Chapter XV: Final Remarks (6)

As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points are--the shape of the trabecular portion, diverging as it does around the infundibulum, and the presence on the parachordal portion of the two large auditory capsules.

These two points indicate, on the hypothesis that infundibulum and oesophagus are convertible terms, that two supporting structures of a cartilaginous nature must have existed in the ancestor of the vertebrate, the first of which surrounded the oesophagus, and the second was in connection with its auditory apparatus.

STRUCTURE OF THE HARD CARTILAGES.

The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of the soft, in so far that it consists of large cells with a comparatively small amount of intercellular substance. Schaffer, who has described it lately, considers that it is a nearer approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the usual sense of the term. Its peculiarities and its differences from the soft are especially well seen by its staining reactions. I have myself been particularly struck with the effect of picrocarmine or combined hæmatoxylin and picric acid {134}staining (Fig. 57). In the case of the soft cartilage the capsular substance stains respectively a brilliant red or blue, while that of the hard cartilage is coloured a deep yellow, so that the junction between the parachordals and the branchial cartilages is beautifully marked out. Then, again, with thionin, which gives so marked a reaction in the case of the soft cartilage, the hard cartilage of the auditory capsule is not stained at all, and in the trabeculæ the deep purple colour is confined to the mucoid cement-substance between the capsules, just as Schaffer has stated. The same kinds of reactions have been described by Schaffer: thus by double staining with hæmalum-eosin the hard cartilage stains red, the soft blue; and he points out that even with over-staining by hæmalum the auditory capsule remains colourless, just as I have noticed with thionin. He infers, precisely as I have done from the thionin reaction, that chondro-mucoid, which is so marked a constituent of the soft cartilage and of the muco-cartilage, is absent or present in but slight quantities in the hard cartilage. Similarly, he points out that double staining with tropoeolin-methyl-violet stains the hard cartilage a bright orange colour, and the soft cartilage a violet.

The evidence, then, shows clearly that a marked chemical difference exists between these two cartilages, which may be expressed by saying that the one contains very largely a basophil substance, which we may speak of as belonging to the class of chondro-mucoid substances, while the other contains mainly an oxyphil substance, probably a chondro-gelatine substance.

We may perhaps go further and attribute this difference of composition to a difference of origin; for whereas the soft cartilage is invariably formed in a special tissue, the muco-cartilage, which shows by its reaction how largely it is composed of a mucoid substance, the hard cartilage is certainly, in the case of the cartilage of the cranium where its origin has been clearly made out, formed in the membranous tissue of the cranium of Ammocoetes--_i.e._ in a tissue which stains light blue with thionin, and contains a gelatinous rather than a mucoid substratum.

The best opportunity of finding out the mode of origin of the hard cartilage is afforded at the time of transformation, when so much of this kind of cartilage is formed anew. Unfortunately, it is very difficult to obtain the early transformation stages, consequently we cannot be said to possess any really exhaustive and {135}definite account of how the new cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a more or less definite account of their formation, and the one striking impression left on the mind of the reader is how their descriptions vary. In one point only are they agreed, and in that I also agree with them, viz. the manner in which the new cranial walls are formed. Schaffer describes the process as the invasion of chondroblasts into the homogeneous fibrous tissue of the cranial walls. Such chondroblasts not only form the cartilaginous framework, but also assimilate the fibrous tissue which they invade, so that finally all that remains of the original fibrous matrix in which the cartilage was formed are these lines of cement-substance between the groups of cartilage cells, which, containing some basophil material, are marked out, as already mentioned (Fig. 57).

We may therefore conclude, from the investigation of Ammocoetes, that the front part of the basi-cranial skeleton arose as two trabecular bars, to which muscles were attached, situated bilaterally with respect to the central nervous system. These bars were composed of tendinous material with a gelatinous rather than a mucoid substratum, in which nests of cartilage-cells were formed, the cartilaginous material formed by these cells being of the hard variety, not staining with thionin, and staining yellow with picro-carmine, etc. By the increase of such nests and the assimilation of the intermediate fibrous material, the original fibro-cartilage was converted into the close-set semi-hyaline cartilage of the trabeculæ and auditory capsules, in which the fibrous material still marks out by its staining-reaction the limits of the cell-clusters.

Such I gather to be Schaffer's conclusions, and they are certainly borne out by my own and Miss Alcock's observations. As far as we have had an opportunity of observing at present, the first process at transformation appears to consist of the invasion of the fibrous tissue of the cranial wall by groups of cells which form nests of cells between the fibrous strands. These nests of cells form round themselves capsular material, and thus form cell-territories of cartilage, which squeeze out and assimilate the surrounding fibrous tissue, until at last all that remains of the original fibrous matrix is the lines of cement-substance which mark out the limits of the various cell-groups.

At present I am inclined to think that both soft and hard cartilage originate in a very similar manner, viz. by the formation of capsular {136}material around the invading chondroblasts, and that the difference in the resulting cartilage is mainly due to the difference in chemical composition of the matrix of the connective tissue which is invaded. Thus the difference may be formulated as follows:--

The hard cartilage is formed by the invasion of chondroblasts into a fibrous tissue, which contains a gelatinous rather than a mucoid substratum, in contradistinction to the soft cartilage which is formed, probably also by the invasion of chondroblasts, in a tissue--the muco-cartilage--which contains a specially mucoid substratum.

Such, then, is the very clearly defined starting-point of the vertebrate skeleton--two distinct formations of different histological and chemical structure,--the one forming a segmented branchial skeleton, the other a non-segmented basi-cranial skeleton.

THE CARTILAGINOUS SKELETON OF LIMULUS.

Among the whole of the invertebrates at present living on the earth, is there any sign of an internal cartilaginous skeleton that will give a direct clue to the origin of the primitive vertebrate skeleton? The answer to this question is most significant: only one animal among all those at present known possesses a cartilaginous skeleton, which is directly comparable with that of Ammocoetes, and here the comparison is very close--only one animal among the thousands of living invertebrate forms, and that animal is the only representative still surviving of the palæostracan group, which was the dominant race when the vertebrate first made its appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage as the branchial skeleton of Ammocoetes, confined to the mesosomatic or branchial region, just as in Ammocoetes, forming, as in Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are situated externally to the branchiæ, as in Ammocoetes. In addition this animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the entosternite or plastron situated, with respect to the oesophagus, similarly to the position of the trabeculæ with respect to the infundibulum in Ammocoetes. Moreover, the cartilaginous cells in this tissue differ from those in the branchial region, in precisely the same manner as the hard cartilage differs from the soft in Ammocoetes.

{137}This plastron, it is true, is found in other animals, all of which are members of the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean Apus--a strange primitive form, which is acknowledged to be the nearest representative of the Trilobita still living on the earth. None of these forms, however, possess any sign of an internal cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are all surviving types of the stage of organization which had been reached in the animal world when the vertebrate first appeared.

THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT CARTILAGE.

Searching through the literature of the histology of the cartilaginous tissues in invertebrate animals, to see whether any cartilage had been described similar to that seen in the branchial cartilages of Ammocoetes, and whether such cartilage, if found, arose in a fibrous tissue resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed muster for a drawing of the branchial cartilage of Ammocoetes. This clue I followed out in the manner described in my former paper in the _Journal of Anatomy and Physiology_, and mapped out the topography of this remarkable tissue.

Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in Fig. 59; and these appendages are very different to the ordinary locomotor appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. 58, consists mainly of a broad, basal part, which carries the gill-book on its under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist, not for locomotor purposes, but because they carry on each segment organs of special importance to the animal (see Chapter XI.). As is seen in Fig. 58, the basal parts of each pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma.

{138}

In each figure the branchial cartilaginous bar, _Br.C._, has been exposed by dissection on one side. _Ent._, entapophysis; _Ent.l._, entapophysial ligament cut across; _Br.C._, branchial cartilaginous bar, which springs from the entapophysis; _H._, heart; _P._, pericardium; _Al._, alimentary canal; _N._, nerve cord; _L.V.S._, longitudinal venous sinus; _Dv._, dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]

At the base of each of these appendages, where it is attached to the body of the animal, the external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked rod of cartilage (_Br.C._), which extends from the body {139}of the animal well into each appendage. This bar of cartilage arises on each side from the corresponding entapophysis (_Ent._), which is the name given to a chitinous spur which projects a short distance (Fig. 58, B) into the animal from the dorsal side, for the purpose of giving attachment to various segmental muscles. These entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of entapophyses, the number of segments in that region, in contradistinction to the prosomatic carapace, which gives no indication on its surface of the number of its components.

Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial ligament (_Ent.l._) (Fig. 58), and in this tissue cartilage is formed. Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a concentration of such groups always takes place at each entapophysis, forming here a solid mass of cartilage, from which the massive cartilaginous bar of each branchial appendage arises.

Further, not only is this cartilage exactly similar to parenchymatous cartilage, as it occurs in the branchial cartilages of Ammocoetes, but also its matrix stains a brilliant purple with thionin in striking contrast to the exceedingly slight light-blue colour of the surrounding perichondrium. In its chemical composition it shows, as might be expected, that it is a cartilage containing a very large amount of some mucin-body.

THE MUCO-CARTILAGE OF LIMULUS.

The resemblance between this structure and that of the branchial bars of Ammocoetes does not end even here, for, as already mentioned, the cartilage originates in a peculiar connective tissue band, the entapophysial ligament, and this tissue bears the same relation in its chemical reactions to the ordinary connective tissue of Limulus, as muco-cartilage does to the white fibrous tissue of Ammocoetes. The white connective tissue of Limulus, as already stated, resembles that of the vertebrate more than does the connective tissue of any other invertebrate, and, similarly to that of Ammocoetes, does not stain, or gives only a light-blue tinge with thionin. The tissue of {140}the entapophysial ligament, on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a perfectly similar soft cartilage is born from it.

The branchial cartilages and the entapophysial ligaments are coloured blue, the branchiæ red. _gl._, generative and hepatic glands surrounding the central nervous system and passing into the base of the flabellum (_fl._).]

One difference, however, exists between the branchial cartilages of these two animals; the innermost axial layer of the branchial bar of Limulus is very apt to contain a specially hard substance, apparently chalky in nature, so that it breaks up in sections, and gives the appearance of a broken-down spongy mass; if, however, the tissue is first placed in a solution of hydrochloric acid, it then cuts easily, and the whole tissue is seen to be of the same structure throughout, the main difference being that the capsular spaces in the axial region are much larger and much more free from cell-protoplasm than are those of the smaller younger cells near the periphery.

{141}I have attempted in Fig. 53 to represent this close resemblance between the segmented branchial skeleton of Limulus and of Ammocoetes, a resemblance so close as to reach even to minute details, such as the thinning out of the cartilage in the subchordal bands and entapophysial ligaments respectively between the places where the branchial bars come off.

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchiæ red. _gl._, glandular substance surrounding the central nervous system and passing into the auditory capsule with the auditory nerve (_VIII._).]

In Fig. 59 I have shown the prosoma and mesosoma of Limulus, and indicated the nerves to the appendages together with the mesosomatic cartilaginous skeleton.

In Fig. 60 I have drawn a corresponding picture of the prosomatic and mesosomatic region of Ammocoetes with the corresponding nerves {142}and cartilages. In this figure the animal is supposed to be slit open along the ventral mid-line and the central nervous system exposed.

THE PROSOMATIC SKELETON OF LIMULUS, COMPOSED OF HARD CARTILAGE.

The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a support for the base of the brain. This skeleton was composed of hard cartilage, and arose in white fibrous tissue containing gelatin rather than mucin.

Is there, then, any peculiar tissue of a cartilaginous nature in Limulus and its allies, situated in the prosomatic region, which is entirely separate from the branchial cartilaginous skeleton, which acts as a supporting internal framework, and contains a gelatinous rather than a mucoid substratum?

It is a striking fact, common to the whole of the group of animals to which our inquiries, deduced from the consideration of the structure of Ammocoetes, have, in every case, led us in our search for the vertebrate ancestor, that they do possess a remarkable internal semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which gives support to a large number of the muscles of that region; which is entirely independent of the branchial skeleton, and differs markedly in its chemical reactions from that cartilage, in that it contains a gelatinous rather than a mucoid substratum.

In Limulus it is a large, tough, median plate, fibrous in character, in which are situated rows and nests of cartilage-cells. The same structure is seen in the plastron of Hypoctonus, of Thelyphonus, and to a certainty in all the members of the scorpion group. Very different is the behaviour of this tissue to staining from that of the branchial region. No part of the plastron stains purple with thionin; it hardly stains at all, or gives only a very slight blue colour. In its chemical composition there is a marked preponderance of gelatin with only a slight amount of a mucin-body. In some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules of the cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while the fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from the picric acid.

All the evidence points to the plastron as resembling the basi-cranial skeleton of Ammocoetes in its composition and in the origin {143}of its cells in a white fibrous tissue. What, then, is its topographical position? It is in all cases a median structure lying between the cephalic stomach and the infra-oesophageal portion of the central nervous system, and in all cases it possesses two anterior horns which pass around the oesophagus and the nerve-masses which immediately enclose the oesophagus (Fig. 61, A). These lateral horns, then, which lie laterally and slightly ventral to the central nervous system, and are called by Ray Lankester and Benham the sub-neural portion of the entosternite, are very nearly in exactly the position of the racquet-shaped head of the trabeculæ in Ammocoetes. It is easy to see that, with a more extensive growth of the nervous material dorsally, such lateral horns might be caused to take up a still more ventral position. Now, these two lateral horns of the plastron of Limulus are continued along its whole length so as to form two thickened lateral ridges, which are conspicuous on the flat surface of the rest of this median plate. In other cases, as in the Thelyphonidæ, the plastron consists mainly of these two lateral ridges or trabeculæ, as they might be called, and Schimkéwitsch, who more than any one else has made a comparative study of the entosternite, describes it as composed in these animals of two lateral trabeculæ crossed by three transverse trabeculæ. I myself can confirm his description, and give in Fig. 61, B, the appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia and part of the infra-oesophageal ganglia fill up the space _Ph._; stretching over the rest of the infra-oesophageal mass is a transverse trabecula, which is very thin; then comes a space in which is seen the rest of the infra-oesophageal mass, and then the posterior part of the plastron, ventrally to which lies the commencement of the ventral nerve-cord.

_Ph._, position of pharynx.]

{144}In these forms, in which the central nervous system is more concentrated towards the cephalic end than in Limulus, the whole of the concentrated brain-mass is separated from the gut only by this thin transverse band of tissue. Judging, then, from the entosternite of Thelyphonus, it is not difficult to suppose that a continuation of the same growth of the brain-region of the central nervous system would cause the entosternite to be separated into two lateral trabeculæ, which would then take up the ventro-lateral position of the two trabeculæ of Ammocoetes.

On the other hand, it might be that two lateral trabeculæ, similar to those of Thelyphonus and situated on each side of the central nervous system, were the original form from which, by the addition of transverse fibres running between the gut and nervous system, the entosternite of Thelyphonus and of the scorpions, etc., was formed. From an extensive consideration of the entosternite in different animals, Schimkéwitsch has come to the conclusion that this latter explanation is the true one. He points out that the lateral trabeculæ can be distinguished from the transverse by their structure, being much more cellular and less fibrous, and the cell-cavities more rounded, or, as I should express it, the two lateral trabeculæ are more cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of structure and from embryological investigations, comes to the conclusion that the entosternite was originally composed of two parts--

1. A transverse muscle corresponding to the adductor muscle of the shell of certain crustaceans, such as Nebalia.

2. A pair of longitudinal mesodermic tendons, which may have been formed originally out of a number of segmentally arranged mesodermic tendons, and are crossed by the fibrils of the transverse muscular bundles.

These paired tendons of the entosternite he considers to correspond to the intermuscular tendons, situated lengthways, which are found in the ventral longitudinal muscles of most arthropods.

It is clear from these observations of Schimkéwitsch, that the essential part of the entosternite consists of two lateral trabeculæ, which were originally tendinous in nature and have become of the nature of cartilaginous tissue by the increase of cellular elements in the matrix of the tissue: these two trabeculæ function as supports for the attachment of muscles, which are specially attached at certain places. At these places transverse fibres belonging to some {145}of the muscular attachments cross between the two longitudinal trabeculæ, and so form the transverse trabeculæ.

I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly resembling those of Hypoctonus, and he also states that they are confined to the lateral portions of the entosternite.

From this evidence it is easy to see that that portion of the basi-cranial skeleton known as the trabeculæ may have originated from the formation of cartilage in the plastron or entosternite of a palæostracan animal. Such an hypothesis immediately suggests valuable clues as to the origin of the cranium and of the rest of the basi-cranial skeleton--the parachordals and the auditory capsules. The former would naturally be a dorsal extension of the more membranous portion of the plastron, in which, equally naturally, cartilaginous tissue would subsequently develop; and the reason why it is impossible to reduce the cranium into a series of segments would be self-evident, for even though, as Schimkéwitsch thinks, the plastron may have been originally segmented, it has long lost all sign of segmentation. The latter would be derived from a second entosternite of the same nature as the plastron, but especially connected with the auditory apparatus of the invertebrate ancestor. The following out of these two clues will be the subject of a future chapter.

In our search, then, for a clue to the origin of the skeletal tissues of the vertebrate we see again that we are led directly to the palæostracan stock on the invertebrate side and to the Cyclostomata on that of the vertebrate; for in Limulus, the only living representative of the Palæostraca, and in Limulus alone, we find a skeleton marvellously similar to the earliest vertebrate skeleton--that found in Ammocoetes. Later on I shall give reasons for the belief that the earliest fishes so far found, the Cephalaspidæ, etc., were built up on the same plan as Ammocoetes, so that, in my opinion, in Limulus and in Ammocoetes we actually possess living examples allied to the ancient fauna of the Silurian times.

{146}SUMMARY.

The skeleton considered in this chapter is not the notochord, but that
composed of cartilage. The tracing downwards of the vertebrate bony and
cartilaginous skeleton to its earliest beginnings leads straight to the
skeleton of the larval lamprey (Ammocoetes), in which vertebræ are not
yet formed, but the cranial and branchial skeleton is well marked.

The embryological and phylogenetic histories are in complete unison to
show that the cranial skeleton is older than the spinal, and this
primitive branchial skeleton is also in harmony with the laws of
evolution, in that its structure, even in the adult lamprey (Petromyzon),
never gets beyond the stage characteristic of embryonic cartilage in the
higher vertebrates.

The simplest and most primitive skeleton is that found in Ammocoetes and
consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton.

The prosomatic skeleton forms a non-segmented basi-cranial skeleton of
the simplest kind--the trabeculæ and the parachordals with their attached
auditory capsules, just as the embryology of the higher vertebrates
teaches us must be the case. There in the free-living, still-existent
Ammocoetes we find the manifest natural outcome of the embryological
history in the shape of simple trabeculæ and parachordals, from which the
whole complicated basi-cranial skeleton of the higher vertebrates arose.

The mesosomatic skeleton, which is formed before the prosomatic,
consisted, in the first instance, of simple branchial bars segmentally
arranged, which were connected together by a longitudinal subchordal bar,
situated laterally on each side of the notochord. These simple branchial
bars later on form the branchial basket-work, which forms an open-work
cage within which the branchiæ are situated.

The cartilages which compose these two skeletons respectively are
markedly different in chemical constitution, in that the first (hard
cartilage) is mainly composed of chondro-gelatin, the second (soft
cartilage) of chondro-mucoid material.

The same kind of difference is seen in the two kinds of connective tissue
which are the forerunners of these two kinds of cartilage. Thus, the
cranial walls in Ammocoetes are formed of white fibrous tissue, an
essentially gelatin-containing tissue; at transformation these are
invaded by chondro-blasts and the cartilaginous cranium, formed of hard
cartilage, results. On the other hand, the forerunner of the branchial
soft cartilage is a very striking and peculiar kind of connective tissue
loaded with mucoid material, to which the name muco-cartilage has been
given.

The enormous interest of this muco-cartilage consists in the fact that it
forms very well-defined plates of tissue, entirely confined to the
head-region, which are not found in any higher vertebrate, not even in
the adult form Petromyzon, for every scrap of the tissue as such
disappears at transformation.

It is this evidence of primitive non-vertebrate tissues, which occur in
the larval but not in the adult form, which makes Ammocoetes so valuable
for the investigation of the origin of vertebrates.

The evidence, then, is extraordinarily clear as to the beginnings of the
vertebrate skeletal tissues.

{147}In the invertebrate kingdom true cartilage occurs but scantily.
There is a cartilaginous covering of the brain of cephalopods. It is
never found in crabs, lobsters, bees, wasps, centipedes, butterflies,
flies, or any of the great group of Arthropoda, except, to a slight
extent, in some members of the scorpion group, and more fully in one
single animal, the King-crab or Limulus: a fact significant of itself,
but still more so when the nature of the cartilage and its position in
the animal is taken into consideration, for the identity both in
structure and position of this internal cartilaginous skeleton with that
of Ammocoetes is extraordinarily great.

Here, in Limulus, just as in Ammocoetes, an internal cartilaginous
skeleton is found, composed of two distinct parts: (1) prosomatic, (2)
mesosomatic. As in Ammocoetes, the latter consists of simple branchial
bars, segmentally arranged, which are connected together on each side by
a longitudinal ligament containing cartilage--the entapophysial ligament.
This cartilage is identical in structure and in chemical composition with
the soft cartilage of Ammocoetes, and, as in the latter case, arises in a
markedly mucoid connective tissue. The former, as in Ammocoetes, consists
of a non-segmental skeleton, the plastron, composed of a white fibrous
connective tissue matrix, an essentially gelatin-containing tissue, in
which are found nests of cartilage cells of the hard cartilage variety.

This remarkable discovery of the branchial cartilaginous bars of Limulus,
together with that of the internal prosomatic plastron, causes the
original difficulty of deriving an animal such as the vertebrate from an
animal resembling an arthropod to vanish into thin air, for it shows that
in the past ages when the vertebrates first appeared on the earth, the
dominant arthropod race at that time, the members of which resembled
Limulus, had solved the question; for, in addition to their external
chitinous covering, they had manufactured an internal cartilaginous
skeleton. Not only so, but that skeleton had arrived, both in structure
and position, exactly at the stage at which the vertebrate skeleton
starts.

What the precise steps are by which chitin-formation gives place to
chondrin-formation are not yet fully known, but Schmiedeberg has shown
that a substance, glycosamine, is derivable from both these skeletal
tissues, and he concludes his observations in the following words: "Thus,
by means of glycosamine, the bridge is formed which connects together the
chitin of the lower animals with the cartilage of the more highly
organized creations."

The evidence of the origin of the cartilaginous skeleton of the
vertebrate points directly to the origin of the vertebrate from the
Palæostraca, and is of so strong a character that, taken alone, it may
almost be considered as proof of such origin.

{148}CHAPTER IV

_THE EVIDENCE OF THE RESPIRATORY APPARATUS_

Branchiæ considered as internal branchial appendages.--Innervation of
branchial segments.--Cranial region older than spinal.--Three-root system
of cranial nerves, dorsal, lateral, ventral.--Explanation of van Wijhe's
segments.--Lateral mixed root is appendage-nerve of invertebrate.--The
branchial chamber of Ammocoetes.--The branchial unit, not a pouch but an
appendage.--The origin of the branchial musculature.--The branchial
circulation.--The branchial heart of the vertebrate.--Not homologous with
the systemic heart of the arthropod.--Its formation from two longitudinal
venous sinuses.--Summary.

The respiratory apparatus in all the terrestrial vertebrates is of the same kind--one single pair of lungs. These lungs originate as a diverticulum of the alimentary canal. On the other hand, the aquatic vertebrates breathe by means of a series of branchiæ, or gills, which are arranged segmentally, being supported by the segmental branchial cartilaginous bars, as already mentioned in the last chapter.

The transition from the gill-bearing to the lung-bearing vertebrates is most interesting, for it has been proved that the lungs are formed by the modification of the swim-bladder of fishes; and in a group of fishes, the Dipnoi, or lung-fishes, of which three representatives still exist on the earth, the mode of transition from the fish to the amphibian is plainly visible, for they possess both lungs and gills, and yet are not amphibians, but true fishes. But for the fortunate existence of Ceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa, it would have been impossible from the fossil remains to have asserted that any fish had ever existed which possessed at the same moment of time the two kinds of respiratory organs, although from our knowledge of the development of the amphibian we might have felt sure that such a transitional stage must have existed. Unfortunately, there is at present no likelihood of any corresponding transitional stage being discovered {149}living on the earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should simultaneously exist in a functional condition; still it seems to me that even if Ceratodus, Lepidosiren, and Protopterus had ceased to exist on the earth, yet the facts of comparative anatomy, together with our conception of evolution as portrayed in the theory of natural selection, would have forced us to conclude rightly that the amphibian stage in the evolution of the vertebrate phylum was preceded by fishes which possessed simultaneously lungs and gills.

In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found in Ammocoetes, was shown to correspond in a marvellous manner to the cartilaginous skeleton of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the consideration of the necessary consequences which logically follow upon the supposition that the branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of Ammocoetes.

INTERNAL BRANCHIAL APPENDAGES.

Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes are confined to the branchial region, their homology of necessity implies an homology of the two branchial regions, and leads directly to the conclusion that the branchiæ of the vertebrate were derived from the branchiæ of the arthropod, a conclusion which, according to the generally accepted view of the origin of the respiratory region in the vertebrate, is extremely difficult to accept; for the branchiæ of Limulus and of the Arthropoda in general are part of the mesosomatic appendages, while the branchiæ of vertebrates are derived from the anterior part of the alimentary canal. This conclusion, therefore, implies that the vertebrate has utilized in the formation of the anterior portion of its new alimentary canal the branchial appendages of the palæostracan ancestor.

{150}

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.]

Let us consider dispassionately whether such a suggestion is _a priori_ so impossible as it at first appears. One of the principles of evolution is that any change which is supposed to have taken place in the process of formation of one animal or group of animals from a lower group must be in harmony with changes which are known to have occurred in that lower group. On the assumption, therefore, that the vertebrate branchiæ represent the branchial portion of the arthropod mesosomatic appendages which have sunk in and so become internal, we ought to find that in members of this very group such inclusion of branchial appendages has taken place. This, indeed, is exactly what we do find, for in all the scorpion tribe, which is acknowledged to be closely related to Limulus, there are no external mesosomatic appendages, but in all cases these appendages have sunk into the body, have disappeared as such, and retained only the vital part of them--the branchiæ. In this way the so-called lung-books of the scorpion are formed, which are in all respects homologous with the branchiæ or gill-books of Limulus. Now, as already mentioned, the lords of creation in the palæostracan times were the sea-scorpions, which, as is seen in Fig. 62, resembled the land-scorpions of the present day in the entire absence of any external appendages on the segments of the mesosomatic region. As they lived in the sea, they must have breathed with gills, and those branchial appendages must have been internal, just as in the land-scorpions of the present time. Indeed, markings have been found on the internal side of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the present time no indication of gill-slits has been found, and we cannot say with certainty how these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion (_Palæophonus Hunteri_), closely resembling the modern scorpion, has been found, which, as Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it {151}breathed is unknown; it shows no signs of stigmata, such as exist in the scorpion of to-day.

Although we possess as yet no certain knowledge of the position of the gill-openings in these ancient scorpion-like forms, what we can say with certainty--and that is the important fact--is, that at the time when the vertebrates appeared, a very large number of the dominant arthropod race possessed internally-situated branchiæ, which had been directly derived from the branchiæ-bearing appendages of their Limulus-like kinsfolk.

This abolition of the branchiæ-bearing appendages as external organs of locomotion, with the retention of the important branchial portion of the appendage as internal branchiæ, is a very important suggestion in any discussion of the way vertebrates have arisen from arthropods; for, if the same principle is of universal application, it leads directly to the conclusion that whenever an appendage possesses an organ of vital importance to the animal, that organ will remain, even though the appendage as such completely vanishes. Thus, as will be shown later, special sense-organs such as the olfactory remain, though the animal no longer possesses antennæ; the important excretory organs, the coxal glands, and important respiratory organs, the branchiæ, are still present in the vertebrate, although the appendages to which they originally belonged have dwindled away, or, at all events, are no longer recognizable as arthropod appendages.

INNERVATION OF BRANCHIAL SEGMENTS.

Passing from _a priori_ considerations to actual facts, it is advisable to commence with the innervation of the branchial segments; for, seeing that the foundation of the whole of this comparative study of the vertebrate and the arthropod is based upon the similarity of the two central nervous systems, it follows that we must look in the first instance to the innervation of any organ or group of organs in order to find out their relationship in the two groups of animals.

The great characteristic of the vertebrate branchial organs is their segmental arrangement and their innervation by the vagus group of nerves, _i.e._ by the hindermost group of the cranial segmental nerves. These cranial nerves are divided by Gegenbaur into two great groups--an anterior group, the trigeminal, which supplies the muscles of mastication, and a posterior group, the vagus, which is essentially {152}respiratory in function. Of these two groups, I will consider the latter group first.

In Limulus the great characteristic of the branchial region is its pronounced segmental arrangement, each pair of branchial appendages belonging to a separate segment. This group of segments forms the mesosoma, and these branchial appendages are the mesosomatic appendages. Anterior to them are the segments of the prosoma, which bear the prosomatic or locomotor appendages. The latter are provided at their base with gnathites or masticating apparatus, so that the prosomatic group of nerves, like the trigeminal group in the vertebrate, comprises essentially the nerves subserving the important function of mastication. As already pointed out, the brain-region of the vertebrate is comparable to the supra-oesophageal and infra-oesophageal ganglia of the invertebrate, and it has been shown (p. 54) how, by a process of concentration and cephalization, the foremost region of the infra-oesophageal ganglia becomes the prosomatic region, and is directly comparable to the trigeminal region in the vertebrate; while the hindermost region is formed from the concentration of the mesosomatic ganglia, and is directly comparable to the medulla oblongata, _i.e._ to the vagus region of the vertebrate brain.

As far, then, as concerns the centres of origin of these two groups of nerves and their exits from the central nervous system, they are markedly homologous in the two groups of animals.

COMPARISON OF THE CRANIAL AND SPINAL SEGMENTAL NERVES.

It has often been held that the arrangements of the vertebrate nervous system differ from those of other segmented animals in one important particular. The characteristic of the vertebrate is the origin of every segmental nerve from two roots, of which one contains the efferent fibres, while the other possesses a sensory ganglion, and contains only afferent fibres. This arrangement, which is found along the whole spinal cord of all vertebrates, is not found in the segmental nerves of the invertebrates; and as it is supposed that the simpler arrangement of the spinal cord was the primitive arrangement from which the vertebrate central nervous system was built up, it is often concluded that the animal from which the vertebrate arose must have possessed a series of nerve-segments, from each of which there arose bilaterally ventral (efferent) and dorsal (afferent) roots.

{153}Now, the striking fact of the vertebrate segmental nerves consists in this, that, as far as their structure and the tissues which they innervate are concerned, the cranial segmental nerves are built up on the same plan as the spinal; but as far as concerns their exit from the central nervous system they are markedly different. A large amount of ingenuity, it is true, has been spent in the endeavour to force the cranial nerves into a series of segmental nerves, which arise in the same way as the spinal by two roots, of which the ventral series ought to be efferent and the dorsal series afferent, but without success. We must, therefore, consider the arrangement of the cranial segmental nerves by itself, separately from that of the spinal nerves, and the problem of the origin of the vertebrate segmental nerves admits of two solutions--either the cranial arrangement has arisen from a modification of the spinal, or the spinal from a simplification of the cranial. The first solution implies that the spinal cord arrangement is older than the cranial, the second that the cranial is the oldest.

In my opinion, the evidence of the greater antiquity of the cranial region is overwhelming.

The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial region, for we see how, quite early in the development, the head is folded off, and the organs in that region thereby completed at a time when the spinal region is only at an early stage of development. We see how the first of the trunk somites is formed just posteriorly to the head region, and then more and more somites are formed by the addition of fresh segments posteriorly to the one first formed. We see how, in Ammocoetes, the first formed parts of the skeleton are the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of more and more spinal segments, organs, such as the heart, which were originally in the head, travel down, and the vagus and lateral-line nerves reach their ultimate destination. Again, we see that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the only shifting being in the spino-occipital region, in fact, at the junction of the cranial and spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in different {154}groups of animals, but that even in the same animal great variations are found, especially in the manner of formation of the limb-plexuses. Such marked meristic variation in the spinal nerves, in contrast to the fixed character of the cranial nerves, certainly points to a more recent formation of the former nerves.

Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog, there is evidence to show that the embryo is derived from two definite centres of growth: the first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the anterior end of the embryo; the second, one which gives rise to the growth in length of the embryo. This secondary area of proliferation coincides with the area of the primitive streak, and he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the anterior end of the primitive streak, and subsequently is found at the level of the most anterior pair of somites.

He then goes on to say--

"From these specimens it seems clear that all those parts in front of the first pair of mesoblastic somites--that is to say, the heart, the brain and medulla oblongata, the olfactory, optic, auditory organs and foregut--are developed from that portion of the unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest of the embryo is formed by the activity of the primitive streak area."

In other words, the secondary area of growth, _i.e._ the primitive streak area, includes the whole of the spinal cord region, while the older primary centre of growth is coincident with the cranial region.

In searching, then, for the origin of the segmental nerves, we must consider the type on which the cranial nerves are arranged rather than that of the spinal nerves.

The first striking fact occurs at the spino-occipital region, where the spinal cord merges into the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to three distinct roots, not two--a dorsal root, a ventral root, and a lateral root. This third root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system {155}continues as the roots of the glossopharyngeal nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles Bell[1] pointed out in 1830, a system of nerves concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication, deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary body-muscles concerned with locomotion and equilibration. Also the centres from which these motor nerves arise are well defined, and form cell-masses in the central nervous system, quite separate from those which give origin to somatic muscles.

This original idea of Charles Bell, after having been ignored for so long a time, is now seen to be a very right one, and it is an extraordinary thing that his enunciation of the dual nature of the spinal roots, which was, to his mind, of subordinate importance, should so entirely have overshadowed his suggestion, that in addition to the dorsal and ventral roots, a lateral system of nerves existed, which were not exclusively sensory or exclusively motor, but formed a separate system of respiratory nerves.

Further, anatomists divide the striated muscles of the body into two great natural groups, characterized by a difference of origin and largely by a difference of appearance. The one set is concerned with the movements of internal organs, and is called visceral, the other is derived from the longitudinal sheet of musculature which forms the myotomes of the fish, and has been called parietal or somatic. The motor nerves of these two sets of muscles correspond with the lateral or respiratory and ventral roots respectively.

Finally, it has been shown that the segments of which a vertebrate is composed are recognizable in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a dorsal and ventral series of commencing muscular segments. Of these the anterior segments of the dorsal series give origin to the striated muscles of the eye which are innervated by the IIIrd (oculomotor), IVth (trochlearis), and VIth (abducens) nerves, while the posterior segments give origin to the {156}muscles from the cranium to the shoulder-girdle, innervated by the XIIth (hypoglossal) nerve. The ventral series of segments give origin to the musculature supplied by the trigeminal, facial, glossopharyngeal, and vagus nerves.

Also, the afferent or sensory nerves of the skin over the whole of this head-region are supplied by the trigeminal nerve, while the afferent nerves to the visceral surfaces are supplied by the vagus, glossopharyngeal and facial nerves.

In van Wijhe's original paper he arranged the segments belonging to the cranial nerves in the following table:--

---------+---------------------------------
Segments.| Ventral nerve-roots and muscles
| derived from myotomes.
---------+---------+-----------------------
1 | III. |M. rectus superior,
| | m. rectus
| | internus, m.
| | rectus inferior,
| | m. obliquus inferior
2 | IV. |M. obliquus
| | superior
3 | VI. |M. rectus externus
4 | -- | --
5 | -- | --
6 | -- | --
7 | XII. |} Muscles from
8 | XII. |} cranium to
9 | XII. |}shoulder-girdle
---------+---------+-----------------------

---------+------------------+--------------------------------
Segments.| Visceral clefts.| Dorsal nerve-roots and muscles.
| |
---------+------------------+--------------+-----------------
1 | | V. N. op- |
| | thalmicus |
| | profundus |
2 | | V. | Masticating
| 1st Mandibular | | muscles.
3 | | VII._1 |{Facial muscles
| | |{(VIII. is dorsal
| 2nd {Hyoid_1 | VII._2 |{branch of VII.)
4 | {Hyoid_2 | |
5 |3rd 1st Branchial | IX. |
6 |4th 2nd " | X._1 |
7 |{5th 3rd " | X._2 |Branchial and
8 |{6th 4th " | X._3} |visceral muscles.
9 |{7th 5th " | X._4 |
---------+------------------+--------------+-----------------

As is seen in the table, van Wijhe attempts to arrange the cranial segmental nerves into dorsal and ventral roots, in accordance with the arrangement in the spinal region. In order to do this he calls the Vth, VIIth, IXth, and Xth nerves dorsal roots, although they are not purely sensory nerves, but contain motor fibres as well.

It is not accidental that he should have picked out for his dorsal roots the very nerves which form Charles Bell's lateral series of roots, inasmuch as this system of lateral roots, apart from dorsal and ventral roots, really is, as Charles Bell thought, an important separate system, dependent upon a separate segmentation in the embryo of the musculature supplied by these roots. This segmentation may receive the name of _visceral_ or _splanchnic_ in contradistinction to _somatic_, since all the muscles without exception belong to the visceral group of striated muscles.

{157}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, are _doubly_ innervated 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 by _single_ roots, 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-roots {158}in 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 Ammocoetes, 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.