These observations of van Wijhe lead directly to the following conclusion. In the cranial region there is evidence of a double set of segments, which may be called somatic and splanchnic. The somatic segments, consisting of the outer skin and the body musculature, aredoublyinnervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve. But the splanchnic segments are innervated bysingleroots, the vagus or Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth nerve, and trigeminal or Vth nerve, which are mixed, containing both sensory and motor fibres, thus differing markedly from the arrangement of the spinal nerves.
From this sketch it follows that the arrangement seen in the spinal cord, would result from the cranial arrangement if this third system of lateral roots were left out. Further, since the cranial system is the oldest, we must search in the invertebrate ancestor for a tripartite rather than a dual system of nerve-roots for each segment; a system composed of a dorsal root supplying only the sensory nerves of the skin-surfaces, a lateral mixed root supplying the system connected with respiration with both sensory and motor fibres, and a ventral root supplying the motor nerves to the body-musculature.
Comparison of the Appendage Nerves of Limulus and Branchipus to the Lateral Root System of the Vertebrate.
If the argument used so far is correct, and this tripartite system of nerve-roots, as seen in the cranial nerves of the vertebrate, really represents the original scheme of innervation in the palæostracan ancestor, then it follows that each segment of Limulus ought to be supplied by three nerves—(1), a sensory nerve supplying its own portion of the skin-surface of the prosomatic and mesosomatic carapaces; (2), a lateral mixed nerve supplying exclusively the appendage of the segment, for the appendages carry the respiratory organs; and (3), a motor nerve supplying the body-muscles of the segment.
It is a striking fact that Milne-Edwards describes the nerve-roots in exactly this manner. The great characteristic of the nerve-rootsin Limulus as in other arthropods is the large appendage-nerve, which is always a mixed nerve; in addition, there is a system of sensory nerves to the prosomatic and mesosomatic carapaces, called by him the epimeral nerves, which are purely sensory, and a third set of roots which are motor to the body-muscles, and possibly also sensory to the ventral surface between the appendages.
Moreover, just as in the vertebrate central nervous system the centres of origin of the motor nerves of the branchial segmentation are distinct from those of the somatic segmentation, so we find, from the researches of Hardy, that a similar well-marked separation exists between the centres of origin of the motor nerves of the appendages and those of the somatic muscles in the central nervous system of Branchipus and Astacus.
In the first place, he points out that the nervous system of Branchipus is of a very primitive arthropod type; that it is, in fact, as good an example of an ancient type as we are likely to find in the present day; a matter of some importance in connection with my argument, since the arthropod ancestor of the vertebrate, such as I am deducing from the study of Ammocœtes, must undoubtedly have been of an ancient type, more nearly connected with the strange forms of the trilobite era than with the crabs and spiders of the present day.
His conclusions with respect to Branchipus may be tabulated as follows:—
1. Each ganglion of the ventral chain is formed mainly for the innervation of the appendages.
2. Each ganglion is divided into an anterior and posterior division, which are connected respectively with the motor and sensory nerves of the appendages.
3. The motor nerves of the appendages arise as well-defined axis-cylinder processes of nerve-cells, which are arranged in well-defined groups in the anterior division of the ganglion.
4. A separate innervation exists for the muscles and sensory surfaces of the trunk. The trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes to them from certain cells on the dorsal surface of each lateral half of the ganglion. These cell-groups are therefore very distinct from those which give origin to the motorappendage-nerves, and, moreover, are not confined to the ganglion, but extend for some distance into the interganglionic region of the nerve-cords which connect together the ganglia of the ventral chain.
Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion that in such a primitive arthropod type as my theory postulates, each segment was supplied with separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted entirely to the innervation of the pair of appendages; that also, in the central nervous system, the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two separate groups of nerve-cells. These nerve-cells in the one case were aggregated into well-defined groups, which formed the centres for the motor nerves of the markedly segmented muscles of the appendages, and in the other case formed a system of more diffused cells, less markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented somatic muscles.
Such an arrangement suggests that in the ancient arthropod type a double segmentation existed, viz. a segmentation of the body, and a segmentation due to the appendages. Undoubtedly, the segments originally corresponded absolutely as in Branchipus, and every appendage was attached to a well-defined separate body-segment. In, however, such an ancient type as Limulus, though the segmentation may be spoken of as twofold, yet the number of segments in the prosomatic and mesosomatic regions are much more clearly marked out by the appendages than by the divisions of the soma; for, in the prosomatic region such a fusion of somatic segments to form the tergal prosomatic carapace has taken place that the segments of which it is composed are visible only in the young condition, while in the mesosomatic region the separate somatic segments, though fused to form the mesosomatic carapace, are still indicated by the entapophysial indentations.
Clearly, then, if the mesosomatic branchial appendages of forms related to Limulus were reduced to the branchial portion of the appendage, and that branchial portion became internal, just as is known to be the case in the scorpion group, we should obtain an animal in which themesosomatic regionwould be characterized by a segmentation predominantly branchial, which might be termed, as in vertebrates, thebranchiomeric segmentation, but yet would showindications of a corresponding somatic ormesomeric segmentation. The nerve supply to these segments would consist of—
1. The epimeral purely sensory nerves to the somatic surface, equivalent in the vertebrate to the ascending root of the trigeminal.
2. The mixed nerves to the internal branchial segments, equivalent in the vertebrate to the vagus, glossopharyngeal, and facial.
3. The motor nerves to the somatic muscles, equivalent in the vertebrate to the original nerve-supply to the somatic muscles belonging to these segments,i.e.to the muscles derived from van Wijhe's 4th, 5th, and 6th somites.
Further, the centres of origin of these appendage-nerves would form centres in the central nervous system separate from the centres of the motor nerves to the somatic muscles, just as the centres of origin of the motor parts of the facial, vagus, and glossopharyngeal nerves form groups of cells quite distinct from the centres for the hypoglossal, abducens, trochlear, and oculomotor nerves.
In fact, if the vertebrate branchial nerves are looked upon as the descendants of nerves which originally supplied branchial appendages, then every question connected with the branchial segmentation, with the origin and distribution of these nerves, receives a simple and adequate solution—a solution in exact agreement with the conclusion that the vertebrate arose from a palæostracan ancestor.
It would, therefore, be natural to expect that the earliest fishes breathed by means of branchial appendages situated internally, and that the evidence for such appendages would be much stronger in them than in more recent fishes.
Although we know nothing of the nature of the respiratory apparatus in the extinct fishes of Silurian times, we have still living, in the shape of Ammocœtes, a possible representative of such types. If, then, we find, as is the case, that the respiratory apparatus of Ammocœtes differs markedly from that of the rest of the fishes, and, indeed, from that of the adult form or Petromyzon, and that that very difference consists in a greater resemblance to internal branchial appendages in the case of Ammocœtes, then we may feel that the proof of the origin of the branchial apparatus of the vertebrate from the internal branchial appendages of the invertebrate has gained enormously.
The Respiratory Chamber of Ammocœtes.
In order to make clear the nature of the branchial segments in Ammocœtes, I have divided the head-part of the animal by means of a longitudinal horizontal section into halves—ventral and dorsal—as shown in Figs. 63 and 64. These figures are each a combination of a section and a solid drawing. The animal was slit open by a longitudinal section in the neighbourhood of the gill-slits, and each half was slightly flattened out, so as to expose the ventral and dorsal internal surfaces respectively. The structures in the cut surface were drawn from one of a series of horizontal longitudinal sections taken through the head of the animal. These figures show that the head-region of Ammocœtes consists of two chambers, the contents of which are different. In front, an oral or stomodæal chamber, which contains the velum and tentacles, is enclosed by the upper and lower lips, and was originally separated by a septum from the larger respiratory chamber, which contains the separate pairs of branchiæ. A glance at the two drawings shows clearly that Rathke's original description of this chamber is the natural one, for he at that time, looking uponAmmocœtes branchialisas a separate species, described the branchial chamber as containing a series of paired gills, with the gill-openings between consecutive gills. His branchial unit or gill, therefore, was represented by each of the so-called diaphragms, which, as seen in Figs. 63, 64, are all exactly alike, except the first and the last. Any one of these is represented in section in Fig.65, and represents a branchial unit in Rathke's view and in mine. Clearly, it may be described as a branchial appendage which projects into an open pharyngeal chamber, so that the series of such appendages divides the chamber into a series of compartments, each of which communicates with the exterior by means of a gill-slit, and with each other by means of the open space between opposing appendages.
Each of these appendages possesses its own cartilaginous bar (Br. cart.), as explained in Chapter III.; each possesses its own branchial or visceral muscles (coloured blue in Figs. 63 and 64), separated absolutely from the longitudinal somatic muscles (coloured dark red in Figs. 63 and 64) by a space (Sp.) containing blood and peculiar fat-cells, etc. Each possesses its own afferent branchial blood-vessel from the ventral aorta, and its own efferent vessel to the dorsal aorta (Fig.65,a. br.andv. br.). Each possesses its own segmental nerve, which supplies its own branchial muscles and no others with motor fibres, and sends sensory fibres to the general surface of each appendage, as also to the special sense-organs in the shape of the epithelial pits (S., Fig.65) arranged along the free edges of the diaphragms; each of these nerves possesses its own ganglion—the epibranchial ganglion.
Fig. 63.—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. 63.—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. 63.—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. 64.—Dorsal half of Head-region of Ammocœtes.Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 64.—Dorsal half of Head-region of Ammocœtes.Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 64.—Dorsal half of Head-region of Ammocœtes.
Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 65.—Section through Branchial Appendage of Ammocœtes.br. cart., branchial cartilage;v. br., branchial vein;a. br., branchial artery;b.s., blood-spaces;p., pigment;S., sense-organ;c., ciliated band;E., I., external and internal borders;m. add., adductor muscle;m.c.s., striated constrictor muscle;m.c.t., tubular constrictor muscle;m.andm.v., muscles of valve.
Fig. 65.—Section through Branchial Appendage of Ammocœtes.
br. cart., branchial cartilage;v. br., branchial vein;a. br., branchial artery;b.s., blood-spaces;p., pigment;S., sense-organ;c., ciliated band;E., I., external and internal borders;m. add., adductor muscle;m.c.s., striated constrictor muscle;m.c.t., tubular constrictor muscle;m.andm.v., muscles of valve.
Fig. 66.—Section through Branchial Appendage of Limulus.br. cart., branchial cartilage;v.br., branchial vein;b.s., blood-spaces formed by branchial artery;P., pigment;m1, posterior entapophysio-branchial muscle;m2, anterior entapophysio-branchial muscle;m3, external branchial muscle.
Fig. 66.—Section through Branchial Appendage of Limulus.
br. cart., branchial cartilage;v.br., branchial vein;b.s., blood-spaces formed by branchial artery;P., pigment;m1, posterior entapophysio-branchial muscle;m2, anterior entapophysio-branchial muscle;m3, external branchial muscle.
The work of Miss Alcock has shown that the segmental branchial nerve supplies solely and absolutely such an appendage or branchialsegment, and does not supply any portion of the neighbouring branchial segments. The nerve-supply in Ammocœtes gives no countenance to the view that the original unit was a branchial pouch, the two sides of which each nerve supplied, but is strong evidence that the original unit was a branchial appendage, which was supplied by asinglenerve with both motor and sensory fibres.
Any observer having before him only this picture of the respiratory chamber of Ammocœtes, upon which to base his view of a vertebrate respiratory chamber, would naturally look upon the branchial unit of a vertebrate as a gilled appendage projecting into the open cavity of the anterior part of the alimentary canal or pharynx. This is not, however, the usual conception. The branchial unit is ordinarily described as a gill-pouch, which possesses two openings or slits, an internal one into the lumen of the alimentary canal, and an external one into the surrounding medium. This view is based upon embryological evidence of the following character:—
The alimentary canal of all vertebrates forms a tube stretching the whole length of the animal; the anterior part of this tube becomes pouched on each side at regular intervals, and the walls of each pouch becoming folded form the respiratory surfaces or gills. The openings of these separate pouches into the central lumen of the gut form the internal gill-pouch openings; the other extremity of the pouch approaches the external surface of the animal, and finally breaks through to form a series of external gill-pouch openings.
From the mesoblastic tissue, between each gill-pouch, there is formed a supporting cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting framework for the internal gill-slit, so that the gills are situated externally to them; the more primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in position, so that the gills are situated internally to them.
From this description of the mode of formation of the respiratory apparatus in water-breathing vertebrates the conception has arisen of the gill-pouch as the branchial unit, a conception which is absolutely removed from all idea of a branchial unit such as is found in an arthropod, viz. an appendage.
This conception of spaces as units pervades the whole of embryology, and is the outcome of the gastrula theory—a theory whichteaches that all animals above the Protozoa are derived from a form which by invagination of its external surface formed an internal cavity or primitive gut. From pouches of this gut other cavities were said to be formed, called cœlomic cavities, and thus arose the group of cœlomatous animals. To speak of the developmental history of animals in terms of spaces; to speak of the atrophy of a cavity as though such a thing were possible, is, to my mind, the wrong way of looking at the facts of anatomy. It resembles the description of a net as a number of holes tied together with string, which is not usually considered the best method of description.
There are two ways in which a series of pouches can be formed from a simple tube without folding, either by a thinning at regular intervals of the original tissue surrounding the tube, or by the ingrowth into the tube of the surrounding tissue at regular intervals, thus—
Fig. 67.—Diagrams to show the two methods of Pouch-formation.A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at intervals.Ep., epiblast;Mes., mesoblast;Hy., hypoblast.
Fig. 67.—Diagrams to show the two methods of Pouch-formation.A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at intervals.Ep., epiblast;Mes., mesoblast;Hy., hypoblast.
Fig. 67.—Diagrams to show the two methods of Pouch-formation.
A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at intervals.Ep., epiblast;Mes., mesoblast;Hy., hypoblast.
In the first case (A) the formation of a pouch is the significant act, and therefore the branchial segments might be expressed in terms of pouches. In the second case (B) the formation of a pouch isbrought about in consequence of the ingrowth of the mesoblastic tissues at intervals; here, although the end-result is the same as in the first case, the pouch-formation is only secondary, the true branchial unit is the mesoblastic ingrowth.
The evidence all points directly to the second method of formation. Thus Shipley, in his description of the development of the lamprey, says—
"The gill-slits appear to me to be the result of the ventral downgrowth of mesoblast taking place only at certain places, these forming the gill-bars. Between each downgrowth the hypoblastic lining of the alimentary canal remains in contact with the epiblast; here the gill-opening subsequently appears about the twenty-second day."
Dohrn describes and gives excellent pictures of the growth of the diaphragms, as the Ammocœtes grows in size, pictures which are distinctly reminiscent of the corresponding illustrations given by Brauer of the growth of the internal gills in the scorpion embryo.
Another piece of evidence confirmatory of the view that the branchial segments are really of the nature of internal appendages, as the result of which gill-pouches are formed, is given by the presence in each of these branchial bars or diaphragms of a separate cœlomic cavity. From the walls of this cavity the branchial muscles and cartilaginous bar are formed.
Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by the formation of somites, which consist of a series of divisions of the cœlom, of which the walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, such cœlomic divisions form two rows—a dorsal and a ventral set. From the walls of the dorsal set the somatic musculature is formed. From those of the ventral set the branchial musculature. From the latter also the branchial cartilaginous bars are formed. Thus Shipley, in his description of the development of the lamprey, says: "The mesoblast between the gills arranges itself into head-cavities, and the walls of these cavities ultimately form the skeleton of the gill-arches."
Similarly, in the arthropod, the segments in the embryo are marked out by a series of cœlomic cavities and Kishinouye has described in Limulus a separate cœlomic cavity for every one of the mesosomatic or branchial segments, and he states that in Arachnidathe segmental cœlomic cavities extend into the limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult condition is reached.
The whole evidence thus points strongly to the conclusion that the true branchial segmental units are the branchial bars or diaphragms, not the pouches between them.
It is possible to understand why such prominence has been given to the conception of the branchial unit as a gill-pouch rather than as a gill-appendage, when the extraordinary change of appearance in the respiratory chamber of the lamprey which occurs at transformation, is taken into consideration. This change is of a very far-reaching character, and consists essentially of the formation of a new alimentary canal in this region, whereby the pharyngeal chamber of Ammocœtes is cut off posteriorly from the alimentary canal, and is confined entirely to respiratory purposes, its original lumen now forming a tube called the bronchus, which opens into the mouth and into a series of branchial pouches.
In Fig.68I give diagrammatic illustrations taken from Nestler's paper to show the striking change which takes place at transformation, (A) representing three branchial segments of Ammocœtes, and (B) the corresponding three segments of Petromyzon. The corresponding parts in the two diagrams are shown by the cartilages (br. cart.), the sense-organs (S), and the branchial veins (V. br.); the corresponding diaphragms are marked by the figures 1, 2, 3 respectively. As is clearly seen, it is perfectly possible in the latter case to describe the respiratory chamber, as Nestler has done, as divided into a series of separate smaller chambers—the gill-pouches—by means of a series of diaphragms or branchial bars. The surface of these gill-pouches is in part thrown into folds for respiratory purposes, and each gill-pouch opens, on the one hand, into the bronchus (Bro.), and, on the other, to the exterior by means of the gill-slit. The branchial unit in Petromyzon is, therefore, according to Nestler and other morphologists, the folded opposed surfaces of two contiguous diaphragms, and each one of the diaphragms is intersegmental between two gill-pouches.
Nestler then goes on to describe the arrangement in Ammocœtes in the same terms, although there is no bronchus or gill-pouch, but only an open chamber into which these gill-bearing diaphragms project, which open chamber serves both for the passage of food andof the water for respiration. This is manifestly the wrong way to look at the matter: the adult form is derived from the larval, notvice versâ, and the transformation process shows exactly how the gills, in Rathke's sense, come together to form the bronchus and so make the gill-pouches of Petromyzon.
When we bear in mind that almost all observers consider that the internal branchiæ of the scorpion group are directly derived from branchial appendages of a kind similar to those of Limulus, it is evident that a branchial appendage such as that of Ammocœtes might also have arisen from such an appendage, because in various respects it is easier to compare the branchial appendage of Ammocœtes, than that of the scorpion group, with that of Limulus.
Fig. 68.—Diagram of three Branchial Segments of Ammocœtes (A) compared with three Branchial Segments after Transformation (B) to show how the Branchial Appendages of Ammocœtes form the Branchial Pouches of Petromyzon.(AfterNestler.)In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the sense-organs (S), are marked out in order to show corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial segment are not distinguished, being represented a uniform black colour.Bro., the bronchus into which each gill-pouch opens.
Fig. 68.—Diagram of three Branchial Segments of Ammocœtes (A) compared with three Branchial Segments after Transformation (B) to show how the Branchial Appendages of Ammocœtes form the Branchial Pouches of Petromyzon.(AfterNestler.)In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the sense-organs (S), are marked out in order to show corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial segment are not distinguished, being represented a uniform black colour.Bro., the bronchus into which each gill-pouch opens.
Fig. 68.—Diagram of three Branchial Segments of Ammocœtes (A) compared with three Branchial Segments after Transformation (B) to show how the Branchial Appendages of Ammocœtes form the Branchial Pouches of Petromyzon.(AfterNestler.)
In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the sense-organs (S), are marked out in order to show corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial segment are not distinguished, being represented a uniform black colour.Bro., the bronchus into which each gill-pouch opens.
In the case of the scorpions, various suggestions have been made as to the manner in which such a conversion may have taken place. The most probable explanation is that given by Macleod, in whicheach of the branchiæ of the scorpion group is directly compared with the branchial part of the Limulus appendage which has sunk into and amalgamated with the ventral surface.
According to this view, the modification which has taken place in transforming the branchial Limulus-appendage into the branchial scorpion-appendage is a further stage of the process by which the Limulus branchial appendage itself has been formed, viz. the getting rid of the free locomotor segments of the original appendage, thus confining the appendage more and more to the basal branchial portion. So far has this process been carried in the scorpion that all the free part of the appendage has disappeared; apparently, also, the intrinsic muscles of the appendage have vanished, with the possible exception of the post-stigmatic muscle, so that any direct comparison between the branchial appendages of Limulus and the scorpions is limited to the comparison of their branchiæ, their nerves, and their afferent and efferent blood-vessels.
In the case of Ammocœtes the comparison must be made not with air-breathing but with water-breathing scorpions, such as existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia, and with the crowd of trilobite and Limulus-like forms which were in past ages so predominant in the sea; forms in some of which the branchial appendages had already become internal, but which, from the very fact of these forms being water-breathers, probably resembled, in respect of their respiratory apparatus, Limulus rather than the present-day scorpion.
On the assumption that the branchial appendages of Ammocœtes, like the branchial appendages of the scorpion group, are to a certain extent comparable with those of Limulus, it becomes a matter of great interest to inquire whether the mode in which respiration is effected in Ammocœtes resembles most that of Limulus or of the scorpion.
The Origin of the Branchial Musculature.
The difference between the movements of respiration in Limulus and those of the scorpions consists in the fact that, although in both cases respiration is effected mainly by dorso-ventral muscles, these muscles are not homologous in the two cases: in the former, the dorso-ventral appendage-muscles are mainly concerned, in the latter, the dorso-ventral somatic muscles.
The paper by Benham gives a full description of the musculature of Limulus, and according to his arrangement the muscles are divided into two sets, longitudinal and dorso-ventral. Of the latter, each mesosomatic segment possesses a pair of dorso-ventral muscles, attached to the mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig.58,Dv.). These muscles are called by Benham the vertical mesosomatic muscles. I shall call them the somatic dorso-ventral muscles, in contradistinction to the dorso-ventral muscles of the branchial appendages. Of the latter, the two chief are the external branchial (Fig.66,m3) and the posterior entapophysio-branchial (Fig.66,m1); a third muscle is the anterior entapophysio-branchial (Fig.66,m2). Of these muscles, the posterior entapophysio-branchial (m1) is closely attached along the branchial cartilaginous bar up to its round-headed termination on the anterior surface of the appendage. The anterior entapophysio-branchial muscle (m2) is attached to the branchial cartilage near the entapophysis.
In the case of the scorpion, as described by Miss Beck, the branchial appendage has become reduced to the branchiæ, and the intrinsic appendage-muscles have entirely disappeared, with the possible exception of the small post-stigmatic muscle; on the other hand, the dorso-ventral somatic muscles, which are clearly homologous with the corresponding muscles of Limulus, have remained, and become the essential respiratory muscles.
Of these two possible types of respiratory movement it is quite conceivable that in the water-breathing scorpions of olden times and in their allies, the dorso-ventral muscles of their branchial appendages may have continued theirrôleof respiratory muscles, and so have given origin to the respiratory muscles of the ancestors of Ammocœtes.
The respiratory muscles of Ammocœtes are three in number, and have been described by Nestler and Miss Alcock as the adductor muscle, the striated constrictor muscle, and the tubular constrictor muscle (Fig.65,m. add.,m.c.s., andm.c.t.). Of these, the constrictor muscle (Fig.71,m. con. str.) is in close contact with its cartilaginous bar, while the adductor (Fig.71,m. add.) is attached to the cartilage only at its origin and insertion, and the tubular muscles (Fig.71,m. con. tub.) have nothing whatever to do with the cartilage at all, being attached ventrally to the connective tissue in the neighbourhoodof the ventral aorta (V.A.), and dorsally to the mid-line between the dorsal aorta (D.A.) and the notochord.
The close relationship of the constrictor muscle to the cartilaginous branchial bar does not favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the scorpion. It is, however, directly in accordance with the view that this muscle is homologous with one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle (m1, Fig.66) of the Limulus appendage, especially when the homology of the Ammocœtes branchial cartilage with the Limulus branchial cartilage is borne in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the Ammocœtes branchial segment as more likely to have been derived from dorso-ventral muscles which belonged originally to a branchial appendage, such as we see in Limulus, than from dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in Limulus and scorpion. In other words, I am inclined to hold the view that the somatic dorso-ventral muscles have disappeared in this region in Ammocœtes, while dorso-ventral appendage-muscles have been retained,i.e.the exact reverse to what has taken place in the air-breathing scorpion.
I am especially inclined to this view because of the manner in which it fits in with and explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates into parietal and visceral, such a division has received general acceptance and, as far as the head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van Wijhe,i.e.to the somatic and splanchnic striated muscles according to my nomenclature. Of these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammocœtes and of the same character throughout; the somatic musculature, on the other hand, is clearly divisible, in the head region, into two sets—a spinal and a cranial set. The somatic muscles innervated by the spinal set of nerves, including in this term the spino-occipital or so-called hypoglossal nerves, are in Ammocœtes most sharply defined from all the other muscles of the body. They form the great dorsal and ventral longitudinalbody-muscles, which extend dorsally as far forward as the nose and are developed embryologically quite distinctly from the others, being formed as muscle-plates (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated musculature of the Ammocœtes must be divided into (1) the visceral muscles; (2) the longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. Of these the 1st, on the view just stated, represent the original appendage-muscles; the 2nd belong to the spinal region, and will be considered with that region; the 3rd represent the original segmental dorso-ventral somatic muscles, which are so conspicuous in the musculature of the Limulus and the scorpion group.
The discussion of this last statement will be given when I come to deal with the prosomatic segments of Ammocœtes. I wish, here, simply to point out that van Wijhe has shown that the eye-muscles develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and therefore represent the somatic muscles belonging to those segments, while no development of any corresponding muscles takes place in the 4th, 5th, and 6th segments; so that if the eye-muscles represent a group of dorso-ventral somatic muscles, such muscles have been lost in the 4th, 5th, and 6th segments. The latter segments are, however, the glossopharyngeal and vagus segments, the branchial musculature of which is derived from the ventral segments of the mesoderm. In other words, van Wijhe's observations mean that the dorso-ventral somatic musculature has been lost in the branchial or mesosomatic region, while the dorso-ventral appendage musculature has been retained, and that, therefore, the mode of respiration in Ammocœtes more closely resembles that of Limulus than of Scorpio.
In addition to these branchial muscles, another and very striking set of muscles is found in the respiratory region of Ammocœtes—the so-called tubular muscles. These muscles are of great interest, but as they are especially connected with the VIIth nerve, their consideration is best postponed to the chapter dealing with that nerve.
Also, in connection with the vagus group of nerves, special sense-organs are found in the skin covering this mesosomatic region, the so-called epithelial pit-organs (Ep. pit., Fig.71). They, too, are ofgreat interest, but their consideration may also better be deferred to the chapter dealing with those special sense-systems known as the lateral line and auditory systems.
Comparison of the Branchial Circulation in Ammocœtes and Limulus.
Closely bound up with the respiratory system is the nature of the circulation of blood through the gills. Before, therefore, proceeding to the consideration of the segments in front of those which carry branchiæ, it is worth while to compare the circulation of the blood in the gills of Limulus and of Ammocœtes respectively.
In all the higher vertebrates the blood circulates in a closed system of capillaries, which unite the arterial with the venous systems. In all the higher invertebrates this capillary system can hardly be said to exist; the blood is pumped from the arterial system into blood spaces or lacunæ, and thus comes into immediate contact with the tissues. From these it is collected into veins, and so returned to the heart. There is, in fact, no separate lymph-system in the higher invertebrates; the blood-system and lymph-system are not yet differentiated from each other. This also is the case in Ammocœtes; here, too, in many places the blood is poured into a lacunar space, and collected thence by the venous system; a capillary system is only in its commencement and a lymph-system does not yet exist. In this part of its vascular system Ammocœtes again resembles the higher invertebrates more than the higher vertebrates.
This resemblance is still more striking when the circulation in the respiratory organs of the two animals is compared. A branchial appendage is essentially an appendage whose vascular system is arranged for the special purpose of aerating blood. In the higher vertebrates such a purpose is attained by the pulmonary capillaries, in Limulus by the division of the posterior surface of the basal part of the appendage into thin lamellar plates, the interior of each of which is filled with blood. The two surfaces of each lamella are kept parallel to each other by means of fibrous or cellular strands forming little pillars at intervals, called by Macleod "colonettes." A precisely similar arrangement is found in the scorpion gill-lamella, as seen in Fig.69, A, taken from Macleod. In Ammocœtes there are no well-defined branchial capillaries, but the blood circulates, as inthe invertebrate gill, in a lamellar space; here, also, as Nestler has shown, the opposing walls of the gill-lamella are held in position by little pillar-like cells, as seen in Fig.69, B, taken from his paper.
In this representative of the earliest vertebrates the method of manufacturing an efficient gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time when vertebrates first appeared. This similarity indicates a close resemblance between the circulatory systems of the two groups of animals, and therefore, to the superficial inquirer, would indicate an homology between the heart of the vertebrate and the heart of the higher invertebrate; but the former is situated ventrally to the gut and the nervous system, while the latter is composed of a long vessel which lies in the mid-dorsal line immediately under the external dorsal covering. Indeed, this ventral position of the heart in the one group of animals and its dorsal position in the other, combined with the corresponding positions of the central nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most important fact which ought to make us hesitate before accepting the homology of the dorsal heart of the arthropod with the ventral heart of the vertebrate—The heart in all invertebrates is a systemic heart,i.e.drives the arterial blood to the different organs of the body, and then the veins carry it back to the respiratory organ, from whence it passes to the heart.
Fig. 69.—Comparison of Branchial Lamellæ of Limulus and Scorpio with Branchial Lamellæ of Ammocœtes.A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocœtes (after Nestler).
Fig. 69.—Comparison of Branchial Lamellæ of Limulus and Scorpio with Branchial Lamellæ of Ammocœtes.A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocœtes (after Nestler).
Fig. 69.—Comparison of Branchial Lamellæ of Limulus and Scorpio with Branchial Lamellæ of Ammocœtes.
A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocœtes (after Nestler).
The only exception to this scheme is found in the vertebrate where the heart is essentially a branchial heart, the blood beingdriven from the heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body. The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to the ventral venous system. This constitutes a very strong reason for believing that the systemic heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the vertebrate heart arise?
Let us first see how the blood is supplied to the gills in Limulus.
Fig. 70.—Longitudinal Diagrammatic Section through the Mesosomatic Region of Limulus, to show the origin of the Branchial Arteries.(AfterBenham.)L.V.S., longitudinal venous sinus, or collecting sinus;a. br., branchial arteries;V.p., veno-pericardial muscles;P., pericardium.
Fig. 70.—Longitudinal Diagrammatic Section through the Mesosomatic Region of Limulus, to show the origin of the Branchial Arteries.(AfterBenham.)L.V.S., longitudinal venous sinus, or collecting sinus;a. br., branchial arteries;V.p., veno-pericardial muscles;P., pericardium.
Fig. 70.—Longitudinal Diagrammatic Section through the Mesosomatic Region of Limulus, to show the origin of the Branchial Arteries.(AfterBenham.)
L.V.S., longitudinal venous sinus, or collecting sinus;a. br., branchial arteries;V.p., veno-pericardial muscles;P., pericardium.
In Limulus the blood flows into the lamellæ from sinuses or blood-spaces (b.s., Fig.66) at the base of each of the lamellæ, which sinuses are filled by a vessel which may be called the branchial artery, since it is the afferent branchial vessel. On each side of the middle line of the ventral surface of the body a large longitudinal venous sinus exists, called by Milne-Edwards the venous collecting sinus,L.V.S., (Fig.70and Fig.58), which gives off to each of the branchial appendages on that side a well-defined afferent branchial vessel—the branchial artery (a. br.). The blood of the branchial artery flows into the blood-spaces between the anterior and posterior laminæ of the appendage and thence into the gill-lamellæ, from which it is collected into an efferent vessel or branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which carries it back to the dorsal heart. The position of the branchial artery and vein is shown in Fig.66, which represents a section through the branchial appendage of Limulus at right angles to the cartilaginous branchial bar (br. cart.), just as Fig.65represents a section through thebranchial appendage of Ammocœtes at right angles to the cartilaginous branchial bar.
Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and Benham concur in showing that in both Limulus and the scorpion group a striking and most useful connection exists between the heart and these two collecting venous sinuses, in the shape of a segmentally arranged series of muscular bands (V.p., Fig. 70 and Fig.58), attached, on the one hand, to the pericardium, and on the other to the venous collecting sinus on each side. These muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,' are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in the scorpion as 'ligaments contractiles,' and considers that they play an important part in assisting the pulmonary circulation; for, he says, "en mettant a nu une portion du cœur, on remarque que ces battements se font sentir sur les ligaments contractiles, et determinent sur les poches pulmonaires une pression qui fait aussitot refluer et remonter le sang dans les vaisseaux pneumocardiaques." Lankester, in discussing the veno-pericardial muscles of Limulus and of the scorpions, says that these muscles probably contract simultaneously with the heart and are of great importance in assisting the flow through the pulmonary system. More recently Carlson has investigated the action of these muscles in the living Limulus and found that they act simultaneously with the muscles of respiration.
Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group, and we may fairly assume that a similar arrangement existed in the extinct forms from which I imagine the vertebrate to have arisen. The further consideration of this group of muscles will be given in Chapter IX.
Passing now to the condition of the branchial blood-vessels of Ammocœtes, we see that the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery which arises segmentally from the ventral aorta (V.A.), as seen in Fig.71(taken from Miss Alcock's paper). From the gill-lamellæ the blood is collected into an efferent or branchial vein (v. br.), whichruns, as seen in Fig.65, along the free edge of the diaphragm, and terminates in the dorsal aorta.
The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries arise segmentally.