mid-line ventrally to form a single terminal plate-like appendage known as the operculum. On its posterior surface the operculum carries the genital duct on each side.
So also in the scorpion group, the operculum is always found and always carries the genital ducts.
A survey of the nature of the opercular appendage demonstrates the existence of three different types—
1. That of Limulus, in which the operculum is free, and carries only the terminations of the genital ducts. In this type the duct on each side opens to the exterior separately (Fig.75).
Fig. 75.—operculum of Limulus To Show the two separate Genital Ducts.
Fig. 75.—operculum of Limulus To Show the two separate Genital Ducts.
Fig. 76.—Operculum of Male Scorpion.Ut., terminal chamber, or uterus.
Fig. 76.—Operculum of Male Scorpion.
Ut., terminal chamber, or uterus.
2. The type of Scorpio, Androctonus, Buthus, etc., in which the operculum is not free, but forms part of the ventral surface of the body-wall, but, like Limulus, carries only the terminations of the genital ducts. In this type the duct on each side terminates in a common chamber (vagina or uterus), which communicates with the exterior by a single external median opening. This common chamber, or uterus (Ut.), extends the whole breadth of the operculum (as seen in Fig.76), and is limited to that segment.
3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the Pedipalpi, in which the operculum forms a part of the ventral surface of the body wall, but no longer covers only the termination of the genital apparatus. It really consists of two parts, a median anterior, which covers the terminal genital apparatus,and a lateral posterior, which covers the first pair of gills, or lung-books, as they are called. In this type (Fig.77) the genital ducts terminate in a common chamber or uterus, the nature of which will be further considered.
As has been pointed out by Blanchard, the terminal genital organs of the scorpions and the Pedipalpi vary considerably in the different genera, especially the male genital organs. The general type of structure is the same, and consists in both male and female of vasa deferentia, which come together to form a common chamber before the actual opening to the exterior. This common chamber has been called in the female scorpion the vagina, or in Thelyphonus the uterus. I shall use the latter term, in accordance with Tarnani's work, and the corresponding chamber in the male will be theuterus masculinus.
A considerable discussion has taken place about the method of action of the external genital organs in the members of the scorpion tribe, into which it is hardly necessary to enter here. The evidence points to the conclusion that in all these forms the operculum covers a median single chamber or uterus, into which the genital ducts open on each side, the main channels of emission being provided with a massive chitinous internal framework. We may feel certain that in the old extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, and that therefore in them also the median portion of the operculum covered a median chamber or uterus composed of the amalgamation of the terminations of the two genital ducts, which were originally separate, as in Limulus.
Fig. 77.—Operculum and Following Segments Of Male Thelyphonus.Opercular segment is marked out by thick black line.Ut. Masc., uterus masculinus;Int. Op., internal opening of uterus into genital chamber;Ext. Op., common external opening to genital chamber (Gen. Ch.) and pulmonary chamber.
Fig. 77.—Operculum and Following Segments Of Male Thelyphonus.Opercular segment is marked out by thick black line.Ut. Masc., uterus masculinus;Int. Op., internal opening of uterus into genital chamber;Ext. Op., common external opening to genital chamber (Gen. Ch.) and pulmonary chamber.
Fig. 77.—Operculum and Following Segments Of Male Thelyphonus.
Opercular segment is marked out by thick black line.Ut. Masc., uterus masculinus;Int. Op., internal opening of uterus into genital chamber;Ext. Op., common external opening to genital chamber (Gen. Ch.) and pulmonary chamber.
The observations of Schmidt, Zittel, and others show that theoperculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, etc., belonged to the type of Thelyphonus, rather than to that of Limulus or Scorpio. In Fig.78I give a picture from Schmidt of the ventral aspect of Eurypterus, and by the side of it a picture of the isolated operculum. Schmidt considers that there were five branchiæ-bearing segments constituting the mesosoma, the foremost of which formed the operculum. Such operculum is often found isolated, and is clearly composed of two lateral appendages fused together in the middle line, of such a nature as to form a median elongated tongue, which lies between and separates the first three pairs of branchial segments. This median tongue, together with the anterior and median portion of the operculum, concealed, in all probability, according to Schmidt, the terminal parts of the genital organs, just as the median part of the operculum in Phrynus and Thelyphonus conceals the complicated terminal portions of the genital organs. The posterior part of the operculum, like that of Phrynus and Thelyphonus, carried the first pair of branchiæ, so Schmidt thinks from the evidence of markings on some specimens.
Fig. 78.—Eurypterus.The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates.M., metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.
Fig. 78.—Eurypterus.The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates.M., metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.
Fig. 78.—Eurypterus.
The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates.M., metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.
Apparently an opercular appendage of this kind is in reality the result of a fusion of the genital operculum with the first branchial appendage in forms such as the scorpion; for, in order that the tergal plates may correspond in number with the sternal in Eurypterus, etc., it is necessary to consider that the operculum is composed of two sternites joined together. Similarly in Thelyphonus, Phrynus, etc., this numerical correspondence is only observed if the operculum is looked upon as double.
A restoration of the mesosomatic region of Eurypterus, viewedfrom the internal surface, might be represented by Fig.79, in which the thick line represents the outline of the opercular segment, and the fainter lines the succeeding branchial segments. The middle and anterior part of the opercular segment carried the terminations of the genital organs; these I have represented, in accordance with our knowledge of the nature of these organs in the present-day scorpions, as a median elongated uterus, bilaterally formed, from which the genital ducts passed, probably as in Limulus, towards a mass of generative gland in the cephalic region, and not as in Scorpio or Thelyphonus, tailwards to the abdominal region.
Fig. 79.—Diagram To indicate the probable nature of the Mesosomatic Segments of Eurypterus.The opercular segment is marked out by the thick black line. The segmentsII.-VI.bear branchiæ, and segmentI.is supposed in the male to carry the uterus masculinus (Ut. Masc.) and the genital ducts.
Fig. 79.—Diagram To indicate the probable nature of the Mesosomatic Segments of Eurypterus.The opercular segment is marked out by the thick black line. The segmentsII.-VI.bear branchiæ, and segmentI.is supposed in the male to carry the uterus masculinus (Ut. Masc.) and the genital ducts.
Fig. 79.—Diagram To indicate the probable nature of the Mesosomatic Segments of Eurypterus.
The opercular segment is marked out by the thick black line. The segmentsII.-VI.bear branchiæ, and segmentI.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 Ammocœtes.
If we compare this mesosomatic region of Eurypterus with that of Ammocœtes, 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 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. What, then, is the nature of the thyroid gland in Ammocœtes?
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 Ammocœtes, 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 Ammocœtes 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 Ammocœtes 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.80the 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.
Fig. 80.—Ventral View of Head Region of Ammocœtes.Th., thyroid gland;M., lower lip, with its muscles.
Fig. 80.—Ventral View of Head Region of Ammocœtes.Th., thyroid gland;M., lower lip, with its muscles.
Fig. 80.—Ventral View of Head Region of Ammocœtes.
Th., thyroid gland;M., lower lip, with its muscles.
The series of transverse sections in Fig.81represents the nature of the organ at different levels in front of and behind the opening into the respiratory chamber; and in Fig.82I 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.
Fig. 81.—Samples from a Complete Series of Transverse Sections through the Thyroid Gland of Ammocœtes.Sections 1 and 2 are anterior to the thyroid opening,Th. o.; sections 3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening before the commencement of the curled portion.
Fig. 81.—Samples from a Complete Series of Transverse Sections through the Thyroid Gland of Ammocœtes.Sections 1 and 2 are anterior to the thyroid opening,Th. o.; sections 3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening before the commencement of the curled portion.
Fig. 81.—Samples from a Complete Series of Transverse Sections through the Thyroid Gland of Ammocœtes.
Sections 1 and 2 are anterior to the thyroid opening,Th. o.; sections 3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening before the commencement of the curled portion.
The series of transverse sections (1-6, Fig.81) show that we are dealing here with a central glandular chamber, C (Fig.81(6) and Fig.82), which opens by the thyroid duct (Th. o.) into the pharyngeal chamber, and is curled upon itself in its more posterior part. This central chamber divides, anteriorly to the thyroid orifice, into two portions, A, A′ (Fig.82), giving origin to two tubes, B, B′, which lie close alongside of, and extend further back than, the posterior limit of the curled portion of the central chamber, C. The structure of the central chamber, C, and, therefore, of the separate coils, is given in both Schneider's and Dohrn's pictures, and is represented in Fig.81(6), which shows the peculiar arrangement and character of the glandular cells typical of this organ, and also the nature of the central cavity, with the arrangement of the ciliated epithelium. The structure of each of the lateral tubes, B, is different from that of the central chamber, in that only half the central chamber is present in them, as is seen by the comparison of the tube B with the tube C in Fig.81(5 and 6), so that we may look upon the central chamber, C, as formed of two tubes, similar in structure to the tubes B, which have come together to form a single chamber by the partial absorption of their walls, the remains of the wall being still visible as the septum, which partially divides the chamber, C, into halves.
In the walls of each of these tubes is situated a continuous glandular line, the structure of the glandular elements being specially characterized by the length of the cells, by the large spherical nucleus situated at the very base of each cell, and by the way in which the cells form a wedge-shaped group, the thin points of all the wedge-shaped cells coming together so as to form a continuous line along the chamber wall. This free termination of the cells of the gland in the lumen of the chamber constitutes the whole method for the secretion of the gland; there is no duct, no alveolus, nothing but this free termination of the cells.
Moreover, sections through the portion A, A′ (Fig.82) show that here, as in the central chamber, C, four of these glandular lines open into a common chamber, but they are not the same four as in the case of the central chamber, for if we name these glandular lines on the left sidea b, a′ b′(Fig.81), and on the right sidec d, c′ d′, then the central chamber has opening into it the glandsa b, c d, while the chambers of A and A′ have opening into them respectivelya b, a′ b′, andc d, c′ d′. Further, the same series of sections shows that the glandsaandbare continuous with the glandsa′andb′respectively across the apex of A, and similarly on the other side, so that the two glandular rowsa bare continuous with the two glandular rowsa′ b′, and we see that thecavity of the portion A or A′ is formed by the bending over of the tube or horn, B or B′, with the partial absorption of the septum so formed between the tube and its bent-over part. If, then, we uncoil the curled-up part of C, and separate the portion, B, on each side from the chamber, C, we see that the so-called thyroid of Ammocœtes may be represented as in Fig.83,i.e.it consists of a long, common chamber, C, which, for reasons apparent afterwards, I will call thepalæo-hysteron, which opens, by means of a large orifice, into the respiratory or pharyngeal chamber. The anterior end of this chamber terminates in two tubes, or horns, B, B′, the structure of which shows that the median chamber, C, is the result of the amalgamation of two such tubes, and consequently in this chamber, orpalæo-hysteron, the glandular lines are symmetrically situated on each side.
Fig. 82.—Diagrammatic Representation of the so-called Thyroid Gland of Ammocœtes.C, central chamber;A, A′, anterior extremity;B, B′, posterior extremity;Th. o., thyroid opening into respiratory chamber;Ps. br., Ps. br′., ciliated grooves, Dohrn's pseudo-branchial grooves.
Fig. 82.—Diagrammatic Representation of the so-called Thyroid Gland of Ammocœtes.C, central chamber;A, A′, anterior extremity;B, B′, posterior extremity;Th. o., thyroid opening into respiratory chamber;Ps. br., Ps. br′., ciliated grooves, Dohrn's pseudo-branchial grooves.
Fig. 82.—Diagrammatic Representation of the so-called Thyroid Gland of Ammocœtes.
C, central chamber;A, A′, anterior extremity;B, B′, posterior extremity;Th. o., thyroid opening into respiratory chamber;Ps. br., Ps. br′., ciliated grooves, Dohrn's pseudo-branchial grooves.
Fig. 83.—Thyroid Gland as it would appear if the Central Chamber were Uncurled and the Two Horns,B,B′,separated from the Central Chamber.
Fig. 83.—Thyroid Gland as it would appear if the Central Chamber were Uncurled and the Two Horns,B,B′,separated from the Central Chamber.
Fig. 83.—Thyroid Gland as it would appear if the Central Chamber were Uncurled and the Two Horns,B,B′,separated from the Central Chamber.
Any explanation, then, of the thyroid gland of Ammocœtes, musttake into account the clear evidence that it is composed of two tubes, which have in part fused together to form an elongated central chamber, in part remain as horns to that chamber, and that in its walls there exist lines of gland-cells of a striking and characteristic nature.
Further, this central chamber, with its horns, is not a closed chamber, but is in communication with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber, as is well known, opens into the respiratory chamber by a funnel-shaped opening—the so-called thyroid duct (Th. o.). In the second place, there exist two ciliated grooves (Ps. br.,Ps. br′.), the pseudo-branchial grooves of Dohrn, which have direct communication with the thyroid chamber. The manner in which these grooves communicate with the thyroid chamber has never, to my knowledge, been described previously to my description in theJournal of Physiology and Anatomy; it is very instructive, for, as I have there shown, each groove enters into the corresponding lateral horn, so that, in reality, there are three openings into the thyroid chamber or palæo-hysteron—a median opening into the central chamber, and a separate opening into each lateral horn.
The system of ciliated grooves on the inner ventral surface of the respiratory chamber of Ammocœtes was originally described by Schneider as consisting of a single median groove, which extends from the opening of the thyroid to the posterior extremity of the branchial chamber, and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run headwards and diverge from each other, becoming more and more lateral, and more and more dorsal, till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon the whole of this system as a single system, for he speaks of "a ciliated groove, which extends from the orifice of the stomach (i.e.anterior intestine) to the orifice of the thyroid, then divides into two, and runs forward right and left of the median ridge, etc." 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.
A series of sections, such as is given in Fig.81, shows the relation of this pair of ciliated grooves to the thyroid better than any elaborate description. In the first place, it is clear that they remain separate up to their termination—they do not join in the middle line to open into the thyroid duct; in the second place, they are separate from the thyroid orifice—they do not terminate at the rim of the orifice, as is the case with the median groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig.81(2)), gradually moving further and further away from the actual opening of the duct into the pharyngeal chamber. During the whole of their course on the wall of the funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of the duct. The direction of the groove (Ps. br.) shifts as it passes deeper and deeper towards the thyroid, until at last, as seen in Fig.81(3 and 4), it is continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or turned-down horn, as I have called it. In other words, the median chamber opens into the pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the central chamber, owing to their being semi-canals, and not complete canals. If they were originally canals, and not grooves, then the thyroid of Ammocœtes would be derived from an organ composed of a large, common glandular chamber, which opened into the respiratory chamber by means of an extensive median orifice, and possessed anteriorly two horns, from each of which a canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.
Dohrn has pointed out that a somewhat similar structure and topographical arrangement is found in Amphioxus and the Tunicata, the gland-cells being here arranged along the hypobranchial groove to form the endostyle and not shut off to form a closed organ, as in the thyroid of Ammocœtes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and in Amphioxus represents the remnants of the more elaborate organ in Ammocœtes, and that, therefore, in order to explain the meaning of these organs in the former animals, we must first find out their meaning in Ammocœtes. Dohrn, however, goes further than this; for just as he considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammocœtes-like form, so he considers Ammocœtes to have arisenfrom a degenerated Selachian; therefore, in order to be logical, he ought to show that the thyroid of Ammocœtes is an intermediate downward step between the thyroid of Selachians and that of Amphioxus and the Tunicates. Here, it seems to me, his argument utterly breaks down; it is so clear that the thyroid of Petromyzon links on to that of the higher fishes, and that the Ammocœtes thyroid is so immeasurably more complicated and elaborate a structure than is that of Petromyzon, as to make it impossible to believe that the Ammocœtes thyroid has been derived by a process of degeneration from that of the Selachian. 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's evidence, then, points strongly to the conclusion that the starting-point of the thyroid gland in the vertebrate series is to be found in the thyroid of Ammocœtes, which has given rise, on the one hand, to the endostyle of Amphioxus and the Tunicata, and on the other, to the thyroid gland of Petromyzon and the rest of the Vertebrata.
The evidence which I have just given of the intimate connection of the two pseudo-branchial grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that morphologically these two grooves and the thyroid must be considered together. His explanation is that the whole system represents a modified pair of branchial segments distinct from those belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in various papers, is, I think, right in so far as the separateness of the thyroid segment is concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid gland is a modified pair of gills.
We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial segments, without any necessity of deriving the thyroid gland from a pair of gills.