Chapter XV: Final Remarks (3)

In the higher animals we are accustomed to speak of certain organs as vital and necessary for the further existence of the animal; these are essentially the central nervous system, the respiratory system, the circulatory system, and the digestive system. Of these four vital systems the first cannot be touched without the chance of degeneration; but that is not the case with the second. The passage from the fish to the amphibian, from the water-breathing to the air-breathing animal, has actually taken place, and was effected by the modification of the swim-bladder to form new respiratory organs--the lung; the old respiratory organs--the gills--becoming functionless, but still persisting in the embryo as vestiges. The necessity arose in consequence of the passage of the animal from water to land, and with this necessity nature found a means of overcoming the difficulty; air-breathing vertebrates arose, and from the very fact of their being able to extend over the land-surfaces, increased in numbers and developed in complexity in the manner already sketched out.

For a respiratory system all that is required is an arrangement {58}by means of which blood should be brought to the surface, so as to interchange its gases with those of the external medium; and it is significant to find that of all vertebrates the Amphibia alone are capable of an effective respiration by means of the skin.

As to the circulatory system, it is exceedingly easily modified. An animal such as Amphioxus has no heart; in some the heart is systemic, in others branchial; in some there are more than one heart; in others there are contractile veins in addition to a heart. There is no difficulty here in altering and modifying the system according to the needs of the individual.

For a digestive system all that is required is an arrangement for the digestion and absorption of food, a mechanism which can arise easily if some of the cells of the skin possess digestive power. Now Miss Alcock has shown that some of the surface-cells of crustaceans secrete a fluid which possesses digestive powers, and she has also shown that certain of the cells in the skin of Ammocoetes possess digestive power.

The difficulty, then, of forming a new digestive system in the passage from the arthropod to the vertebrate is very much the same as the difficulty in forming a new respiratory system in the passage from the water-breathing fish to the air-breathing amphibian--a change which does not strike us as inconceivable, because we know it has taken place.

The whole argument so far leads to the conclusion that vertebrates arose from ancient forms of arthropods by the formation of a new alimentary canal, and the enclosure of the old canal by the growing central nervous system. If this conclusion is true, then it follows that we possess a well-defined starting-point from which to compare the separate organs of the arthropod with those of the vertebrate, and if, in consequence of such working hypothesis, each organ of the arthropod is found in the vertebrate in a corresponding position and of similar structure, then the truth of the starting-point is proved as fully as can possibly be expected by deductive methods. It is, in fact, this method of comparative anatomy which has proved the descent of man from the ape, the frog from the fish, etc.

Let us, then, compare all the organs of such a low vertebrate as Ammocoetes with those of an arthropod of the ancient type.

{59}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 Ammocoetes, 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 Ammocoetes 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--Ammocoetes. 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 Ammocoetes 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 Ammocoetes more {60}valuable 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 Ammocoetes 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 former gnathostomatous condition either in Petromyzon or, in its larval state, Ammocoetes. 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 all {61}resemblance 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 Ammocoetes, the tentacles, the muco-cartilage, the tubular muscles, all these structures, so striking in the Ammocoetes 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 Ammocoetes-like ancestors, even though Myxine, Amphioxus, and the tunicates be all stages on the downward grade from those same Ammocoetes-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 AMMOCOETES 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 Ammocoetes with that of arthropods, especially of Limulus and of the scorpion-group.

{62}The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-oesophageal 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-oesophageal 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-oesophageal 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-oesophageal 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-oesophageal in position, but is situated on the oesophageal commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in reality the first ganglion of the infra-oesophageal series, and not to belong to the supra-oesophageal group.

_Ant. I._ and _Ant. 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 extinct {63}Eurypteridæ--in fact, in the Palæostraca generally--there are two median eyes in addition to the lateral eyes, which were innervated from these ganglia.

In Ammocoetes, then, if the supra-infundibular portion of the brain really corresponds to the supra-oesophageal 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, Ammocoetes.

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.

{64}A review of the animal kingdom as a whole leads to the conclusion
that the upward development of animals from an original coelenterate
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 oesophagus into an alimentary canal which is situated dorsally to the
central nervous system. Thus the oesophagus pierces the central nervous
system and divides it into two parts, the supra-oesophageal ganglia and
the infra-oesophageal 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 oesophagus.

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 oesophagus 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 oesophagus 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 oesophagus (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
the _Enteropneusta_, the best known example of which is called
_Balanoglossus_; 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?

{65}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 Ammocoetes stage of the lamprey, because it
was formerly considered to be a separate species and received the name of
Ammocoetes. 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
Ammocoetes on the other are {66}available 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 oesophagus of the invertebrate, the
cerebral hemispheres and basal ganglia the supra-oesophageal ganglia, the
_crura cerebri_ the oesophageal commissures, and the infra-infundibular
part of the brain the infra-oesophageal ganglia. It is immediately
apparent that just as the invertebrate oesophagus 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 coelenterate involved the piercing of the anterior
portion of the central nervous system by the oesophagus, 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 been
{67}compelled to pass the whole of his food through the narrow
oesophageal 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.

{68}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 Ammocoetes 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 the {69}hypodermal 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 an _upright 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 the {70}tapetum. 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 an _inverted 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.

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 be {71}contiguous 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 a _compound 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.

{72}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.'

_ABCD_, as in Fig. 28. _Op. g. I._ and _Op. 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.

{73}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:--

Layer of rods and cones }
External nuclear layer } retina proper } Ectodermic part
External molecular layer }}
Internal nuclear layer } ganglion of retina }
Internal molecular layer }} } retinal } neurodermic
Optic nerve-cell layer } ganglion of optic nerve} ganglion} part
Layer of optic nerve }
fibres

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 crustacean {74}retina, 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 is _a priori_ most 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, {75}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 Ammocoetes 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 Ammocoetes, 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 Ammocoetes 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 are {76}always 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 AMMOCOETES.

The evidence of Ammocoetes 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. 20 is seen, and the mass of the right _ganglion 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 left _ganglion habenulæ_ is present, though much smaller than on {77}the 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.

_l.m._, upper lip muscles; _m.c._, muco-cartilage; _n._, nose; _na.c._, nasal cartilage; _pn._, right pineal eye and nerve; _g.h.r._, right _ganglion habenulæ_; _s.m._, somatic muscles; _cr._, membranous wall of cranium; _ch._, choroid plexus; _gl._, glandular substance and pigment filling up brain-case.]

{78}

On the right side the nerve end-cells have been drawn free from pigment.]

On the left side the eye is drawn as it appeared in the section. On the right side I have removed the pigment from the nerve end-cells, and drawn the eye as, in my opinion, it would appear if it were functional.]

This difference between right and left indicates a greater degeneration on the left side, and points distinctly to a close relationship between the nerve-masses known as _ganglia habenulæ_ and the median eyes. In my opinion this ganglion is, in part, at all events, the optic ganglion of the median eye on each side. It is built up on the same type as the optic ganglia of invertebrate simple eyes, with a cortex of small round cells and a medulla of fine nerve-fibres. Into this ganglion, on the right side, there passes a very well-defined nerve--the nerve of the dorsal eye. The eye itself with its nerve, _pn._, and its optic ganglion, _g.h.r._, is beautifully shown by means of a horizontal section through the head of Ammocoetes (Fig. 31). Originally, as described by Scott, the eye stood vertically {79}above its optic ganglion, and presented an appearance remarkably like Fig. 32, which represents one of the simple eyes and optic ganglia of a larva of Acilius as described by Patten; then, with the forward growth of the upper lip, the right pineal eye was dragged forward and its nerve pulled horizontally over the _ganglion habenulæ_. For this reason the eye, nerve, and ganglion are better shown in a nearly horizontal than in a transverse section.

The optic nerve belonging to this eye is most evident and clearly shown in Fig. 31, and in the series of consecutive sections which follow upon this section; no doubt can arise as to the structure in question having been the nerve of the eye, even though, as is possible, it does not contain any functional nerve-fibres.

_pn._2_, left or ventral pineal eye; _pn._1_, last remnant of right, or dorsal pineal eye; _g.h.r._, right _ganglion habenulæ_; _g.h.l._1_, _g.h.l._3_, parts of left _ganglion habenulæ_; _pi._, fold of _pia mater_ which separates the left _ganglion habenulæ_ from the left pineal eye; _f._, strands of nerve-fibres connecting the left eye with its ganglion, _g.h.l._3_; _V_3_, third ventricle; _V.aq._, ventricle of aquæduct.]

The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in appearance, being much less evidently an eye. Fig. 34 is one of the same series of horizontal sections as Fig. 31, _pn._1_ being the last remnant of the right, or dorsal, eye, while _pn._2_ shows the left, or ventral, eye with its connection with the left _ganglion habenulæ_.

{80}In a series of sections I have followed the nerve of the right pineal eye to its destination, as described in my paper in the _Quarterly Journal of Microscopical Science_, and have found that it enters into the _ganglion habenulæ_ just as the nerve to any simple eye enters into its optic ganglion. This nerve, as I have shown, is a very distinct, well-defined nerve, with no admixture of ganglion-cells or of connective tissue, very different indeed to the connection between the left pineal eye and its optic ganglion. Here there is no defined nerve at all; but the cells of the _ganglion habenulæ_ stretch right up to the remains of the eye itself. Seeing, then, that both the eye and ganglion on this side have reached a much further grade of degeneration than on the right side, it may be fairly concluded that the original condition of these two median eyes is more nearly represented by the right eye, with its well-defined nerve and optic ganglion, than by the left eye, or by the eyes in lizards and other animals which do not show so well-defined a nerve as is possessed by Ammocoetes. Quite recently Dendy has examined the two median eyes in the New Zealand lamprey _Geotria australis_. In this species the second eye is much better defined than in the European lamprey, and its connection with the _ganglion habenulæ_ is more nerve-like. In neither eye, however, is the nerve so clean cut and isolated as is the nerve of the dorsal, or right, eye in the Ammocoetes stage of _Petromyzon Planeri_; in both, cells resembling those of the cortex of the _ganglion habenulæ_ and connective tissues are mixed up with the nerve-fibres which pass from each eye to its respective optic ganglion.

THE RIGHT PINEAL EYE OF AMMOCOETES.

The optic fibres of the right median eye of Ammocoetes are connected with a well-defined retina, the limits of which are defined by the white pigment so characteristic of this eye. This pigment is apparently calcium phosphate, which still remains as the 'brain-sand' of the human pineal gland. The cells, which are hidden by this pigment, were described by me in 1890 as the retinal end-cells with large nuclei. In 1893, Studniçka examined them more closely, and concluded that the retinal cells are of two kinds: the one, nerve end-cells, the sensory cells proper; the other, pigmented epithelial cells, which surround the sense-cells. The sense-cells may contain some of the white pigment, but not so much as the other cells. Similarly, in the {81}median eyes of Limulus, Lankester and Bourne find it difficult to determine how far the retinal end-cells contain pigment and how far that pigment really is in the cells surrounding these nerve end-cells.

The interior of the eye presents the appearance of a cavity in shape like a cornucopia, the stalk of which terminates at the place where the nerve enters. This cavity is not empty, but the posterior part of it is filled with the termination of the nerve end-cells of the retina, as pointed out by me and confirmed by Studniçka. These terminations are free from pigment, and contain strikingly translucent bodies, which I have described in my paper in the _Quarterly Journal_, and called rhabdites, for they present the same appearance and are situated in the same position as are many of the rhabdites on the terminations of the retinal end-cells of arthropod eyes. Studniçka has also seen these appearances, and figures them in his second paper on the nerve end-cells of the pineal eye of Ammocoetes.

Up to this point the following conclusions may be drawn:--

1. Ammocoetes possesses a pair of median eyes, just as was the case with
the most ancient fishes, and with the members of the contemporary
palæostracan group.

2. The retina of one of these eyes is well-defined and upright, not
inverted, and therefore in this respect agrees with that of all median
eyes.

3. The presence of nerve end-cells, with pigment either in them or in
cells around them, to the unpigmented ends of which translucent bodies
resembling rhabdites are attached, is another proof that this retina
agrees with that of the median eyes of arthropods.

4. The simple nature of the nerve with its termination in an optic
ganglion closely resembling in structure an arthropod optic ganglion,
together with Studniçka's statement that the nerve end-cells pass
directly into the nerve, points directly to the conclusion that this
retina is a simple, not a compound, retina, and that it therefore in this
respect also agrees with the retina of all median eyes.

With respect to this last conclusion, neither I myself nor Studniçka have been able to see any definite groups of cells between the nerve end-cells and the optic nerve such as a compound retina necessitates.