Life History of the Lamprey—not a Degenerate Animal.
The striking peculiarity of the lamprey is its life-history. It lives in fresh water, spending a large portion of its life in the mud during the period of its larval existence: then comes a somewhat sudden transformation-stage, characterized, as in the lepidopterous larva, by a process of histolysis, by which many of the larval tissues are destroyed and new ones formed, with the result that the larval lamprey, or Ammocœtes, is transformed into the adult lamprey, or Petromyzon. This transformation takes place in August, at all events in the neighbourhood of Cambridge, and later in the year the transformed lamprey migrates to the sea, grows in size and maturity, and returns to the river the following spring up to its spawning beds, where it spawns and forthwith dies. How long it lives in the Ammocœtes stage is unknown; I myself have kept some without transformation for four years, and probably they live in the rivers longer than that before they change from their larval state. It is absolutely certain that very much the longest part of the animal's life is spent in the larval stage, and that with the maturity of the sexual organs and the production of the fertilized ova the life of the individual ends.
Now, the striking point of this transformation is that it produces an animal more nearly comparable with higher vertebrates than is the larval form; in other words, the transformation from larva to adult is in the direction of upward progress, not of degeneration. It is, therefore, inaccurate to speak of the adult lamprey as degenerate from a higher race of fishes represented by its larval form—Ammocœtes. Its transformation does not resemble that of the tunicates, but rather that of the frog, so that, just as in the case of the tadpole, the peculiarities of its larval form may be expected to afford valuable indications of its immediate ancestry. The very peculiarities to which attention must especially be paid are those discarded at transformation, and, as will be seen, these are essentially characteristic of the invertebrate and are not found in the higher vertebrates. In fact, the transformation of the lamprey from the Ammocœtes to the Petromyzon stage may be described as the casting off of many of its ancestral invertebrate characters and the putting on of the characteristics of the vertebrate type. It is this double individuality of the lamprey, together with its long-continued existence in the larval form, which makes Ammocœtes morevaluable than any other living vertebrate for the study of the stock from which vertebrates sprang.
Many authorities hold the view that the lamprey, like Amphioxus, must be looked upon as degenerate, and therefore as no more suitable for the investigation of the problem of vertebrate ancestry than is Amphioxus itself. This charge of degeneracy is based on the statement that the lamprey is a parasite, and that the eyes in Ammocœtes are under the skin. The whole supposition of the degeneracy of the Cyclostomata arose because of the prevailing belief of the time that the earliest fishes were elasmobranchs, and therefore gnathostomatous. From such gnathostomatous fishes the cyclostomes were supposed to have descended, having lost their jaws and become suctorial in habit in consequence of their parasitism.
The charge of parasitism is brought against the lamprey because it is said to suck on to fishes and so obtain nutriment. It is, however, undoubtedly a free-swimming fish; and when we see it coming up the rivers in thousands to reach the spawning-beds, and sucking on to the stones on the way in order to anchor itself against the current, or holding on tightly during the actual process of spawning, it does not seem justifiable to base a charge of degeneration upon a parasitic habit, when such so-called habit simply consists in holding on to its prey until its desires are satisfied. If, of course, its suctorial mouth had arisen from an ancestral gnathostomatous mouth, then the argument would have more force.
Dohrn, however, gives absolutely no evidence of a formergnathostomatouscondition either in Petromyzon or, in its larval state, Ammocœtes. He simply assumes that the Cyclostomata are degenerated fishes and then proceeds to point out the rudiments of skeleton, etc., which they still possess. Every point that Dohrn makes can be turned round; and, with more probability, it can be argued that the various structures are the commencement of the skeletal and other structures in the higher fishes, and not their degenerated remnants. Compare the life-history of the lamprey and of the tunicate. In the latter case we look upon the animal as a degenerate vertebrate, because the larval stage alone shows vertebrate characteristics; when transformation has taken place, and the adult form is reached, the vertebrate characteristics have vanished, and the animal, instead of reaching a higher grade, has sunk lower in the scale, the central nervous system especially having lost allresemblance to that of the vertebrate. In the former case a transformation also takes place, a marvellous transformation, characterized by two most striking facts. On the one hand, the resulting animal is more like a higher vertebrate, for, by the formation of new cartilages, its cranial skeleton is now comparable with that of the higher forms, and the beginnings of the spinal vertebræ appear; by the increased formation of nervous material, its brain increases in size and complexity, so as to compare more closely with higher vertebrate brains; its eyes become functional, and its branchiæ are so modified, simultaneously with the formation of the new alimentary canal in the cranial region, that they now surround branchial pouches which are directly comparable to those of higher vertebrates. On the other hand, the transformation process is equally characterized by the throwing off of tissues and organs, one and all of which are comparable in structure and function with corresponding structures in the Arthropoda—the thyroid of the Ammocœtes, the tentacles, the muco-cartilage, the tubular muscles, all these structures, so striking in the Ammocœtes stage, are got rid of at transformation. Here is the true clue. Here, in the throwing off of invertebrate characters, and the taking on of a higher vertebrate form, especially a higher brain, not a lower one, Petromyzon proclaims as clearly as is possible that it is not a degenerate elasmobranch, but that it has arisen from Ammocœtes-like ancestors, even though Myxine, Amphioxus, and the tunicates be all stages on the downward grade from those same Ammocœtes-like ancestors.
As to the eyes, they are functional in the adult form and as serviceable as in any fish. There is no sign of degeneracy; it is only possible to speak of a retarded development which lasts through the larval stage.
Comparison of Brain of Ammocœtes with that of an Arthropod.
Seeing that the steady progress of the development of the central nervous system is the most important factor in the evolution of animals, it follows that of all organs of the body, the central nervous system must be most easily comparable with that of the supposed ancestor. I will, therefore, start by comparing the brain of Ammocœtes with that of arthropods, especially of Limulus and of the scorpion-group.
The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-œsophageal portion of the invertebrate brain in so far that in both cases here is the seat of the will. Voluntary action is as impossible to the arthropod deprived of its supra-œsophageal ganglia as to the vertebrate deprived of its cerebrum. It corresponds, also, in that from it arise the nerves of sight and smell and no other nerves; this is also the case with the supra-œsophageal ganglia, for from a portion of these ganglia arise the nerves to the eyes and the nerves to the first antennæ, of which the latter are olfactory in function. Thus, in the accompanying figure, taken from Bellonci, it is seen that the supra-œsophageal ganglia consist of a superior segment corresponding to the cerebrum, a middle segment from which arise the nerves to the lateral eyes and to the olfactory antennæ, corresponding to the basal ganglia of the brain and the optic lobes, and, according to Bellonci, of an inferior segment from which arise the nerves to the second pair of antennæ. This last segment is not supra-œsophageal in position, but is situated on the œsophageal commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in reality the first ganglion of the infra-œsophageal series, and not to belong to the supra-œsophageal group.
Fig. 27.—The Brain ofSphæroma serratum. (AfterBellonci.)Ant. I.andAnt. II., nerves to 1st and 2nd antennæ.f.br.r., terminal fibre layer of retina;Op. g. I., first optic ganglion;Op. g. II., second optic ganglion;O.n., optic nerve-fibres forming an optic chiasma.
Fig. 27.—The Brain ofSphæroma serratum. (AfterBellonci.)Ant. I.andAnt. II., nerves to 1st and 2nd antennæ.f.br.r., terminal fibre layer of retina;Op. g. I., first optic ganglion;Op. g. II., second optic ganglion;O.n., optic nerve-fibres forming an optic chiasma.
Fig. 27.—The Brain ofSphæroma serratum. (AfterBellonci.)
Ant. I.andAnt. II., nerves to 1st and 2nd antennæ.f.br.r., terminal fibre layer of retina;Op. g. I., first optic ganglion;Op. g. II., second optic ganglion;O.n., optic nerve-fibres forming an optic chiasma.
Further, in Limulus, in the scorpion-group, and in all the extinctEurypteridæ—in fact, in the Palæostraca generally—there are two median eyes in addition to the lateral eyes, which were innervated from these ganglia.
In Ammocœtes, then, if the supra-infundibular portion of the brain really corresponds to the supra-œsophageal of the palæostracan group, we ought to find, as indeed is the case, an optic apparatus consisting of two lateral eyes and two median eyes, innervated from the supra-infundibular brain-mass, and an olfactory apparatus built up on the same lines as in the scorpion-group, also innervated from this region. If, in addition, it be found that those two median eyes are degenerate eyes of the same type as the median eyes of Limulus and the scorpion-group, then the evidence is so strong as to amount to a proof of the correctness of the theory. This evidence is precisely what has been obtained in recent years, for the vertebrate did possess two median eyes in addition to the two lateral ones, and these two median eyes are degenerate eyes of the type found in the median eyes of arthropods and are not of the vertebrate type. Moreover, as ought also to be the case, they are most evident, and one of the pair is most nearly functional in the lowest perfect vertebrate, Ammocœtes.
Of all the discoveries made in recent years, the discovery that the pineal gland of the vertebrate brain was originally a pair of median eyes is by far the most important clue to the ancestry of the vertebrate, for not only do they correspond exactly in position with the median eyes of the invertebrates, but, being already degenerate and functionless in the lowest vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct clue possible to the nature of the pre-vertebrate stage. It is especially significant that in Limulus they are already partially degenerated. What, then, ought to be the structure and relation to the brain of the median and lateral eyes of the vertebrate if they originated from the corresponding organs of some one or other member of the palæostracan group?
This question will form the subject of the next chapter.
Summary.
The object of this book is to attempt to find out from what group of invertebrates the vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which evolution takes place.A review of the animal kingdom as a whole leads to the conclusion that the upward development of animals from an original cœlenterate stock, in which the central nervous system consists of a ring of nervous material surrounding the mouth, has led, in consequence of the elaboration of the central nervous system, to a general plan among the higher groups of invertebrates in the topographical arrangement of the important organs. The mouth is situated ventrally, and leads by means of the œsophagus into an alimentary canal which is situated dorsally to the central nervous system. Thus the œsophagus pierces the central nervous system and divides it into two parts, the supra-œsophageal ganglia and the infra-œsophageal ganglia. This is an almost universal plan among invertebrates, but apparently does not hold for vertebrates, for in them the central nervous system is always situated dorsally and the alimentary canal ventrally, and there is no piercing of the central nervous system by an œsophagus.Yet a remarkable resemblance exists between the central nervous system of the vertebrate and that of the higher invertebrates, of so striking a character as to compel one school of anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the infundibulum, passes to the ventral surface of the brain in the very position where the œsophagus would have been if that brain had belonged to an annelid or an arthropod. This school of anatomists therefore concluded that this infundibular tube represented the original invertebrate œsophagus which had become closed and no longer opened into the alimentary canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal of the vertebrate is ventral to the central nervous system, and not dorsal, as in the invertebrate, it follows that the remains of the original invertebrate mouth into which the œsophagus (in the vertebrate the infundibular tube) must have opened must be searched for on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the front of the other.The difficulties in the way of accepting such reversal of surfaces have proved insuperable, and another school has arisen which suggests that evolution has throughout proceeded on two lines, the one forming groups of animals in which the central nervous system is pierced by the food-channel and the gut therefore lies dorsally to it, the other in which the central nervous system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the highest products of the evolution have become markedly segmented animals, in the former, annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is based is the existence of low forms of animals, known as theEnteropneusta, the best known example of which is calledBalanoglossus; they are looked upon as aberrant annelid forms by many observers.This theory does not attempt to explain the peculiarities of the tube of the vertebrate central nervous system, or to account for the extraordinary resemblance between the structure and arrangement of the central nervous systems of vertebrates and of the highest invertebrate group.Neither of these theories is satisfactory or has secured universal acceptance. The problem must be considered entirely anew. What are the guiding principles in this investigation?The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form—man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration.If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group—the Arthropoda—for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group.The evidence of geology points to the same conclusion, for the evidence of the rocks shows that before the highest mammal—man—appeared, the dominant race was the mammalian quadruped, from whom the highest mammal of all—man—sprung; then comes, in Mesozoic times, the age of reptiles which were dominant when the mammal arose from them. Preceding this era we find in Carboniferous times that the amphibian was dominant, and from them the next higher group—the reptiles—arose. Below the Carboniferous come the Devonian strata with their evidence of the dominance of the fish, from whom the amphibian was directly evolved. The evidence is so clear that each succeeding higher form of vertebrate arose from the highest stage reached at the time, as to compel one to the conclusion that the fishes arose from the race which was dominant at the time when the fishes first appeared. This brings us to the Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions, king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite age, and the whole period, from the first appearance of the trilobite to the time of dwindling away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to include both trilobites and the higher scorpion and king-crab forms evolved from them. The evidence of geology then points directly and strongly to the origin of vertebrates from the Palæostraca—arthropod forms which were not crustacean and not arachnid, but gave origin both to the modern-day crustaceans and arachnids. The history of the rocks further shows that these ancient fishes, when they first appeared, resembled in a remarkable manner members of the palæostracan group, so that again and again palæontologists have found great difficulty in determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the earth one member of this remarkable group—the Limulus, or King-Crab. On the vertebrate side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, because it was formerly considered to be a separate species and received the name of Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral problems, so that it is most fortunate for the solution of the ancestry of vertebrates that Limulus on the one side and Ammocœtes on the other areavailable for thorough investigation and comparison. There are no trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the trilobite organization among living crustaceans.So strongly do all these different lines of argument point to the origin of vertebrates from arthropods as to make it imperative to reconsider the position of that school of anatomists who derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn the animal over, but re-consider the position, the infundibular tube of the vertebrate still representing the œsophagus of the invertebrate, the cerebral hemispheres and basal ganglia the supra-œsophageal ganglia, thecrura cerebrithe œsophageal commissures, and the infra-infundibular part of the brain the infra-œsophageal ganglia. It is immediately apparent that just as the invertebrate œsophagus leads into the large cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the third ventricle, which, together with the other ventricles, forms in the embryo a large anterior dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the animal unreversed, and immediately the whole mystery of the tubular nature of the central nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of the vertebrate.The manner in which the nervous material has invaded the walls of the tube is clearly shown both by the study of the comparative anatomy of the central nervous system in the vertebrate and also by its development in the embryo.This theory implies that the vertebrate alimentary canal is a new formation necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the general plan of the evolution of the invertebrate from the cœlenterate involved the piercing of the anterior portion of the central nervous system by the œsophagus, while, at the same time, upward progress meant brain-development; brain-development meant concentration of nervous matter at the anterior end of the animal, with the result that in the highest scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube that nothing but fluid pabulum can pass through into the stomach; the whole group have become blood-suckers. These kinds of animals—the sea-scorpions—were the dominant race when the vertebrates first appeared: here in the natural competition among members of the dominant race the difficulty must have become acute. Further upward evolution demanded a larger and larger brain with the ensuing consequence of a greater and greater difficulty of food-supply. Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still beencompelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. This, then, is the working hypothesis upon which this book is written. If this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it follows that we have various fixed points of comparison in the central nervous systems of the two groups of animals from which to search for further clues. It further follows that from such starting-point every organ of importance in the body of the arthropod ought to be visible in the corresponding position in the vertebrate, either as a functional or rudimentary organ. The subsequent chapters will deal with this detailed comparison of organs in the arthropod and vertebrate respectively.
The object of this book is to attempt to find out from what group of invertebrates the vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which evolution takes place.
A review of the animal kingdom as a whole leads to the conclusion that the upward development of animals from an original cœlenterate stock, in which the central nervous system consists of a ring of nervous material surrounding the mouth, has led, in consequence of the elaboration of the central nervous system, to a general plan among the higher groups of invertebrates in the topographical arrangement of the important organs. The mouth is situated ventrally, and leads by means of the œsophagus into an alimentary canal which is situated dorsally to the central nervous system. Thus the œsophagus pierces the central nervous system and divides it into two parts, the supra-œsophageal ganglia and the infra-œsophageal ganglia. This is an almost universal plan among invertebrates, but apparently does not hold for vertebrates, for in them the central nervous system is always situated dorsally and the alimentary canal ventrally, and there is no piercing of the central nervous system by an œsophagus.
Yet a remarkable resemblance exists between the central nervous system of the vertebrate and that of the higher invertebrates, of so striking a character as to compel one school of anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the infundibulum, passes to the ventral surface of the brain in the very position where the œsophagus would have been if that brain had belonged to an annelid or an arthropod. This school of anatomists therefore concluded that this infundibular tube represented the original invertebrate œsophagus which had become closed and no longer opened into the alimentary canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal of the vertebrate is ventral to the central nervous system, and not dorsal, as in the invertebrate, it follows that the remains of the original invertebrate mouth into which the œsophagus (in the vertebrate the infundibular tube) must have opened must be searched for on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the front of the other.
The difficulties in the way of accepting such reversal of surfaces have proved insuperable, and another school has arisen which suggests that evolution has throughout proceeded on two lines, the one forming groups of animals in which the central nervous system is pierced by the food-channel and the gut therefore lies dorsally to it, the other in which the central nervous system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the highest products of the evolution have become markedly segmented animals, in the former, annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is based is the existence of low forms of animals, known as theEnteropneusta, the best known example of which is calledBalanoglossus; they are looked upon as aberrant annelid forms by many observers.
This theory does not attempt to explain the peculiarities of the tube of the vertebrate central nervous system, or to account for the extraordinary resemblance between the structure and arrangement of the central nervous systems of vertebrates and of the highest invertebrate group.
Neither of these theories is satisfactory or has secured universal acceptance. The problem must be considered entirely anew. What are the guiding principles in this investigation?
The evolution of animal life on this earth can clearly, on the whole, be described as a process of upward progress culminating in the highest form—man; but it must always be remembered that whole groups of animals such as the Tunicata have been able to survive owing to a reverse process of degeneration.
If there is one organ more than another which increases in complexity as evolution proceeds, which is the most essential organ for upward progress, surely it is the central nervous system, especially that portion of it called the brain. This consideration points directly to the origin of vertebrates from the most highly organized invertebrate group—the Arthropoda—for among all the groups of animals living on the earth in the present day they alone possess a central nervous system closely comparable with that of vertebrates. Not only has an upward progress taken place in animals as a whole, but also in the tissues themselves a similar evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely those of the arthropod than of any other invertebrate group.
The evidence of geology points to the same conclusion, for the evidence of the rocks shows that before the highest mammal—man—appeared, the dominant race was the mammalian quadruped, from whom the highest mammal of all—man—sprung; then comes, in Mesozoic times, the age of reptiles which were dominant when the mammal arose from them. Preceding this era we find in Carboniferous times that the amphibian was dominant, and from them the next higher group—the reptiles—arose. Below the Carboniferous come the Devonian strata with their evidence of the dominance of the fish, from whom the amphibian was directly evolved. The evidence is so clear that each succeeding higher form of vertebrate arose from the highest stage reached at the time, as to compel one to the conclusion that the fishes arose from the race which was dominant at the time when the fishes first appeared. This brings us to the Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions, king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite age, and the whole period, from the first appearance of the trilobite to the time of dwindling away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to include both trilobites and the higher scorpion and king-crab forms evolved from them. The evidence of geology then points directly and strongly to the origin of vertebrates from the Palæostraca—arthropod forms which were not crustacean and not arachnid, but gave origin both to the modern-day crustaceans and arachnids. The history of the rocks further shows that these ancient fishes, when they first appeared, resembled in a remarkable manner members of the palæostracan group, so that again and again palæontologists have found great difficulty in determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the earth one member of this remarkable group—the Limulus, or King-Crab. On the vertebrate side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, because it was formerly considered to be a separate species and received the name of Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral problems, so that it is most fortunate for the solution of the ancestry of vertebrates that Limulus on the one side and Ammocœtes on the other areavailable for thorough investigation and comparison. There are no trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the trilobite organization among living crustaceans.
So strongly do all these different lines of argument point to the origin of vertebrates from arthropods as to make it imperative to reconsider the position of that school of anatomists who derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn the animal over, but re-consider the position, the infundibular tube of the vertebrate still representing the œsophagus of the invertebrate, the cerebral hemispheres and basal ganglia the supra-œsophageal ganglia, thecrura cerebrithe œsophageal commissures, and the infra-infundibular part of the brain the infra-œsophageal ganglia. It is immediately apparent that just as the invertebrate œsophagus leads into the large cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the third ventricle, which, together with the other ventricles, forms in the embryo a large anterior dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the animal unreversed, and immediately the whole mystery of the tubular nature of the central nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of the vertebrate.
The manner in which the nervous material has invaded the walls of the tube is clearly shown both by the study of the comparative anatomy of the central nervous system in the vertebrate and also by its development in the embryo.
This theory implies that the vertebrate alimentary canal is a new formation necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the general plan of the evolution of the invertebrate from the cœlenterate involved the piercing of the anterior portion of the central nervous system by the œsophagus, while, at the same time, upward progress meant brain-development; brain-development meant concentration of nervous matter at the anterior end of the animal, with the result that in the highest scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube that nothing but fluid pabulum can pass through into the stomach; the whole group have become blood-suckers. These kinds of animals—the sea-scorpions—were the dominant race when the vertebrates first appeared: here in the natural competition among members of the dominant race the difficulty must have become acute. Further upward evolution demanded a larger and larger brain with the ensuing consequence of a greater and greater difficulty of food-supply. Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still beencompelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. This, then, is the working hypothesis upon which this book is written. If this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it follows that we have various fixed points of comparison in the central nervous systems of the two groups of animals from which to search for further clues. It further follows that from such starting-point every organ of importance in the body of the arthropod ought to be visible in the corresponding position in the vertebrate, either as a functional or rudimentary organ. The subsequent chapters will deal with this detailed comparison of organs in the arthropod and vertebrate respectively.
CHAPTER II
THE EVIDENCE OF THE ORGANS OF VISION
Different kinds of eye.—Simple and compound retinas.—Upright and inverted retinas.—Median eyes.—Median or pineal eyes of Ammocœtes and their optic ganglia.—Comparison with other median eyes.—Lateral eyes of vertebrates compared with lateral eyes of crustaceans.—Peculiarities of the lateral eye of the lamprey.—Meaning of the optic diverticula.—Evolution of vertebrate eyes.—Summary.
Different kinds of eye.—Simple and compound retinas.—Upright and inverted retinas.—Median eyes.—Median or pineal eyes of Ammocœtes and their optic ganglia.—Comparison with other median eyes.—Lateral eyes of vertebrates compared with lateral eyes of crustaceans.—Peculiarities of the lateral eye of the lamprey.—Meaning of the optic diverticula.—Evolution of vertebrate eyes.—Summary.
The Different Kinds of Eye.
In all animals the eyes are composed of two parts. 1. A set of special sensory cells called the retina. 2. A dioptric apparatus for the purpose of forming an image on the sensory cells. The simplest eye is formed from a modified patch of the surface-epithelium; certain of the hypodermal cells, as they are called, elongate, and their cuticular surface becomes bulged to form a simple lens. These elongated cells form the retinal cells, and are connected with the central nervous system by nerve-fibres which constitute an optic nerve; the cells themselves may contain pigment.
The more complicated eyes are modifications of this type for the purpose of making both the retina and the dioptric apparatus more perfect. According to a very prevalent view, these modifications have been brought about by invaginations of the surface-epithelium. Thus if ABCD (Fig.28) represents a portion of the surface-epithelium, the chitinous cuticle being represented by the dark line, with the hypodermal cells beneath, and if the part C is modified to form an optic sense-plate, then an invagination occurring between A and B will throw the retinal sense-cells with the optic nerve further from the surface, and the layers B and A between the retina and the source of light will be available for the formation of the dioptric apparatus. The lens is now formed from the cuticular surface of A, and thehypodermal cells of A elongate to form the layer known by the name of corneagen, or vitreogen, the cells of B remaining small and forming the pre-retinal layer of cells. The large optic nerve end-cells of the retinal layer, C, take up the position shown in the figure, and their cuticular surface becomes modified to form rods of varying shape called rhabdites, which are attached to the retinal cells. Frequently the rhabdites of neighbouring cells form definite groups, each group being called a rhabdome. Whatever shape they take it is invariably found that these little rods (bacilli), or rhabdites, are modifications of the cuticular surface of the cells which form the retinal layer. Also, as must necessarily be the case from the method of formation, the optic nerve arises from the nuclear end of the retinal cells, never from the bacillary end. As in the case first mentioned, so in this case, the light strikes direct upon the bacillary end of the retinal cells; such eyes, therefore, are eyes with anupright retina.
Fig. 28.—Diagram of Formation of an Upright Simple Retina.
Fig. 28.—Diagram of Formation of an Upright Simple Retina.
Fig. 28.—Diagram of Formation of an Upright Simple Retina.
It may happen that the part invaginated is the optic sense-plate itself, as would be the case if in the former figure, instead of C, the part B was modified to form a sense-plate. This will give rise to an eye of a character different from the former (Fig.29). The optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as is seen, the light passes through the retinal layer to thetapetum. The cuticular surface of the retinal cells forming the rods or bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal layer.
Such an eye, in contradistinction to the former type, is an eye with aninverted retina; but still the same law holds as in the former case—the optic nerve-fibres enter at the nuclear ends of the cells, and the rods are formed from the cuticular surface.
In these eyes the pigmented tapetal layer is believed to act as a looking-glass; the dioptric apparatus throws the image on to its shiny surface, from whence it is reflected directly on to the rods, which are in close contact with the tapetum. A similar process has been suggested in the case of the mammalian lateral eye, with its inverted retina. Johnson describes the post-retinal pigmented layer as being frequently coloured and shiny, and imagines that it reflects the image directly back on to the rods.
Fig. 29.—Diagram of Formation of an Inverted Simple Retina.The arrow shows the direction of the source of light in this as in the preceding figure. In both figures the cuticular rhabdites are represented by thick black lines.
Fig. 29.—Diagram of Formation of an Inverted Simple Retina.The arrow shows the direction of the source of light in this as in the preceding figure. In both figures the cuticular rhabdites are represented by thick black lines.
Fig. 29.—Diagram of Formation of an Inverted Simple Retina.
The arrow shows the direction of the source of light in this as in the preceding figure. In both figures the cuticular rhabdites are represented by thick black lines.
Thus we see that eyes can be placed in different categories,e.g.those with an upright retina and those with an inverted retina; also, according to the presence or absence of a tapetum, eyes have been grouped as tapetal or non-tapetal. All the eyes considered so far are called simple eyes, or ocelli; and a number of ocelli may becontiguous though separate, as in the lateral eyes of the scorpion. They may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a very large number of cases, retain each its own dioptric apparatus, and therefore the external appearance of the compound eye represents not a single lens, but a large number of facets, as is seen in the eyes of insects. Owing to these differences, eyes have been divided into simple and compound, and into facetted and non-facetted.
Yet another complication occurs in the formation of eyes, which is, perhaps, the most important of all: the retinal portion of the eye, instead of consisting of simple retinal cells, with their accompanying rhabdites, may include within itself a portion of the central nervous system.
The rationale of such a formation is as follows: The external covering of the body is formed by a layer of external epithelial cells—the ectodermal cell-layer—and an underlying neural layer, of which the latter gives origin to the central nervous system. As development proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm the sensory nerves of the skin. That part of the neural layer which underlies the optic plate forms the optic ganglion, and when the central nervous system leaves the surface to take up its deeper position, the strand of nerve-fibres known as the optic nerve, is left connecting it with the retinal cells as seen in Figs. 28, 29. It may, however, happen that part of the optic ganglion remains at the surface, in close connection with the retinal end-cells, when the rest of the central nervous system sinks inwards. The retina of such an eye is composed of the combined optic ganglion and retinal end-cells; the strand of nerve-fibres which is left as the connection between it and the rest of the brain, which is also called the optic nerve, is not a true peripheral nerve, as in the first case, but rather a tract of fibres connecting two parts of the brain, of which one has remained at the periphery. Such a retina, in contradistinction to the first kind, may be called acompound retina.
The optic ganglion, as seen in eyes with a simple retina, consists of a cortical layer of small, round nerve-cells, and an internal medulla of fine nerve-fibres, which form a thick network known as 'Punctsubstanz,' or in modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from other parts of the brain connect them with the optic ganglion.
At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous system is considered to be composed of a number of separate nerve-units, called neurones, each neurone consisting of a nerve-cell with its various processes; one of these—the neuraxon—constitutes the nerve-fibre belonging to that nerve-cell, the other processes—the dendrites—establish communication with other neurones. The place where these processes come together is called a synapse, and the tangle of fine fibres formed at a number of synapses forms the 'neuropil.'
Fig. 30.—Diagram of Formation of an Upright Compound Retina.ABCD, as in Fig.28.Op. g. I.andOp. g. II., two optic ganglia which combine to form the retinal ganglion,Rt. g.
Fig. 30.—Diagram of Formation of an Upright Compound Retina.ABCD, as in Fig.28.Op. g. I.andOp. g. II., two optic ganglia which combine to form the retinal ganglion,Rt. g.
Fig. 30.—Diagram of Formation of an Upright Compound Retina.
ABCD, as in Fig.28.Op. g. I.andOp. g. II., two optic ganglia which combine to form the retinal ganglion,Rt. g.
When, therefore, a compound retina is formed by the amalgamation of the ectodermal part—the retinal cells proper—with the neurodermic part—to which the name 'retinal ganglion' may be given,—such a retina consists of neuropil substance and nerve-cells, as well as the retinal end-cells. In all such compound retinas, the retinal ganglion is not single, but two optic ganglia at least are included in it, so that there are two sets of nerve-cells and two synapses are always formed; one between the retinal end-cells and the neurones of the first optic ganglion, which may be called the ganglion of the retina, the other between the first and second ganglia, which, seeing that the neuraxons of its cells form the optic nerve, may be called the ganglion of the optic nerve. The 'neuropil' formed by these synapses forms the molecular layers of the compound retina, and the cells themselves form the nuclear layers. Thus an upright compound retina, formed in the same way as the upright simple retina, would be illustrated by Fig.30.
Further, in precisely the same way as in the case of the simple retina, such a compound retina may be upright or inverted. Thus, in the lateral eyes of crustaceans and insects, a compound retina of this kind is formed, which is upright; while in the vertebrates the compound retina of the lateral eyes is inverted.
The compound retina of vertebrates is usually described as composed of a series of layers, which may be analyzed into their several components as follows:—
The difference between the development of these two types of eye—those with a simple retina and those with a compound retina—has led, in the most natural manner, to the conception that the retina is developed, in the higher animals, sometimes from the cells of the peripheral epidermis, sometimes from the tissue of the brain—two modes of development termed by Balfour 'peripheral' and 'cerebral.' An historical survey of the question shows most conclusively that all investigators are agreed in ascribing the origin of the simple retina to the peripheral method of development, the retina being formed from the hypodermal cells by a process of invagination, while the cerebral type of development has been described only in the development of the compound retina. The natural conclusion from this fact is that, in watching the development of the compound retina, it is more difficult to differentiate the layers formed from the epidermal retinal cells and those formed from the epidermal optic ganglion-cells, than in the case of the simple retina, where the latter cells withdraw entirely from the surface. This is the conclusion to which Patten has come, and, indeed, judging from the text-book of Korschelt and Heider, it is the generally received opinion of the day that, as far as the Appendiculata are concerned, the retina, in the true sense—the retinal end-cells, with their cuticular rods,—is formed, in all cases, from the peripheral cells of the hypodermal layer, the cuticular rods being modifications of the general cuticular surface of the body. The apparent cerebral development of the crustaceanretina, as quoted from Bobretsky by Balfour, is therefore in reality the development of the retinal ganglion, and not of the retina proper.
There is, I imagine, a universal belief that the natural mode of origin of a sense-organ, such as the eye, must always have been from the cells forming the external surface of the animal, and that direct origin from the central nervous system isa priorimost improbable. It is, therefore, a matter of satisfaction to find that the evidence for the latter origin has universally broken down, with the single exception of the eyes of vertebrates and their degenerated allies; a fact which points strongly to the probability that a reconsideration of the evidence upon which the present teaching of the origin of the vertebrate eye is based will show that here, too, a confusion has arisen between that part formed from the epidermal surface and that from the optic ganglion.
The Median or Pineal Eyes.
Undoubtedly, in recent times, the most important clue to the ancestry of vertebrates has been given by the discovery that the so-called pineal gland in the vertebrate brain is all that remains of a pair of median or pineal eyes, the existence of which is manifest in the earliest vertebrates; so that the vertebrate, when it first arose, possessed a pair of median eyes as well as a pair of lateral eyes. The ancestor of the vertebrate, therefore, must also have possessed a pair of median eyes as well as a pair of lateral eyes.
Very instructive, indeed, is the evidence with regard to these median eyes, for one of the great characteristics of the ancient palæostracan forms is the invariable presence of a pair of median eyes as well as a pair of lateral eyes. In the living representative of such forms—Limulus—the pair of median eyes (Fig.5) is well shown, and it is significant that here, according to Lankester and Bourne, these eyes are already in a condition of degeneration; so also in many of the Palæostraca (Fig.7) the lateral eyes are the large, well-developed eyes, while the median eyes resemble those of Limulus in their insignificance.
We see, then, that in the dominant arthropod race at the time when the fishes first appeared, the type of eyes consisted of a pair of well-developed lateral eyes and a pair of insignificant, partially degenerated, median eyes. Further, according to all palæontologists,in the best-preserved head-shields of the most ancient fishes, especially well seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed, between which, in the mid-line, are seen a pair of small, insignificant median eyes.
The evidence of the rocks, therefore, proves that the pair of median eyes which were originally the principal eyes (Hauptaugen), had already, in the dominant arthropod group been supplanted by a pair of lateral eyes, and had, in consequence, become small and insignificant, at the time when vertebrates first appeared. This dwindling process thus initiated in the arthropod itself has steadily continued ever since through the whole development of the vertebrates, with the result that, in the highest vertebrates, these median or pineal eyes have become converted into the pineal gland with its 'brain-sand.'
In the earliest vertebrate these median eyes may have been functional; they certainly were more conspicuous than in later forms. Alone among living vertebrates the right median eye of Ammocœtes is so perfect and the skin covering it so transparent that I have always felt doubtful whether it may not be of use to the animal, especially when one takes into consideration the undeveloped state of the lateral eyes in this animal, hidden as they are under the skin. Thus the one living vertebrate which is comparable with these extinct fishes is the one in which one of the pineal eyes is most well defined, most nearly functional.
Before passing to the consideration of the structure of the median eyes of Ammocœtes, it is advisable to see whether these median eyes in other animals, such as arachnids and crustaceans, belong to any particular type of eyes, for then assuredly the median eyes of Ammocœtes ought to belong to the same type if they are derived from them.
In the specialized crustacean, as in the specialized vertebrate, the median eyes have disappeared, at all events in the adult, but still exist in the primitive forms, such as Branchipus, which resemble the trilobites in some respects. On the other hand, the median eyes have persisted, and are well developed in the arachnids, both scorpions and spiders possessing a well-developed pair. The characteristics of the median eyes must then be especially sought for in the arachnid group.
Both scorpions and spiders possess many eyes, of which two arealways separate and median in position, while the others form lateral groups; all these eyes possess a simple retina and a simple corneal lens. Grenacher was the first to point out that in the spiders two very distinct types of eye are found. In the one the retina is upright; in the other the retina is inverted, and the eye possesses a tapetal layer. The distribution of these two types is most suggestive, for the inverted retina is always found in the lateral eyes, never in the two median eyes; these always possess a simple upright retina.
In the crustaceans, the lateral eyes differ also from the median eyes, but not in the same way as in the arachnids; for here both types of eye possess an upright retina, but the retina of the lateral eyes is compound, while that of the median eyes is simple. In other words, the median eyes are in all cases eyes with a simple upright retina and a simple cuticular lens, while the retina of the lateral eyes is compound or may be inverted, according as the animal in question possesses crustacean or arachnid affinities. The lateral eye of the vertebrate, possessing, as it does, an inverted compound retina, indicates that the vertebrate arose from a stock which was neither arachnid nor crustacean, but gave rise to both groups—in fact, was a member of the great palæostracan group. What, then, is the nature of the median eyes in the vertebrate?
The Median Eyes of Ammocœtes.
The evidence of Ammocœtes is so conclusive that I, for one, cannot conceive how it is possible for any zoologist to doubt whether the parietal organ, as they insist on calling it, had ever been an eye, or rather a pair of eyes.
Anyone who examines the head of the larval lamprey will see on the dorsal side, in the median line, first, a somewhat circular orifice—the unpaired nasal opening; and then, tailwards to this, a well-marked circular spot, where the skin is distinctly more transparent than elsewhere. This spot coincides in position with the underlying dorsal pineal eye, which shines out conspicuously owing to the glistening whiteness of its pigment. Upon opening the brain-case the appearance as in Fig.20is seen, and the mass of the rightganglion habenulæ(G.H.R.), as it has been called, stands out conspicuously as well as the right or dorsal pineal eye (Pn.). Both eye and ganglion appear at first sight to be one-sided, but further examination shows that a leftganglion habenulæis present, though much smaller than onthe right side. In connection with this is another eye-like organ—the left or ventral pineal eye,—much more aborted, much less like an eye than the dorsal one; so also there are two bundles of peculiar fibres called Meynert's bundles, which connect this region with the infra-infundibular region of the brain; of these, the right Meynert's bundle is much larger than the left.