Fig. 42.—Ideal Diagram of the Layers in a Crustacean Eye.The retina is divided into an epithelial part,C(the layer of retinular cells and rhabdomes), and a neurodermal or cerebral part, which is formed of,A, the ganglion of the optic nerve, and,B, the ganglion of the retina. 1, optic nerve fibres which cross at their entrance into the retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer of crystalline cones; 9, cornea.
Fig. 42.—Ideal Diagram of the Layers in a Crustacean Eye.The retina is divided into an epithelial part,C(the layer of retinular cells and rhabdomes), and a neurodermal or cerebral part, which is formed of,A, the ganglion of the optic nerve, and,B, the ganglion of the retina. 1, optic nerve fibres which cross at their entrance into the retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer of crystalline cones; 9, cornea.
Fig. 42.—Ideal Diagram of the Layers in a Crustacean Eye.
The retina is divided into an epithelial part,C(the layer of retinular cells and rhabdomes), and a neurodermal or cerebral part, which is formed of,A, the ganglion of the optic nerve, and,B, the ganglion of the retina. 1, optic nerve fibres which cross at their entrance into the retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer of crystalline cones; 9, cornea.
The comparison of this figure (Fig.42) with that of the Petromyzon retina (Fig.41) shows how great is the similarity of the latter with the arthropod type, and how the very points in which it deviates from the recognized vertebrate type are explainable by comparison with that of the arthropod. The most striking difference between the retinas in the two figures is that the layer of terminal nerve fibres (5, Fig.42), which, after all, are only the elongated terminations of the retinal cells belonging to Parker's neurones of the first order, is very much longer than in Petromyzon or in any vertebrate, for the external molecular layer (6, Fig.41) (Müller's layer of Nervenansätze) is very short and inconspicuous (in Fig.41it is drawn too thick).
Turning from the retina to the fibres of the optic nerve we again find a remarkable resemblance, for in Ammocœtes, as pointed out byLangerhans and carefully figured by Kohl, a crossing of the fibres of the optic nerve occurs as the nerve leaves the retina, just as is so universally the case in all compound retinas. To this crossing Kohl has given the namechiasma nervi optici, in distinction to the cerebral chiasma, which he callschiasma nervorum opticorum. Further, we find that even this latter chiasma is well represented in the arthropod brain; thus Bellonci in Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a true optic chiasma, the only difference in opinion being, whether the crossing of the optic nerves is complete or not. Especially instructive are Bellonci's figures and description. He describes the brain of Sphæroma as composed of three segments—a superior segment, the cerebrum proper, a middle segment, and an inferior segment; the optic fibres, as is seen in Fig.39, after crossing, pass direct into the middle segment, in the ganglia of which they terminate. From this segment also arises the nerve to the first antenna of that side—i.e.the olfactory nerve. The optic part, then, of this middle segment is clearly the brain portion of the optic ganglionic apparatus, and may be called the optic lobes, in contradistinction to the peripheral part, which is usually called the optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as already mentioned. These optic lobes are therefore homologous with the optic lobes of the vertebrate brain.
The resemblance throughout is so striking as to force one to the conclusion that the retina of the vertebrate eye is a compound retina, composed of a retina and retinal ganglion of the type found in arthropods. From this it follows that the development of the vertebrate retina ought to show the formation of (1) an optic plate formed from the peripheral epidermis and not from the brain; (2) a part of the brain closely attached to this optic plate forming the retinal ganglion, which remains at the surface when the rest of the optic ganglion withdraws; (3) an optic nerve formed in consequence of this withdrawal, as the connection between the retinal and cerebral parts of the optic ganglion.
This appears to me exactly what the developmental process does show according to Götte's investigations. He asserts that the retina arises from an optic plate, being the optical portion of his 'Sinnes-platte.' At an early stage this is separated by a furrow (Furche) from the general mass of epidermal cells which ultimately form the brain. This separation then vanishes, and the retina and brain-massbecome inextricably united into a mass of cells, which are still situated at the surface. By the closure of the cephalic plate and the withdrawal of the brain away from the surface, a retinal mass of cells is left at the surface connected with the tubular central nervous system by the hollow optic diverticulum or primary optic vesicle. If we regard only the retinal and nervous elements, and for the moment pay no attention to the existence of the tube, Götte's observation that the true retina has been formed from the optic plate (Sinnes-platte) to which the retinal portion of the brain (retinal ganglion) has become firmly fixed, and that then the optic nerve has been formed by the withdrawal of the rest of the brain (optic lobes), is word for word applicable to the description of the development of the compound retina of the arthropod eye, as has been already stated.
The Significance of the Optic Diverticula.
The origin of the retina from an optic epidermal plate in vertebrates, as in all other animals, brings the cephalic eyes of all animals into the same category, and leaves the vertebrate eye no longer in an isolated and unnatural position. In one point the retina of the vertebrate eye differs from that of a compound retina of an invertebrate; in the former, a striking supporting tissue exists, known as Müller's fibres, which is absent in the latter. This difference of structure is closely associated with another of the same character as in the central nervous system, viz. the apparent development of the nervous part from a tube. We see, in fact, that the retinal and nervous arrangements of the vertebrate eye are comparable with those of the arthropod eye, in precisely the same way and to the same extent as the nervous matter of the brain of the vertebrate is comparable with the brain of the arthropod. In both cases the nervous matter is, in structure, position, and function, absolutely homologous; in both cases there is found in the vertebrate something extra which is not found in the invertebrate—viz. a hollow tube, the walls of which, in the case of the brain, are utilized as supporting tissues for the nerve structures. The explanation of this difference in the case of the brain is the fundamental idea of my whole theory, namely, that the hollow tube is in reality the cephalic stomach of the invertebrate, around which the nervous brain-matter was originally grouped in precisely the same manner as in the invertebrate. What, then, are the optic diverticula?
"The formation of the eye," as taught by Balfour, "commences with the appearance of a pair of hollow outgrowths from the anterior cerebral vesicle. These outgrowths, known as the optic vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they soon, however, become partially constricted, and form vesicles united to the base of the brain by comparatively narrow, hollow stalks, the rudiments of the optic nerves."
"After the establishment of the optic nerves, there takes place (1) the formation of the lens, and (2) the formation of the optic cup from the walls of the primary optic vesicle."
He then goes on to explain how the formation of the lens forms the optic cup with its double walls from the primary optic vesicle, and says—
"Of its double walls, the inner, or anterior, is formed from the front portion, the outer, or posterior, from the hind portion of the wall of the primary optic vesicle. The inner, or anterior, which very speedily becomes thicker than the other, is converted into the retina; in the outer, or posterior, which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigment-layer of the choroid."
The difficulties in connection with this view of the origin of the eye are exceedingly great, so great as to have caused Balfour to discuss seriously Lankester's suggestion that the eye must have been at one time within the brain, and that the ancestor of the vertebrate was therefore a transparent animal, so that light might get to the eye through the outer covering and the brain-mass; a suggestion, the unsatisfactory nature of which Balfour himself confessed. Is there really evidence of any part of either retina or optic nerve being formed from the epithelial lining of the tube?
This tube is formed as a direct continuation of the tube of the central nervous system, and we can therefore apply to it the same arguments as have been used in the discussion of the meaning of the latter tube. Now, the striking point in the latter case is the fact that the lining membrane of the central canal is in so many parts absolutely free from nervous matter, and so shows, as in the so-called choroid plexuses, its simple, non-nervous epithelial structure. This also we find in the optic diverticulum. Where there is no evidence of any invasion of the tube by nervous elements, there it retains its simple non-nervous character of a tube composed of a single layer ofepithelial cells—viz. in that part of the tube which, as Balfour says, remains thin, in which pigment is eventually deposited, and which ultimately becomes the tesselated pigment-layer of the choroid. Nobody has ever suggested that this pigment-layer is nervous matter, or ever was, or ever will be, nervous matter; it is in precisely the same category as the membranous roof of the brain in Ammocœtes, which never was, and never will be, nervous matter. Yet, according to the old embryology both in the case of the eye and the brain, the pigment-layer and the so-called choroid plexuses are a part of the tubular nervous system.
Turning now to the optic nerve, Balfour describes it as derived from the hollow stalk of the optic vesicle. He says—
"At first the optic nerve is equally continuous with both walls of the optic cup, as must of necessity be the case, since the interval which primarily exists between the two walls is continuous with the cavity of the stalk. When the cavity within the optic nerve vanishes, and the fibres of the optic nerve appear, all connection is ruptured between the outer wall of the optic cup and the optic nerve, and the optic nerve simply perforates the outer wall, and becomes continuous with the inner one."
In this description Balfour, because he derived the optic nerve fibres from the epithelial wall of the optic stalk, of necessity supposed that such fibres originally supplied both the outer and inner walls of the optic cup and, therefore, seeing that when the fibres of the optic nerve appear they do not supply the outer wall, he supposes that their original connection with the outer wall is ruptured, because a discontinuity of the epithelial lining takes place coincidently with the appearance of the optic nerve-fibres, and, according to him, the optic nerve simply perforates the outer wall and becomes continuous with the inner one. This last statement is very difficult to understand. I presume he meant that some of the fibres of the optic nerve supplied from the beginning the inner wall of the optic cup, but that others which originally supplied the outer wall were first ruptured, then perforated the outer wall, and finally completed the supply to the inner wall or retina.
This statement of Balfour's is the necessary consequence of his belief, that the epithelial cells of the optic stalk gave rise to the fibres of the optic nerve. If, instead of this, we follow Kölliker and His, who state that the optic nerve-fibres are formed outside theepithelial walls of the optic stalk, and that the cells of the latter form supporting structures for the nerve-fibres, then the position of the optic nerve becomes perfectly simple and satisfactory without any rupturing of its connection with the outer wall and subsequent perforation, for the optic nerve-fibres from their very first appearance pass directly to supply the retina—i.e.the inner wall of the optic cup and nothing else.
They pass, as is well known, without any perforation by way of the choroidal slit to the inner surface of the inner wall (retina) of the optic cup; then, when the choroidal slit becomes closed by the expansion of the optic cup, the optic nerve naturally becomes situated in the centre of the base of the cup and spreads over its inner surface as that surface expands.
A section across the optic cup at an early stage at the junction of the optic stalk and optic cup would be represented by the upper diagram in Fig. 43; at a later stage, when the choroidal slit is closed, by the lower diagram.
Fig. 43.—Diagram of the Relation of the Optic Nerve to the Optic Cup.The upper diagram represents a stage before the formation of the choroidal slit, the lower one the stage of closure of the choroidal slit.R., retina;O.n., optic nerve;p., pigment epithelium.
Fig. 43.—Diagram of the Relation of the Optic Nerve to the Optic Cup.The upper diagram represents a stage before the formation of the choroidal slit, the lower one the stage of closure of the choroidal slit.R., retina;O.n., optic nerve;p., pigment epithelium.
Fig. 43.—Diagram of the Relation of the Optic Nerve to the Optic Cup.
The upper diagram represents a stage before the formation of the choroidal slit, the lower one the stage of closure of the choroidal slit.R., retina;O.n., optic nerve;p., pigment epithelium.
The evident truth of this manner of looking at the origin of the optic nerve is demonstrated by the appearance of the optic nerve in Ammocœtes and Petromyzon. In the latter, although the development is complete, and the eye, and consequently also the optic nerve-fibres, are fully functional, there is still present in the axial core of the nerve a row of epithelial cells (Axenstrang) which are altered so as to form supporting structures, in the same way as a row of epithelial cells in the retina is altered to form the system of supporting cells known by the name of the Müllerian fibres.
The origin of this axial core of cells is perfectly clear, as has been pointed out by W. Müller. He says—
"The development of the optic nerve shows peculiarities inPetromyzon of such a character as to make this animal one of the most valuable objects for deciding the various controversial questions connected with the genesis of its elements. The lumen of the stalk of the primary optic vesicle is obliterated quite early by a proliferation of its lining epithelium. Also the original continuity of this epithelium with that of the pigment-layer is at an early period interrupted at the point of attachment of the optic stalk. This interruption occurs at the time when the fibres of the optic nerve first become visible."
Further on he says—
"The epithelium of the optic stalk develops entirely into supporting cells, which in Petromyzon fill up the original lumen and so form an axial core (Axenstrang) to the nerve-fibres which are formed entirely outside them; the projections of these supporting cells are directed towards the periphery, and so separate the bundles of the optic nerve-fibres. The mesodermal coat of the optic stalk takes no part in this separation; it simply forms the connective tissue sheath of the optic nerve. The development of the optic nerve in the higher vertebrates also obeys the same law, as I am bound to conclude from my own observations."
The evidence, then, of Ammocœtes is very conclusive. Originally a tube composed of a single layer of epithelial cells became expanded at the anterior end to form a bulb. On the outside of this tube or stalk the fibres of the optic nerve make their appearance, arising from the ganglion-cell layer of the retina, and, passing over the surface of the epithelial tube at the choroidal fissure, proceed to the brain by way of the optic chiasma. Owing to the large number of fibres, their crossing at the junction of the stalk with the bulb, and the narrowness at this neck, the obliteration of the lumen of the tube which takes place in the stalk is carried out to a still greater extent at this narrow part. The result of this is that all continuity of the cell-layers of the original tube of the optic stalk with those of both the inner and outer walls of the bulb is interrupted, and all that remains in this spot of the original continuous line of cells which connected the tube of the stalk with that of the bulb are possibly some of the groups of cells which are found scattered among the fibres of the optic nerve at their entrance into the retina. Such separation of the originally continuous elements of the epithelial wall of the optic stalk, which is apparent only at this neck of the nerve in Petromyzon, takes placealong the whole of the optic nerve in the higher vertebrates, so that no continuous axial core of cells exist, but only scattered supporting cells.
If further proof in support of this view be wanted, it is given by the evidence of physiology, which shows that the fibres of the optic nerve are not different from other nerve-fibres of the central nervous system, but that they degenerate when separated from their nerve-cell, and that the nerve-cell of which the optic nerve-fibre is a process is the large ganglion-cell of the ganglionic layer of the retina. The origin of the ganglionic layer of the retina cannot therefore be separated from that of the optic nerve-fibres. If the one is outside the epithelial tube, so is the other, and what holds true of the ganglionic layer must hold good of the rest of the retinal ganglion and, from all that has been said, of the retina itself. We therefore come to the conclusion that the evidence is distinctly in favour of the view, that the retina and optic nerve in the true sense are structures which originally were outside a non-nervous tube, but, just like the central nervous system as a whole, have amalgamated so closely with the elements of this tube as to utilize them for supporting structures. One part of this non-nervous tube, its dorsal wall, like the corresponding part of the brain-tube, still retains its original character, and by the deposition of pigment has been pressed into the service of the eye to form the pigmented epithelial layer.
We can, however, go further than this, for we know definitely in the case of the retina what the fate of the epithelial cells lining this tube has been. They have become the system of supporting structures known as Müllerian fibres.
The epithelial layer of the primary optic vesicle can be traced into direct continuity with the lining epithelium of the brain cavity, as a single layer of epithelial cells in the core of the optic nerve, forming the optic stalk, which, in consequence of close contact, becomes the well-known axial layer of supporting cells. This epithelial layer of the optic stalk then expands to form the optic bulb, the outer or dorsal wall of which still remains as a single layer of epithelium and becomes the layer of pigment epithelium. This layer of epithelium becomes doubled on itself by the approximation of the inner or ventral wall of the optic cup to the outer or dorsal wall in consequence of the presence of the lens, and still remaining a single layer, forms thepars ciliaris retinæ; then suddenly, at theoraserrata, the single epithelial layer vanishes, and the layers of the retina take its place. It has long been known, however, that even throughout the retina this single epithelial layer still continues, being known as the fibres of Müller. This is how the fact is described in the last edition of Foster's "Text-book of Physiology," p. 1308—
"Stretching radially from the inner to the outer limiting membrane in all regions of the retina are certain peculiar-shaped bodies known as the radial fibres of Müller. Each fibre is the outcome of the changes undergone by what was at first a simple columnar epithelial cell. The changes are, in the main, that the columnar form is elongated into that of a more or less prismatic fibre, the edges of which become variously branched, and that while the nucleus is retained the cell substance becomes converted into neuro-keratin. And, indeed, at theora serratathe fibres of Müller may be seen suddenly to lose their peculiar features and to pass into the ordinary columnar cells which form thepars ciliaris retinæ."
Fig. 44.—Diagram representing the Single-layered Epithelial Tube of the Vertebrate Eye after Removal of the Nervous and Retinal Elements.O.n., axial core of cells in optic nerve;p., pigment epithelium;p.c.r., pars ciliaris retinæ;m.f., Müllerian fibres;l., lens.
Fig. 44.—Diagram representing the Single-layered Epithelial Tube of the Vertebrate Eye after Removal of the Nervous and Retinal Elements.O.n., axial core of cells in optic nerve;p., pigment epithelium;p.c.r., pars ciliaris retinæ;m.f., Müllerian fibres;l., lens.
Fig. 44.—Diagram representing the Single-layered Epithelial Tube of the Vertebrate Eye after Removal of the Nervous and Retinal Elements.
O.n., axial core of cells in optic nerve;p., pigment epithelium;p.c.r., pars ciliaris retinæ;m.f., Müllerian fibres;l., lens.
It is then absolutely clear that the essential parts of the eye may be considered as composed of two parts—
1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the accompanying Fig.44, and the left side of Fig.41.
2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described. In this part supporting elements are found, just as in any other compound retina, of the nature of neuroglia, which are independent of the Müllerian fibres.
Of these two parts we have already seen that the second is to all intents and purposes a compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is continuous with the single-layered epithelial tube of the central nervous system—i.e.with the cephalic part of the gut of the arthropod ancestor—it follows with certainty that the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the wall of which, near the anterior extremity, the compound retina has amalgamated on either side, just as the infra-œsophageal ganglia have amalgamated with the ventral wall of the main gut-tube. In this way, and in this way alone, does the interpretation of the structure of the vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions already arrived at.
The question therefore arises:—Have we any grounds for believing that the ancient forms of primitive crustaceans and primitive arachnids, which were so abundant in the time when the Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the consideration of the vertebrate eye strongly indicates must have been the case?
The beautiful pictures of Blanchard, and his description, show how, on the arachnid side, paired diverticula of the stomach are nearly universal in the group. Thus, although they are not present in the scorpions, still, in the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked characteristic of the stomach-region is the presence of four pairs of cœcal diverticula, which spread laterally over the prosomatic region. In the spiders the number of such diverticula increases, and the whole prosomatic region becomes filled up with these tubes. Blanchard considers that they form nutrient tubes for the direct nutrition of the organs in the prosoma, especially the important brain-region of the central nervous system. He points out that these animals are blood-suckers, and that, therefore, their food is already in a suitable form for purposes of nutrition when it is taken in by them, so that, as it were, the anterior part of the gut is transformed into a series of vessels or diverticula conveying blood directly to the important organs in the prosoma, by means of which they obtain nourishment in addition to their own blood-supply.
The universality of such diverticula among the arachnids makes it highly probable that their progenitors did possess an alimentary canal with one or more pairs of anterior diverticula. In thevertebrate, however, the paired diverticula are associated with a compound retina, a combination which does not occur among living arachnids; we must, therefore, examine the crustacean group for the desired combination, and naturally the most likely group to examine is the Phyllopoda, especially such primitive forms as Branchipus and Artemia, for it is universally acknowledged that these forms are the nearest living representatives of the trilobites. If, therefore, it be found that the retina and optic nerve in Artemia is in specially close connection with an anterior diverticulum of the gut on each side, then it is almost certain that such a combination existed also in the trilobites.
Fig. 45.—Section through one of the two Anterior Diverticula of the Gut in Artemia and the Retinal Ganglion.The section is through the extreme anterior end of the diverticulum, thus cutting through many of the columnar cells at right angles to their axis.Al., gut diverticulum;rt. gl., retinal ganglion.
Fig. 45.—Section through one of the two Anterior Diverticula of the Gut in Artemia and the Retinal Ganglion.The section is through the extreme anterior end of the diverticulum, thus cutting through many of the columnar cells at right angles to their axis.Al., gut diverticulum;rt. gl., retinal ganglion.
Fig. 45.—Section through one of the two Anterior Diverticula of the Gut in Artemia and the Retinal Ganglion.
The section is through the extreme anterior end of the diverticulum, thus cutting through many of the columnar cells at right angles to their axis.Al., gut diverticulum;rt. gl., retinal ganglion.
Fig. 46.—The Brain, Eyes, and Anterior Termination of the Alimentary Canal of Artemia, viewed from the Dorsal Aspect.Br., brain;l.e., lateral eyes;c.e., median eyes;Al., alimentary canal.
Fig. 46.—The Brain, Eyes, and Anterior Termination of the Alimentary Canal of Artemia, viewed from the Dorsal Aspect.Br., brain;l.e., lateral eyes;c.e., median eyes;Al., alimentary canal.
Fig. 46.—The Brain, Eyes, and Anterior Termination of the Alimentary Canal of Artemia, viewed from the Dorsal Aspect.
Br., brain;l.e., lateral eyes;c.e., median eyes;Al., alimentary canal.
Fig. 47.—A, The Formation of the Retina of the Eye of Ammocœtes(afterScott);B, The Formation of the Retina of the Eye of Ammocœtes, on my theory.R., retina;l., lens;O.n., optic nerve fibres;Al., cephalic end of invertebrate alimentary canal;V., cavity of ventricles of brain;Al.d., anterior diverticulum of alimentary canal;op.d., optic diverticulum.
Fig. 47.—A, The Formation of the Retina of the Eye of Ammocœtes(afterScott);B, The Formation of the Retina of the Eye of Ammocœtes, on my theory.R., retina;l., lens;O.n., optic nerve fibres;Al., cephalic end of invertebrate alimentary canal;V., cavity of ventricles of brain;Al.d., anterior diverticulum of alimentary canal;op.d., optic diverticulum.
Fig. 47.—A, The Formation of the Retina of the Eye of Ammocœtes(afterScott);B, The Formation of the Retina of the Eye of Ammocœtes, on my theory.
R., retina;l., lens;O.n., optic nerve fibres;Al., cephalic end of invertebrate alimentary canal;V., cavity of ventricles of brain;Al.d., anterior diverticulum of alimentary canal;op.d., optic diverticulum.
My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the course of his work he cut serial sections through the whole animal, and, as mentioned in my paper in theJournal of Anatomy and Physiology, he discovered that the retinal ganglion of each lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion (Fig.45). In this animal there are only two gut-diverticula, which are situated most anteriorly. I have plotted out this series of sections by means of a camera lucida, with the result that the retina appears as a bulging attached ventro-laterally to the extremity of each gut-diverticulum, as is shown in Fig.46. It is instructive to compare with this figure Scott's picture of the developing eye in Ammocœtes, where he figures the retina asa bulging attached ventrally to the extremity of the narrow tube of the optic diverticulum. In Fig.47, A, I reproduce this figure of Scott, and by the side of it, Fig.47, B, I have represented the origin of the vertebrate eye as I believe it to have occurred.
We see, then, this very striking fact, that in the most primitive of the Crustacea, not only are there two anterior diverticula of the gut, but also the retinal ganglion of the lateral eye is in specially close connection with the end of the diverticulum on each side. In fact, we find in the nearest living representative of the trilobites a retina and retinal ganglion and optic nerve, closely resembling that of the vertebrate, in close connection with an epithelial tube which has nothing to do with the organ of sight, but is one of a pair of anterior gut-diverticula. It is impossible to obtain more decisive evidence that the trilobites possessed a pair of gut-diverticula surrounded to a greater or less extent by the retina and optic nerve of each lateral eye.
Such anterior diverticula are commonly found in the lower Crustacea; they are usually known by the name of liver-diverticula, but as they take no part in digestion, and, on the contrary, represent that part of the gut which is most active in absorption, the term liver is not appropriate, and it is therefore better to call them simply the pair of anterior diverticula. Our knowledge of their function in Daphnia is given in a paper by Hardy and M‘Dougall, which does not appear to be widely known. Hardy succeeded in feeding Daphnia with yolk of egg in which carmine grains were mixed, and was able in the living animal to watch the whole process of deglutition, digestion, and absorption. The food, which is made into a bolus, is moved down to the middle region of the gut, and there digestion takes place. Then by an antiperistaltic movement the more fluid products of the digestion-process are sent right forward into the two anterior diverticula, where the single layer of columnar cells lining these diverticula absorbs these products, the cells becoming thickly studded with fat-drops after a feed of yolk of egg. The carmine particles, which were driven forward with the proteid- and fat-particles, are not absorbed, but are at intervals driven back by contractions of the anterior diverticula to the middle region of the gut.
These observations prove most clearly that the anterior diverticula have a special nutrient function, being the main channels by which new nutrient material is brought into the body, and, aspointed out by the authors, it is a remarkable exception in the animal kingdom that absorption should occur in that portion of the gut which is anterior to the part in which digestion occurs. In all these animals the two anterior diverticula extend forwards over the brain, and, as we have seen in Artemia, the anterior extremity of each one is so intimately related to a part of the brain—viz. the retinal ganglion—as to form a lining membrane to that mass of nerve-cells. It follows, therefore, that the nutrient fluid absorbed by the cells of this part of the gut-diverticulum must be primarily for the service of the retinal ganglion. In fact, the relations of this anterior portion of the gut to the brain as a whole suggest strongly that the marked absorptive function of this anterior portion of the gut exists in order to supply nutrient material in the first place to the most vital, most important organ in the animal—the brain and its sense-organs. This conclusion is borne out by the fact that in these lower crustaceans the circulation of blood is of a very inefficient character, so that the tissues are mainly dependent for their nutrition on the fluid immediately surrounding them. It stands to reason that the establishment of the anterior portion of the gut as a nutrient tube to the brain would necessitate a closer and closer application of the brain to that tube, so that the process of amalgamation of the brain with the single layer of columnar epithelial cells which constitutes the wall of the gut (which we see in its initial stage in the retinal ganglion of Artemia), would tend rapidly to increase as more and more demands were made upon the brain, until at last both the supra- and infra-œsophageal ganglia, as well as the retinal ganglia and optic nerves, were in such close intimate connection with the ventral wall of the anterior portion of the gut and its diverticula as to form a brain and retina closely resembling that of Ammocœtes.
Such an origin for the lateral eyes of the vertebrate explains in a simple and satisfactory manner why the vertebrate retina is a compound retina, and why both retina and optic nerve have an apparent tubular development.
At the same time one discrepancy still exists which requires consideration—viz. in no arthropod eye possessing a compound retina is the retina inverted. All the known cases of inversion among arthropods occur in eyes, the retina of which is simple, and are all natural consequences of the process of invagination by whichthe retina is formed. On the other hand, eyes with an inverted compound retina are not entirely unknown among invertebrates, for the eyes of Pecten and of Spondylus possess a retina which is inverted after the vertebrate fashion and still may be spoken of as compound rather than simple. It is clear that an invagination, the effect of which is an inversion of the retinal layer, would lead to the same result, whether the retinal optic nerves were short or long, whether, in fact, a retinal ganglion existed or not. Undoubtedly the presence of the retinal ganglion tends greatly to obscure any process of invagination, so that, as already mentioned, many observers, with Parker, consider the retina of the crustacean lateral eye to be formed by a thickening only, without any invagination, while Reichenbach says an obscure invagination does take place at a very early stage. So in the vertebrate eye most observers speak only of a thickening to form the retina, but Götte's observation points to an invagination of the optic plate at an early stage. So also in the eye of Pecten, Korschelt and Heider consider that the thickening, by which the retina is formed according to Patten, in reality hides an invagination process by means of which, as Bütschli suggests, an optic vesicle is formed in the usual manner. The retina is formed from the anterior wall of this vesicle, and is therefore inverted.
The origin of the inverted retina of the vertebrate eye does not seem to me to present any great difficulty, especially when one takes into consideration the fact that the retina is inverted in the arachnid group, only in the lateral eyes. The inversion is usually regarded as associated with the tubular formation of the vertebrate retina, and it is possible to suppose that the retina became inverted in consequence of the involvement of the eye with the gut-diverticulum. I do not myself think any such explanation is at all probable, because I cannot conceive such a process taking place without a temporary derangement—to say the least of it—of the power of vision, and as I do not believe that evolution was brought about by sudden, startling changes, but by gradual, orderly adaptations, and as I also believe in the paramount importance of the organs of vision for the evolution of all the higher types of the animal kingdom, I must believe that in the evolution from the Arthropod to the Cephalaspid, the lateral eyes remained throughout functional. I therefore, for my own part, would say that the inversion of theretina took place before the complete amalgamation with the gut-diverticulum, that, in fact, among the proto-crustacean, proto-arachnid forms there were some sufficiently arachnid to have an inverted retina, and at the same time sufficiently crustacean to possess a compound retina, and therefore a compound inverted retina after the vertebrate fashion existed in combination with the anterior gut-diverticula. Thus, when the eye and optic nerve sank into and amalgamated with the gut-diverticulum, neither the dioptric apparatus nor the nervous arrangements would suffer any alteration, and the animal throughout the whole process would possess organs of vision as good as before or after the period of transition.
Further, not only the retina but also the dioptric apparatus of the vertebrate eye point to its origin from a type that combined the peculiarities of the arachnids and the crustaceans. In the former it is difficult to speak of a true lens, the function of a lens being undertaken by the cuticular surface of the cells of the corneagen (Mark's 'lentigen'), while in the latter, in addition to the corneal covering, a true lens exists in the shape of the crystalline cones. Further, these crustacean lenses are true lenses in the vertebrate sense, in that they are formed by modified hypodermal cells, and not bulgings of the cuticle, as in the arachnid. We see, in fact, that in the compound crustacean eye an extra layer of hypodermal cells has become inserted between the cornea and the retina to form a lens. So also in the vertebrate eye the lens is formed by an extra layer of the epidermal cells between the cornea and the retina. The fact that the vertebrate eye possesses a single lens, though its retina is composed of a number of ommatidia, while the crustacean eye possesses a lens to each ommatidium, may well be a consequence of the inversion of the vertebrate retina. It is most probable, as Korschelt and Heider have pointed out, that the retina of the arachnid eyes is composed of a number of ommatidia, just as in the crustacean eyes and in the inverted eyes it is probable that the image is focussed on to the pigmented tapetal layer, and thence reflected on to the percipient visual rods. In such a method of vision a single lens is a necessity, and so it must also be if, as I suppose, eyes existed with an inverted compound retina. Owing to the crustacean affinities of such eyes, a lens would be formed and the retina would be compound: owing to the arachnid affinities, the retina would be inverted and the hypodermal cells which formed the lens would be massedtogether to form a single lens, instead of being collected in groups of four to form a series of crystalline cones.
To sum up: The study of the vertebrate eyes, both median and lateral, leads to most important conclusions as to the origin of the vertebrates, for it shows clearly that whereas, as pointed out in this and subsequent chapters, their ancestors possessed distinct arachnid characteristics, yet that they cannot have been specialized arachnids, such as our present-day forms, but rather they were of a primitive arachnid type, with distinct crustacean characteristics: animals that were both crustacean and arachnid, but not yet specialized in either direction: animals, in fact, of precisely the kind which swarmed in the seas at the time when the vertebrates first made their appearance. In the opinion of the present day, the ancestral forms of the Crustacea, which were directly derived from the Annelida, may be classed as an hypothetical group the Protostraca, the nearest approach to which is a primitive Phyllopod.
"Starting from the Protostraca," say Korschelt and Heider, "according to the present condition of our knowledge, we may, as has been already remarked, assume three great series of development of the Arthropodan stock, by the side of which a number of smaller independent branches have been retained. One of these series leads through the hypothetical primitive Phyllopod to the Crustacea; the second through the Palæostraca (Trilobita, Gigantostraca, Xiphosura) to the Arachnida; the third through forms resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and the Tardigrada must probably be regarded as smaller independent branches of the Arthropodan stock."
To these "three great series of development of the Arthropodan stock" the evidence of Ammocœtes shows that a fourth must be added, which, starting also from the Protostraca, and closely connected with the second, palæostracan branch, leads through the Cephalaspidæ to the great kingdom of the Vertebrata. Such a direct linking of the earliest vertebrates with the Annelida through the Protostraca is of the utmost importance, as will be shown later in the explanation of the origin of the vertebrate cœlom and urinary apparatus.
Summary.
The most important discovery of recent years which gives a direct clue to the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing how perfect one of them still is in Ammocœtes. The vertebrate ancestor, then, possessed two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of the origin of the vertebrate from Palæostracans it is universally admitted that all the Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which are the earliest fishes known, are all said to show the presence of a pair of median eyes as well as of a pair of lateral eyes. This evidence directly suggests that the structure of both the median and lateral vertebrate eyes ought to be very similar to that of the median and lateral arthropod eyes. Such is, indeed, found to be the case.The retina of the simplest form of eye is formed from a group of the superficial epidermal cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.In other cases a portion of the optic ganglion remains at the surface, when the brain sinks inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve. Such a retina may be called a compound retina and characterizes the lateral eyes of both crustaceans and vertebrates. Also, owing to the method of formation of the retina by invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed, may be directed towards the source of light or away from it. In the first case the retina may be called upright, in the second inverted.Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the arachnids, but not of the crustaceans.The evidence shows that all the invertebrate median eyes possess a simple upright retina, and in structure are remarkably like the right median or pineal eye of Ammocœtes; while the lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a compound inverted retina.This retina, however, is extraordinarily similar in its structure to the compound crustacean retina, and these similarities are more accentuated in the retina of the lateral eye of Petromyzon than that of the higher vertebrates.The evidence afforded by the lateral eye of the vertebrate points unmistakably to the conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid characters—belonged, therefore, to a group of animals which gave rise to both the crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which is regarded as the ancestor of both the crustaceans and arachnids.In two respects the retina of the lateral eyes of vertebrates differs from that of all arthropods, for it possesses a special supporting structure, the Müllerian fibres, which do not exist in the latter, and it is developed in connection with a tube, the optic diverticulum, which is connected on each side with the main tube of the central nervous system. These two differences are in reality one and the same, for the Müllerian fibres are the altered lining cells of the optic diverticulum, and this tube has the same significance as the rest of the tube of the nervous system; it is something which has nothing to do with the nervous portion of the retina but has become closely amalgamated with it. The explanation is, word for word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate possessed two anterior diverticula of its alimentary canal which were in close relationship to the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly allied to the trilobites, does possess two anterior diverticula of the gut which are in extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on each side.The evidence of the optic apparatus of the vertebrate points most remarkably to the derivation of the Vertebrata from the Palæostraca.
The most important discovery of recent years which gives a direct clue to the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing how perfect one of them still is in Ammocœtes. The vertebrate ancestor, then, possessed two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of the origin of the vertebrate from Palæostracans it is universally admitted that all the Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which are the earliest fishes known, are all said to show the presence of a pair of median eyes as well as of a pair of lateral eyes. This evidence directly suggests that the structure of both the median and lateral vertebrate eyes ought to be very similar to that of the median and lateral arthropod eyes. Such is, indeed, found to be the case.
The retina of the simplest form of eye is formed from a group of the superficial epidermal cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.
In other cases a portion of the optic ganglion remains at the surface, when the brain sinks inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve. Such a retina may be called a compound retina and characterizes the lateral eyes of both crustaceans and vertebrates. Also, owing to the method of formation of the retina by invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed, may be directed towards the source of light or away from it. In the first case the retina may be called upright, in the second inverted.
Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the arachnids, but not of the crustaceans.
The evidence shows that all the invertebrate median eyes possess a simple upright retina, and in structure are remarkably like the right median or pineal eye of Ammocœtes; while the lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a compound inverted retina.
This retina, however, is extraordinarily similar in its structure to the compound crustacean retina, and these similarities are more accentuated in the retina of the lateral eye of Petromyzon than that of the higher vertebrates.
The evidence afforded by the lateral eye of the vertebrate points unmistakably to the conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid characters—belonged, therefore, to a group of animals which gave rise to both the crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which is regarded as the ancestor of both the crustaceans and arachnids.In two respects the retina of the lateral eyes of vertebrates differs from that of all arthropods, for it possesses a special supporting structure, the Müllerian fibres, which do not exist in the latter, and it is developed in connection with a tube, the optic diverticulum, which is connected on each side with the main tube of the central nervous system. These two differences are in reality one and the same, for the Müllerian fibres are the altered lining cells of the optic diverticulum, and this tube has the same significance as the rest of the tube of the nervous system; it is something which has nothing to do with the nervous portion of the retina but has become closely amalgamated with it. The explanation is, word for word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate possessed two anterior diverticula of its alimentary canal which were in close relationship to the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly allied to the trilobites, does possess two anterior diverticula of the gut which are in extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on each side.
The evidence of the optic apparatus of the vertebrate points most remarkably to the derivation of the Vertebrata from the Palæostraca.
CHAPTER III
THE EVIDENCE OF THE SKELETON