Book - The Pineal Organ (1940) 24
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Chapter 24 Relation of the Median to the Lateral Eyes
In considering this question it will be necessary first to review briefly the development and structure of median and lateral eyes in the simplest forms of invertebrates, and to trace in living representatives of phyla in which bilateral symmetry has been evolved, the gradual differentiation and perfection of the paired lateral eyes of the higher types, in association with habits of life requiring accurate vision and quick perception, as for instance in the dragon-fly or cuttle-fish ; and to contrast the eyes of these with the type of eye which suffices for their free-swimming larvae, or the adult animal of lower types which have not advanced much from the larval form, and in which the adult creature lives in an environment similar to that of the larva, e.g. as in some of the non-parasitic Turbellaria. It will be necessary also to consider the conditions of life, which it may be assumed existed in the earliest stages of the ancestral history or phylogeny of these groups ; that is to say, before a terrestrial existence was possible or an active predatory and carnivorous life could have been maintained, either in the open sea, on land, or in the air. Such a life is possible only in animals with very highly evolved sensory organs, central nervous system, and organs of locomotion, such as fins, jointed-limbs, or wings. A comparison also may be made between the sense-organs of the more highly evolved types and those which live a burrowing life in the mud at the bottom of shallow waters, such as in ponds or tidal waters round the coast, or of those which lead a parasitic existence. If this is done, it will at once become evident that enormous periods of time must have elapsed in order to allow the evolutionary changes which have occurred in the higher types of animals to take place, and also that in tracing back the origin of any particular phylum, it is not the adult species of the more highly differentiated types of the orders that are likely to prove of value in assessing the relationship of different groups, but the larval forms and more particularly those species in which the larvae have a free-swimming existence in the water or on moist herbage. These considerations limit the extent of the inquiry very considerably, and imply that there can be no very close relationship between such divergent and specialized types as those of the higher Crustacea and Mollusca ; or between the higher vertebrate classes and either of the two invertebrate phyla mentioned. At the same time a comparison of the very early stages of development does indicate a descent of these highly differentiated and specialized types from an extremely remote common stock, of which we have no geological record, but which it may be supposed existed as a simple form of animal adapted for life in water or in the mud or sand at the bottom of the sea, fresh water lakes, or estuaries of rivers. We may accordingly examine such types as are living to-day under similar conditions. As examples we may mention such animals as Planaria or simple forms of Annelids ; in these we find a type of sense-organ capable of reacting to light in an animal which shows bilateral symmetry of the body generally and more particularly of the central nervous system, e.g. as in Planaria or the Annelid Hcemopis sanguisuga (Fig. 16, Chap. 3, p. 21). The essential elements for the higher types of eye are present in these simple forms, namely : pigment, a refractive medium, and neuro-sensory cells connected by nerve-fibres with the central nervous system ; we see also in Hcemopis the commencement of the differentiation of a pair of aggregate lateral eyes, and between these two simple eyes formed by the modification of a single cell. It may be conceived that the lateral paired eyes, each consisting of a group of modified cells and being situated in a more favourable position than the median eyes, would in the process of time become still more differentiated, and that in place of single receptive or neuro-sensory cells containing pigment in one part, a clear refractive mucoid material in another, and terminating in a nerve-fibre, which originates from its deep aspect, special pigment cells and refractive cells would be set apart : the pigment cells for the absorption of superfluous rays of light, the clear refractive cells for the dioptric mechanism, e.g. " lentigen cells " and " cone cells," while others will serve as retinal or receptor cells and still others for the conducting of visual impulses through one or more optic ganglia to the brain.
As the requirements became greater, still further adaptive changes ensued ; thus a hard protective covering or scleral tunic was developed around the sensitive epithelial retina ; muscles were evolved to enable independent movements of the eyeball to take place ; also an outer chitinous or cartilaginous capsule which enclosed an orbital cavity, and which at the same time provided a framework for the attachment of the specialized orbital muscles (Figs. 91 and 92, Chap. 11, pp. 129, 130) in some instances, e.g. in Sepia (Fig. 36, Chap. 3, p. 50, and Fig. 121, Chap. 12, p. 164), a movable iris-like fold and intraocular muscles were developed for regulating the amount of light entering the eye, and possibly accommodation of the lens for near or distant vision ; also a " secondary cornea " growing as a fold over the front of the eyeball and enclosing a chamber, resembling the anterior chamber of a vertebrate eye, but differing from it in its mode of formation and in communicating through an unclosed opening with the exterior and thus being filled with sea-water. We have thus, by means of what is termed " parallel development," an eye which resembles in many respects a vertebrate eye, yet differs from the vertebrate type in some very important structural details, which indicate that the differentiation and perfection of the cephalopod type of eye, must have taken many ages to complete and that the divergence of the parent stem of these higher molluscs must have taken place at a very early period in the phylogeny of the invertebrate stock. This conclusion is supported by reference to the ontogenetic development of Sepia, which was worked out by Koelliker and also by geological evidence. Embryology shows that there is a marked deviation in the course of development of the higher Mollusca from that of the simpler types, such as the Amphineura, which include Chiton (Fig. 115, Chap. 12, p. 157) and the Aplacophora, in which the bilateral symmetry of the central nervous system — which consists of paired lateral and ventral cords provided with serially arranged ganglia and united by transverse commissural bands, as in Turbellaria — remains undisturbed ; and, further, that the remarkable specialization in form which characterizes each of the higher classes of the Mollusca has not been evolved. While on the geological side, judging from the similarity of the phragmacone of living cephalopods to the ammonites, and belemnites of the Lias strata which belong to the lower series of the Jurassic period, it may be inferred that the Cephalopoda of that time were not only highly differentiated as regards their soft parts, including the eyes, but much larger than any living species. It seems obvious also that the period during which this remarkable differentiation took place in the higher types of the Cephalopoda must have extended much farther back than the Lias, in which it seems to have already attained a maximum.
If one were to judge from the higher types of Mollusca only, the differences which exist in the eyes and general anatomy of these types might lead one to think that the whole phylum of Mollusca was totally different from the phyla of other invertebrates. This is not so, however, as is most clearly seen in the ontogeny and in the adult structure of the Amphineura. The development of this class was specially studied by Kowalevsky in Chiton. In this animal the early stages of development show a typical blastocyst, gastrula, mouth (blastopore), proctodeum and anus, mesoderm formation, paired eyes and otocysts, cerebral ganglia, lateral (pleuro-visceral) and ventral (pedal) nerve cords — all of which point to a primary community of origin of the Mollusca, with other phyla of the invertebrates, and also indicate, when compared with similar stages of development in the embryos of vertebrates, that the pre-vertebrate stock from which the vertebrates arose passed through the same stages in development and had sensory organs and nervous system which was built up on essentially the same plan as that previously mentioned in connection with Planaria.
The study of the eyes of molluscs emphasizes another question bearing upon the history of the paired lateral eyes of the larva. In some instances in place of further development and differentiation taking place in the course of ontogeny, a reverse process is observed, namely degeneration. Thus in certain Lamellibranchia and in Chiton, cephalic eyes appear temporarily during the development of the larva and later disappear when, having become covered by the shell, they are rendered useless. This disappearance of the larval eyes is found chiefly in those animals which burrow in the mud, live in deep sea water, or are parasitic. In some cases they are replaced by eyes of a different type, which are secondarily acquired, e.g. the eyes on the edge of the mantle in some bivalve lamellibranchs, such as Pecten (Fig. 106, Chap. 12, p. 148) and on the back of Chiton (Fig. 115, Chap. 12, p. 157). In Chiton some of the megalaesthet.es, or large sensory organs, become transformed into what appear to be eyes. Each of these eyes is covered by a pigmented layer of cells which envelops a modified aesthete ; superficially is an arched layer of the tegmentum which forms a cornea beneath which is a lens, and a cell layer which is regarded as a retina. The individual cells of the retina are connected by nerve fibres with the nerves of the ordinary aesthetes .
In connection with the degeneration of the larval lateral eyes of Pecten and Chiton, it may be noted that the simple form of eye found in Nautilus (Fig. 112, Chap. 12, p. 152), which consists of merely a spherical optic pit, opening to the exterior by a constricted orifice with neither lens, iris-fold nor cornea, is probably an instance of arrested development of a formerly more highly developed organ rather than the retention of a primitively simple form. It may be presumed that this arrest occurs at an early stage in its formation, and it may be regarded as an inherited condition which originated in association with a change in the habits of the adult animal from a creeping or possibly actively swimming creature to a passive state in which the animal floats about enclosed in the last compartment of its rigid spiral shell (Fig. no, Chap. 12, p. 151).
The distinction between a simple structure, which is simple because it has retained its original simple form unchanged throughout its ancestral life-history, and one which is simple as the result of degeneration is in some cases very difficult and can only be decided by a very careful consideration of the animal as a whole, in all its bearings, structural and functional, embryological and phylogenetic. Unfortunately, as far as we are aware, the ontogenetic development of Nautilus has not yet been studied. The complicated structure of the adult tetrabranch Nautilus and a consideration of this structure in relation with the general principles and facts of comparative anatomy, combined with a study of the various types of extinct Nautilidse, appear to warrant the general conclusion that the mobile shell-less forms with actively functioning eyes come first in the order of evolution, and that fixation or the development of a shell is usually followed by the degeneration of the locomotive organs and of the eyes. At the same time it is almost certain that the eyes of the Nautilidse never attained the high degree of evolution which has been reached in the cuttle-fishes. The shell was developed at a very early period in the phylogenetic history of the Nautilidae, which are pre-eminently Palaeozoic in their distribution, and it seems likely that the truth lies between the two suppositions, and that the eye of Nautilus is simple partly because it has undergone degeneration and partly because it has retained its primarily simple form without having undergone a high degree of specialization.
Eyes of Mollusca
Among the various types of eye which are met with in the Mollusca it is worth while noting, with reference to the inverted eyes of Pecten, the singular compound eyes of the upright type which are found round the edge of the mantle of the " Noah's-ark " shell-fish (Area Noce). These appear on the summit of small rounded projections, and consist of a dome-shaped epithelial cap which covers and encloses a central mesodermal core. In microscopical sections (Fig. 40, Chap. 3, p. 58) the epithelial cap is seen to be composed of a single layer of tall columnar cells, which are arranged in groups or units, each of which is composed of one central visual cell surrounded by six cylindrical pigment cells, thus forming an ommatidium resembling somewhat the ommatidia of certain arthropods. The ommatidia of Area are, however, separated by slender unpigmented interstitial cells. The sensory or visual cells are conical in form, the base of the cone being slightly convex and directed outward, while the apex is directed inwards and rests on a subepithelial limiting membrane. A clear spherical nucleus is present in the superficial segment of the cell, whereas the tapering inner part of the cell has a rod-like structure. Although they resemble in certain points the simple forms of aggregate eyes of some arthropods, they are regarded as having been evolved independently and as having no genetic relation with these. Moreover, although there is a marked difference between the mantle eyes of Area and those of Pecten, the single layer of columnar pigment cells which form the iris-like zone which surrounds the cornea and lens in Pecten resemble the columnar pigment cells of the ommatidia in the eyes of Area, and they indicate a stage in the development of the eye previous to the differentiation of specialized visual cells from pigment cells. A comparison of the two types of eye also indicates the manner in which the eyes of Pecten can be developed from a simple type such as that of Area by invagination of the superficial central area of modified epithelium into the subjacent mesodermal layer which forms the central core of the tentacle. This inference is confirmed by the work of Kishinouye on the development of the marginal or mantle-eyes of Cardium nuticum (cockle), which when fully developed closely resemble those of Pecten, but differ in the existence of a " choroid " layer between the retina and tapetum and by the mesoblastic origin of the pigment layer (Fig. 243). According to the description given by Kishinouye (1894), Dot h right and left mantle edges are beset with dark-brown almost black pigment. They unite at the posterior end of the shell and form a triangular pigment area surrounding the siphonal openings. Over this area the right and left valves of the shell do not meet closely, but leave a rather wide gap. In this triangular interval the tentacles are arranged in irregular rows round the siphonal apertures. The larger and longer, which are about 100 in number, bear the eyes. They are bent away from the siphonal openings and each of them has a long band of black pigment on the siphonal side, i.e. the side exposed to the light. The eye appears as a black spot on the siphonal side of the tip of the tentacle, opposite the position of the eye in Cardium edule. A vertical section through the adult eye shows superficially a thin layer of pavement epithelium ; a central area of this epithelium, the " cornea," is unpigmented and clear ; this is surrounded by a zone of pigmented cells continuous externally with the general epithelium covering the tentacle. Beneath the cornea is a lens which is composed of flattened ectodermal cells. Its vertical diameter is considerably greater than the transverse diameter, and it is slightly constricted in the centre. In contact with the lower pole of the lens is a bilaminar, cup-shaped retina which consists of a superficial stratum of columnar cells, the outer deeper ends of which have a rod-like structure, whereas the inner segments of the cells, which are in contact superficially with the lens, are clear and contain a vesicular nucleus. The outer superficial stratum of the doublelayered retinal cup is formed by a layer of cubical epithelial cells, which is continuous, at the margin of the cup, with the layer of visual cells. Kishinouye compared this layer with the outer layer of hexagonal pigment cells of the vertebrate retina, and speaks of it as the " choroid " layer. It is covered externally by a non-cellular layer or tapetum and a thin stratum of mesoblastic cells containing pigment. Both of these layers are pierced by the fibres of the optic nerve, which is connected with the viscero-parietal ganglion. Each eye is protected by a triangular fold of the epithelium, which forms an overlapping screen. The first stage in the development of these eyes is represented by a depression of the modified central area of epithelium at the apex of an ocular tentacle which forms a hollow cup. Later, by proliferation of the cells at the bottom of the cup, a solid mass of epithelium is formed which becomes differentiated into a superficial part, the lens, and a deeper layer, the retina, one side of the cup becomes raised to form the triangular protective screen, the inner limb of this fold is continuous with the opposite lip which covers the lens and forms the cornea. The retinal portion appears to be invaginated in much the same way as in the formation of the secondary optic vesicle in vertebrates.
This brief summary of the structure and development of the various types of molluscan eyes indicates that the higher orders of Mollusca have deviated very widely from the simple forms which represent the ancestral parent stock ; also that the eyes of different orders of the Mollusca differ very widely not only from each other but from those of other Phyla of the invertebrates and from the vertebrates. The eyes of the cuttle-fish show a high degree of differentiation and have a superficial resemblance to the eyes of vertebrates, but they lack the delicate mechanisms for accommodation of the lens and adaptation of the pupil, and they differ essentially in their mode of development and structure from the vertebrate eyes. Median eyes are only seen in the larval stages of the lower classes of Mollusca ; and the early stages of ontogeny having been abbreviated or suppressed, they have entirely disappeared in the cephalopods. In the phylogeny of the phylum Mollusca, different types of eye have been evolved in different regions of the body, such as the back and edge of the mantle, by methods which involve folding and invagination of the surface layers ; these are similar to those which occur in other phyla of the invertebrates and in the vertebrates, but have been evolved by processes of parallel development and are not genetically related, and it is only in the case of the paired eyes of the simpler types of molluscs and their trochophore and veliger larvae that the genetic relation with other phyla can be traced with any degree of certainty.
The Median and Lateral Eyes of Arthropoda
It is with reference to certain classes of the Arthropoda that the most enthusiastic claims were made for the existence of a close relationship between vertebrates and invertebrates, and in view of the similarity in structure and position of the median eyes of invertebrates and the median or pineal eyes of vertebrates, it was thought that the common origin which was claimed for the Entomostracan and the pineal eyes constituted one of the most important pieces of evidence in favour of the vertebrate stock having arisen from an arthropod ancestor. The controversy was chiefly centred on the general resemblance in the form and structure of certain palaeozoic Merostomata and their living representatives, e.g. Apus, Limulus, and Scorpio, to the Ostracodermata and their living representatives, namely, the cyclostomes. The Merostomata which were specially cited included Bunodes, Eurypterus, and Pterygotus, which along with the " trilobite larva " of Limulus, were compared with Cephalaspis and Auchenaspis (Thyestes). A closer study by recent workers (Stensio, Kaier) involving the comparison of the central and peripheral nervous systems, the sense-organs, and other parts of living cyclostomes (see Fig. 22, Chap. 3, p. 28) with reconstructed casts of the cranial cavities of certain Palaeozoic fishes (Anaspida, Cephalaspidomorpha, Fig. 245), has definitely shown that the claims made by earlier authors of a close relationship between these ancient fishes and the cyclostomes were well founded. Moreover, a more exact knowledge of the mode of development of the median and lateral eyes of invertebrates has led to a better understanding of the differences which exist between the fully developed eyes of different types of the adult animal ; also the mode of development of the more complicated types from the simpler, and the way in which an eye commencing as an epithelial pit and primarily purely dermal in origin may by a process of inrolling of the skin in the region of the neural crest be carried towards the median plane and ultimately included in the membranous roof of the fore-brain vesicle (Fig. 246), where later being cut off from the skin in the process of closure of the neural tube, it finally appears as a tubular evagination of the brain. The distal end of this tube, which corresponds to the bottom of the primary dermal pit, is usually dilated and forms a vesicle in the walls of which there are a variable number of retinal placodes (two, three, or four), which are commonly grouped or fused into a single median eye. These arise as one or two pairs, the tri-placodal type being formed by the complete fusion of one pair of placodes and the incomplete fusion of the other pair, which usually lie dorsal to the single placode (Figs. 248, 249). In the process of inrolling of the ocellar pit either the superficial or the deep limb of the fold may be developed as the sensitive or retinal layer (Fig. 246, A and B). If the deeper limb of the fold is modified to form the retina the superficial or pre-retinal layer may atrophy or it may be utilized in the formation of a lens. Whereas if the more superficial limb of the fold is transformed into the sensitive layer, the deeper or post-retinal stratum may give rise to a pigment layer or a reflecting membrane (tapetum). A more common arrangement is for only the deeper part of a symmetrically formed optic pit to be modified as a sensitive or retinal area, the cells at the sides of the pit becoming elongated and transformed into lentigen, or vitreogen cells, which secrete a clear, viscous fluid. This condenses and forms a non-cellular or vitreous lens in the cavity of an ocellar pit which is usually closed by the union of the margins of the fold over the mouth of the pit and the formation of a cuticular and cellular lens superficial to the vitreous lens (Fig. 248).
Now in Apus and Branchipus Patten has demonstrated a very interesting phase of the development of the triplacodal type of median or " Ento
Fig. 248. — Cross-section of Parietal Eye Vesicle of Apus. (After Patten.) a. rt. : anterior retina. c.e.v. : common cavity of lateral and
c. : chitinous plug closing the opening
of the exterior.
common cavity to the
parietal eye vesicles. /. rt. : left retina. pg. c. : large pigment cells bounding
cavity of parietal eye vesicle.
mostracan eye." The retinal placodes, which are four in number and purely dermal in origin, are depressed so that they come to lie at the bottom of separate pits, below the general surface of the surrounding ectoderm. They approach the median plane and become infolded so as to lie in the walls of a small common pit which opens into a larger common chamber which is enclosed by the forward growth of a transverse fold of the skin. This covers over both the median and the lateral eyes. The narrow mouth of this common chamber, which appears as a small pore in the skin, becomes closed by a chitinous plug (Fig. 248), and the opening of the smaller pit in the walls of which the retinal placodes of the median eye are imbedded also becomes closed, only a small recess in the floor of the larger common chamber being left as an indication of the site of the original communication (Fig. 247). The median or parietal eye of Apus, according to Patten's description, forms a closed chamber with a retinal placode on each side wall, and two unpaired placodes one on its posterior and the other on its inner wall. Each placode consists of a single row of large colourless, columnar cells. Their distal ends are buried in a dense mass of dark brown or black pigment, their proximal ends are colourless. As in Branchipus, there are two large cells which
Fig. 249. — Triple Eye of Calanella mediterranea, a Free-swimming Copepod Crustacean, tyjwt, : from below. (After Grenacher.)
n. front : frontal nerve. p. : pigment plates of the paired eyes.
n. opt. : optic nerve. ret. c. : retinal cells.
p. 1 : pigment plate of unpaired portion.
appear to give rise to the greater part of the pigment that fills the cavity of the vesicle. When the pigment is partially dissolved it is seen that each retinal cell is capped with a large brush-like mass of fine fibres (retinidium).
Before commencing the detailed comparison of the median with the lateral eyes of arthropods it will be necessary to define the different types of eye which are met with in this phylum, which includes the Crustacea, Insecta, and Arachnoidea. Patten subdivided the eyes of Arthropoda into four types, namely :
1. Paired larval ocelli. 3. Frontal ocelli or stemmata.
2. Parietal eyes. 4. Lateral or compound eyes.
Other authors combine Patten's second and third types into a single group, namely median eyes, or speak of Patten's second group as the Entomostracan eye, and the third group as the hexapod type or frontal eyes of insects.
Patten contrasted the principal characters of his four types in the following manner :
-Triplacodal Parietal Eye of Branchipus from Patten.)
A — Axial section from front to back.
B — Median sagittal section. The three-lobed vesicle consists of right and left ectoparietal eyes and a single (probably bilateral) entoparietal eye. The cavity is completely cut off from the exterior in the adult animal. The distal ends of the retinal cells, which contain imperfectly formed rods, are turned towards the lumen of the cavity.
ant. : anterior end. n. ec. p.e. : nerve of ectoparietal eye.
ec. p.e. : ectoparietal eye. n
ect. p. : ectodermal pit. ;/
en. p.e. : entoparietal eye.
f.o. : frontal organ.
g.c. : ganglion cell.
en. p.e. : nerve of entoparietal eye. & g.f.o. : nerve and ganglion of frontal organ. c. : nucleus of pigment cell, posterior end.
i. " The larval ocelli (Fig. 78, Chap. 11, p. 117), of which there may be six pairs. These are present in the active larvae of most insects, but disappear during the metamorphosis (Coleoptera, Lepidoptera, Neuroptera, Hymenoptera). They are cup-like infoldings of the ectoderm, with upright or horizontal retinal cells or rods."
" In the insects the retinal cells are never completely inverted, and the ocelli never form unpaired eyes enclosed in a common chamber or vesicle."
2. " The parietal eye (Entomostracan eye) (Fig. 250). In the Crustacea and Arachnida two pairs of ocelli unite to form an unpaired ocellar vesicle or parietal eye. The ocellar placodes remain more or less distinct and form the side walls of the dilated anterior or distal end of the vesicle. The proximal or posterior end is generally tubular, and may open on the outer surface of the head or it may merge with the pallial folds and open into the forebrain vesicle. The parietal eye usually persists through life, and it may be the largest and most important one functionally."
3. " The frontal eyes or stemmata (Figs. 241 and 242, Chap. 23, pp. 343, 345) of insects consist of two pairs of placodes that form a median triocular group. They arise during the metamorphosis, or at any rate after the embryonic period, and are quite independent of the primary ocelli. They are never involved in a pallial fold or a common vesicle, and the retinal cells are apparently always upright. They are functional eyes only in adult insects, or in the late larval stages."
" In the arachnids and Crustacea (phyllopods, Entomostraca) the frontal eyes are present in a highly modified form as two sets of frontal organs, two paired and one unpaired. In Limulus they become the olfactory organs. In spiders and scorpions they are apparently absent. Their nerve-roots arise from the median anterior surface of the forebrain or from the anterior surface of the optic ganglia and hemisphere."
4. " The lateral or compound eyes are found in adult Insecta, Crustacea and Arachnida, including the trilobites and merostomes. Like the stemmata, their relation to the primary head segments cannot be easily determined, because at the time that the cephalic lobes are most clearly segmented, as in the embryonic stages of Acilius and the scorpion, the lateral eyes are absent and they do not appear, if at all, until near the close of larval life. In Limulus they belong to the cheliceral segment ; in insects they appear to belong to the antennal segment. The development of the lateral eyes is essentially the same in all arthropods. They are developed from large, crescentic placodes lying near the posterolateral margin of the cephalic lobes, close to the infolding for the optic ganglion, but they never lie inside the fold, and the visual cells are never inverted. The entire visual layer is formed from a single layer of primary ectoderm. The placodes are frequently divided, or may be entirely separated into two distinct parts, which differ in their histological characters and in function (Hymenoptera, Neuroptera, Coleoptera). One part may be especially well developed in males (Ephemeridse), or one may serve for vision under water and the other for vision in air."
Now Patten in using the term " parietal eye " to denote the entomostracan type of median eye, definitely assumed not only that this type of invertebrate eye resembled in its structure and mode of development the parietal eye of vertebrates but that this correspondence of the parietal eyes was merely one of many structural and developmental resemblances between the entomostracan and vertebrate phyla. He, further, presumed that there was a long period of functional activity of the median eye or eyes of vertebrates, during which the lateral eyes passed through a corresponding period of inactivity while they were evolving and before, as he states, " they again became functional." During this period he believed that the parietal eyes were the only functional visual organs. This latter concept is, we believe, for many reasons untenable, of which we may mention : (1) that the general tendency is for inactive or nonfunctioning organs to regress rather than evolve ; and (2) that the oldest fossil fishes which are known, such as the Anaspidee and Cephalaspidae, lived during the same period side by side with the giant forms of marine Merostomata, e.g. Eurypterus and Pterygotus. In both the ostracoderm fishes and the Merostomata many of the principal distinguishing characters of the two phyla were already developed, including those relating to the cranial skeleton, the brain and cranial nerves, the sense-organs, branchial and vascular systems (Figs. 238, 245), and we may therefore conclude that although the parietal eyes of living Cyclostomes and Entomostraca resemble each other in certain respects and their special characters appear to have persisted for an immensely long period without undergoing any essential changes in structure, there has during this period been a simultaneous and gradual evolution with differentiation of the lateral eyes of both invertebrates and vertebrates. As the differentiation of each type became more pronounced, namely, the invertebrate and vertebrate types, so they diverged more and more from the simple form of eye, from which they both originated, and from each other ; whereas the median or parietal eyes tended to regress rather than evolve, except in certain genera among Arthropoda, Monorhina, Amphibia, and Reptilea. These evolutionary changes have resulted in two widely different types of eye, one of which is upright and faceted, while the other has an inverted retina and single lens protected by a smooth transparent cornea. Thus at the present day it is difficult to believe that these two types of eye could have arisen from a common ancestral form. Intermediate stages are found between simple ocelli and the compound faceted eyes of invertebrates, but the special characters of the vertebrate eye seem to have been already evolved in the earliest fishes of which we have any geological record. It is true that the geological evidence is circumstantial rather than direct, but the close agreement in the structural details of the cranial skeletons of many of the extinct Palaeozoic fishes with their living representatives — showing that the brain, cranial nerves, vestibule, and semicircular canals along with the branchial and vascular systems were in essential points alike — fully warrants the assumption that the lateral eyes of the extinct ostracoderm fishes and living Cyclostomata were essentially alike. In other words, that the lateral eyes of the cephalaspid and other fossil fishes were of the inverted type found in living species, and that they were functional. Moreover, judging from the small size of the parietal foramina or impressions in many of the extinct Palaeozoic fishes as compared with the size of the orbital cavities ; and that in many cases the parietal canal did not even pierce the roof of the skull, it must be concluded that the lateral eyes were the chief organs of vision, and that in those cases in which the parietal canal did not pierce the vault of the skull, the parietal eye was completely functionless as a visual organ.
The replacement of the simple ocellar eyes of the larva by the compound eyes of the imago has been clearly demonstrated in the water beetle, Dytiscus marginalis, by Giinther, and has already been alluded to, p. 117. It will be necessary, however, to describe in detail the differentiation of the crescentic or kidney-shaped area of epithelium called the " optic plate " or " rudiment of the lateral eye " in order to compare the ommatidia of the lateral eye with the ocelli which precede it (Figs. 78, 79, 80, 81, Chap. 11, pp. 11 7-1 19). The cells of the optic plate elongate and also proliferate and those in the centre of the plate become grouped into cylindrical unit-systems or retinulce. Their protoplasm becomes clear, and some of the nuclei assume a deeper position. Each group consists of eight cells, one central and seven peripheral. The central cell and six of the peripheral cells take part in the formation of a retinula, while the seventh cell of the peripheral series is pushed out of the system. The central cell, which is flask-shaped in its basal portion where the nucleus is situated, tapers into a fine rod as it approaches the surface. The remaining six cells with the central basal cell form the visual portion of the retinula or rhabdome. Superficial to each retinula and between it and the ectoderm are especially modified clear ectoderm cells containing vacuoles, which give rise to the crystalline cones and together with these form the ommatidia. Each crystalline cone is formed by the amalgamation of four clear rods and each of these rods is derived from a refractile vesicle which is contained in one of the clear ectoderm cells. Later the four rods cohere and give rise to a crystalline cone, as in the crustacean, Palcemon (Fig. 37, Chap. 3, p. 52). The cells between adjacent ommatidia extend through the entire thickness of the ectoderm and secrete pigment, while superficially the cuticle, which covers the whole area, is secreted by small corneagen or lentigen cells. The cuticular layer is thickened over the distal ends of each ommatidium to form a plano-convex cuticular lens.
Now in comparing the compound lateral eyes with the simple eyes of the larva, it may be presumed that each ommatidium of the compound eye represents an ocellus and that the whole compound eye is formed by an aggregate of modified ocelli. The process of pit formation by a downgrowth of epithelial cells, however, appears to have been abbreviated in the ontogenetic development of the ommatidia, and although a potential lumen may be considered to exist in the central axis of each ommatidium, there is no actual cavity containing a vitreous mass, as in the simple ocellus (Fig. 38, Chap. 3, p. 53). There is nevertheless a distinct resemblance between the single basal cell of the rhabdome surrounded by the six peripheral cells of each retinula of the compound eye and the two large visual cells, with the adjacent long, slender cells surrounding it, which lie at the bottom of the pit of the simple ocellus. The probability of compound eyes being formed by an aggregation of simple ocelli is, moreover, supported by intermediate conditions between compound and simple eyes, such as are seen in the lateral and central eyes of Euscorpius (Figs. 94, 95, Chap. 11, pp. 132, 133) and Limulus (Figs. 86, 87. Chap. 11, pp. 124, 125), in both of which there is a combination of retinular or rhabdome formation with a continuous corneal surface, suggesting fusion of phylogenetically separate ocelli. The schizochroal eyes of Phacops, a Trilobite of the Devonian period, also support the view that the compound lateral eyes arise phylogenetically by fusion of adjacent ocelli into a single optic plate, for in Phacops the ommatidia are definitely isolated and separated by interstitial areas of the test (Fig. 98, Chap. 11, p. 137), the corneal facets thus being discontinuous, as contrasted with the more common continuous corneal surface of the holochroal type.
The various stages by which the simple upright eyes of the lower types of arthropods appear to have been transformed in the course of phylogeny into the paired compound eyes of the higher types has been studied by Lankester, Parker, Patten, Watase, and others, who have also endeavoured to fill in and explain certain intermediate stages between the larval and adult forms, which are often absent in those arthropods in which there is a definite metamorphosis in the life history of the animal, as for instance in many of the Insecta. In tracing these changes, it will be necessary first to review briefly some of the causes of these alterations in form and structure.
Starting with the assumption that the effect of light on protoplasm is to produce a chemical change that results in the formation of pigment, and further that this process renders the cell at once capable of absorbing light and reacting to it, we may note next that this reaction is a chemical change which is accompanied by decomposition of a part of the protoplasm of the cell during the manufacture of the pigment, and further that the work done by the action of the rays of light involves an expenditure of energy which affects the nerve-endings in the base of the cell. Thus one part of the cell becomes differentiated as a receptor and transmitter of nerve-impulses, and another the pigmented part for the absorption of light. Later special cells or parts of cells are differentiated to form the refractile elements, namely, the vitreous, the cuticular lenses or facets, and the rods and cones ; these lie for the most part superficially and at the clear distal ends of the cells or of the ommatidia. The intracellular pigment or the specialized pigment cells are displaced towards or are formed in the peripheral parts of the cells, or ommatidia ; whereas the neuro-sensory cells tend to sink beneath the level of the surrounding epithelium, and in this way a pit or downgrowth of epithelium is produced from which nerve-fibres pass to a sub-epithelial plexus or join to form an optic nerve. This process of pit-formation or downgrowth of the neuro-sensory cells may occur singly as in the formation of a simple ocellus, or as an aggregate of unit systems within a definite optic area. The unit systems or retinulae may lie close together, being covered by a continuous layer of the cuticle, which in some cases is thickened over each retinula so as to form a corneal lens, as in the lateral eyes of Limulus (Fig. 86, Chap. 11, p. 124), or to a less extent as in the minute planoconvex lenses which are present in the adult lateral eyes of Agelena and Dytiscus marginalis (Fig. 81, Chap. 11, p. 119), or again in the faceted eyes of many insects. In some cases the units may be widely separated from each other, as in the schizochroal type of trilobite eye (Fig. 98), this form being generally regarded as the more primitive as compared with the holochroal forms in which the corneal lenses form a continuous surface. In many cases there lies in front of or superficial to the retinular layer, and between it and the cuticle, a stratum of modified epithelial or " hypodermal " cells. This layer is usually produced by the infolding of the epithelial cells surrounding the mouth of a single visual pit, as in the larval eye of Dytiscus (Fig. 4, Chap. 3, p. 10), or as in the median eyes of Limulus (Fig. 87, Chap. 11, p. 125), and Scorpio (Fig. 251), by a single layer which covers the whole sensory placode. In these cases the retinal layer of the larva is, when first formed, frequently inverted, and the eye consists of three layers, pre-retinal, retinal, and post-retinal, a condition which is somewhat similar to that found in the lateral eyes of vertebrates, but which must not be regarded as foreshadowing the evolution of the lateral vertebrate type, as the two types of eye occur at the ends of two widely divergent stems or phyla of the animal kingdom, and have most probably arisen independently.
The folding of the epithelial layers which has produced the reversal of the retinal layer must be considered as a mechanical process which in the invertebrate phylum Arthropoda has involved the median paired eyes of certain groups — for instance Apus, Branchipus, Scorpio, Limulus — whereas in other examples the median paired eyes have retained the primary upright character, as in Acilius (Fig. 82, Chap. 11, p. 120), the blowfly (Fig. 9, Chap. 3, p. 14), and the median or parietal eyes of vertebrates (Fig. 134, Chap. 17, p. 188, Fig. 183, Chap. 20, p. 259, and Fig. 252.
Besides this differentiation in the extent of the superficial area of the
Fig. 251. — A Transverse Section of the Right Median Eye of an Adult Scorpion. (After G. H. Parker.)
Compare with Fig. 253, D. The primary cavity has been obliterated by adhesion of the retinal and post-retinal layers, along the line marked fis.
c, c'., c." : outer, middle, and inner
layers of cuticle. hyp. : hypodermis. lens : cuticular lens. nib. : basement membrane. m. pr. r. : pre-retinal membrane. mes. : thin mesodermal layer between
retina and pre-retina.
n. pr. r.c. : nucleus of pre-retinal cell.
pig. : pigment.
po. r. : post-retina.
pr. r. : preretina (lentigen or vitreous
layer), r. : retina. rhb. : rhabdites. scl. : sclera.
derma which is involved in the formation of an optic placode, there is a tendency in the more highly differentiated eyes to increase in depth. The central neuro-sensory cells become more and more depressed and at the same time elongated ; moreover, their distal ends become modified to form vertical rods, whereas the distal ends of the cells lining the sides of the pit form short rods which tend to become horizontal, and near the mouth of the pit secrete a vitreous mass which serves as a lens. In the adult eyes of the higher types of insects and many of the Crustacea the unit systems or ommatidia tend to elongate still further and undergo further specialization. In some cases this elongation of the ommatidia is attended by cell division, so that the retinal layer becomes changed from the primary single layer of epithelial cells to one consisting of two or more layers of cells, while in others the increase in depth is considered to be brought about by simple elongation of the cells, which extend the whole distance from basement membrane to cuticle (Patten). In the eyes of many of the Crustacea there is an extension outward so that the retinal layer becomes raised on the summit of a movable stalk which contains a
Fig. 252. — Longitudinal Section of Sphenodon Embryo (II, Stage S 1 ). (After Dendy, showing Pineal Region.)
C. Ab. : commissura abberans. C.P. : commissura posterior. C.S. : commissura superior. Ch. P. : choroid plexus. D.S. : dorsal sac. Par. : paraphysis.
Par. PI. : parietal plug.
P.E. : pineal eye.
P.R. : pineal recess.
P.S. : pineal sac.
S.C.O. : subcommissural organ.
S.O.C. : supraoccipital cartilage.
series of secondary neurones ; these form one, two, or three optic ganglia which connect the retinal ganglion with the supra-oesophageal ganglion or fore-brain. Intermediate conditions are found between the more primitive and less specialized types in which only one optic ganglion is present, as in Branchipus, and the more highly evolved genera such as Astacus, in which three are present.
In the median eyes (parietal or entomostracan) the development of the placodes does not extend beyond the one-layered simple type of retina, and in some the visual layer becomes inverted and the placodes of opposite sides become fused in the median plane.
1 See reference, p. 262. 24
G. H. Parker, in his description of the eyes of scorpions (1886), made some important observations on their mode of development and of the changes which take place in the relation of the fibres of the optic nerve
Fig. 253. — Sections Illustrating the Development of the Median Eyes of a Scorpion. (After G. H. Parker.)
A — Right face of an approximately vertical section of a young embryo. The lower part of the section is in the median plane ; the upper part is slightly to the right of the median plane. The opening of the cavity has commenced and is seen at the upper part of the fissure. The retina is inverted, its morphologically superficial surface being situated on its deep aspect and directed towards the brain, from which it is separated by the post-retinal layer.
B C, D, E, F — Transverse sections at successively higher levels, through the same region of a slightly older embryo, when the cavity has been fully opened out. It shows the division of the stem of the Y-shaped cavity into two limbs, which constitute the cavities of the right and left median eyes. It also shows the emergence of the fibres of the optic nerve from the superficial aspect of the retina. In the adult scorpion this relation is reversed.
cav. : cavity.
enc. : brain.
fis. : lower end of cleft-like fissure
beneath the fold forming the retina
and pre-retina. hyp. : hypodermis. mb. : basement membrane. mes. : mesoderm.
in the transitional period between the larva and the fully developed animal. He also made some interesting comparisons between the median and the lateral eyes. He noted that the median eyes are situated close
n.f. : nerve-fibres.
nl. mes. : nucleus of mesoderm cell.
n. opt. : optic nerve.
po. r. : post-retina.
pr. r. : pre-retina.
r. : retina.
sep. : septum.
to the sagittal plane a little in front of the centre of the shield ; they are two in number and always symmetrically placed. The lateral eyes form two isolated groups one on either side at the edge of the shield, where its anterior border meets its lateral margin. In different genera the number of eyes in each group varies from two to seven. Two kinds of lateral eyes can be distinguished, the larger or " principal " and the smaller or " accessory." No essential difference exists between these two groups. As in the spiders, the vitreous and retinal layers are separate (Fig. 251). These apparently two-layered eyes were described by Lankester and Bourne as diplostichous. Locy and Patten, however, recognized that these eyes are in reality three layered or triplostichous. Patten (1886) also claimed that the median eyes of scorpions were formed from a cup-like involution of the ectoderm. Parker fully confirmed
Fig. 254. — A — Section through Early Stage of One of the Developing Median Eyes of Agelena. B — Section through a Later Stage of a Developing Median Eye. (After Kishinouye.)
br. 3 : third lobe of brain. ch. g. : cheliceral ganglion. oc. m. : mouth of ocellar pit of median eye.
pr. : post-retinal layer. r. : retina. vit. : vitreous.
these earlier observations, and showed that inversion of the retina takes place during the development of the median eyes of the scorpion, Centrums, as has been demonstrated in the median eyes of the spider Agelena by Locy and Kishinouye (Fig. 254, A), but in the scorpion the two optic pits or sacs are united by a common stalk (Fig. 253, B, C, D, E, p. 370), whereas in the spider they appear as independent involutions. In the scorpion the fibres of the optic nerve arise at first from the superficial surface of the retinal layer (Fig. 253, D, E, F), and pass out beneath the superficial layer of the fold of ectoderm which covers the retina (Fig. 253, A, pr.r. and F, «./.). This superficial layer of the fold is known as the pre-retinal layer, and gives rise to the vitreous or lentigen stratum. During the later stages of development there is little change in the point of exit of the fibres of the optic nerve ; it simply shifts from the embryonic postero-lateral position to the postero-ventral position of the adult. There is, however, a very important and significant change in the course of the intracapsular fibres, which apparently shift from an attachment at the outer ends of the retinal cells to the inner or deep ends of these cells, which is the position they occupy in the adult. The migration of the fibres takes place at the same time that the nuclei recede into the deeper parts of the eye, and seems to be controlled by the growth of the rhabdomeres. The exact process by which the change from the inverted embryonic position of the retina to the final adult position takes place are still obscure ; but it is quite clear that as a result of the involution of the retinal area its morphologically deep surface, from which the nervefibres primarily arise, becomes turned towards the light, and that its originally superficial surface, in which the clear refractile rods become developed in upright eyes, is in the early larval stages turned away from the light. This apparently anomalous condition is changed during the later stages of development, so that eventually the nerve-fibres are found leaving the deep ends of the retinal cells, and in the adult animal the rods or rhabdomeres are pre-nuclear and directed towards the light. It is also certain that the retinas of both median and lateral eyes are strictly hypodermal in origin and not neural. Owing to the manner in which the involution takes place the median eye is of the three-layered, triplostichous type, the superficial layer of the hypodermis or pre-retina giving rise to the vitreous body and cuticular lens ; the middle layer forming the inverted retina and the third or deep layer forming the sclera, which becomes intimately fused with the retina. The retina contains two kinds of cells, the visual or neuro-sensory cells and pigment cells. Pigment is also contained in the neuro-sensory cells. The walls of these cells develop pre-nuclear rhabdomeres, and a nerve-fibre emerges from their deep ends.
Parker draws the following conclusions with regard to the adaptive structural changes which follow the inversion of the retina in the median eyes of scorpions and spiders : " The striking similarity in the structure and development of the median eyes of scorpions and the median eyes of spiders has already been indicated. In both cases the retina by a process of involution has become inverted. The question whether the retina was functional during the involution of the eye was answered in the affirmative by the phases noted in the development of the optic nerve. At least the fact that the fibres of the optic nerve are at first attached to the morphological deep ends of the retinal cells, and only at a later date come to emerge from the opposite end, is most easily explainable on the supposition that the retina was functional before involution. The primitive eye would then consist of a single layer of retinal cells, from the deep ends of which the nerve-fibres emerge. Admitting that in the ancestral eye the rhabdomes were in their usual position, namely, at the outer end of each retinal cell, an inversion of the retina would not only place the optic fibres on the front face of the retina, but the rhabdomeres would come to occupy the deep ends of the cells. The prenuclear rhabdomeres of the median eyes in scorpions must then be secondary structures, developed in such a way as to replace functionally the older post-nuclear structures. The sphaeospheres, as Mark has suggested, may represent the remains of post-nuclear rhabdomeres. These are then to be regarded as disappearing, and the fact that in some species of scorpions they are present while in others they are absent, would favour this view."
' The possible relation of the median eyes to the lateral eyes in scorpions has already suggested itself, for in pointing out the probable nature of the phylogenetic antecedent of the median eyes a condition has been implied which agrees with the essential features of the lateral eyes. Of all the eyes in spiders and scorpions, the lateral eyes in scorpions are undoubtedly the least complicated, and they may be looked upon as deviating least from the ancestral type."
Fig. 255. — Section through One of the Developing Lateral Eyes of a Spider, Agelena. (After Kishinouye.)
The retina is upright, and the nerve-fibres issue from the base of the retinal papilla.
n.f : nerve-fibres. r. oc. I. : retina of lateral eye.
These considerations, taken in connection with papers published by Mark (1887) and Patten (1889) on the mode of development of the eyes of Apus, Branchipus, spiders, and scorpions, provide a clear explanation of the means by which a pre-retinal cellular and cuticular lens are formed in continuity with a retinal and post-retinal layer, by a simple folding of the hypodermis in such a way that the developing eye appears in transverse section as an S-shaped bend (Fig. 254, A : compare Fig. 255, which shows the position of the nerve-fibres in the upright lateral eyes of the spider Agelena). The middle segment of the bend is inverted and becomes the retina, and when the visual cells become differentiated their outer ends, which give rise to the rods and which were primarily directed towards the surface, become deep ; whereas their inner ends, which terminate in nerve-fibres, become superficial and lie in contact with the cellular or vitreous lens which is developed as a modification of the ordinary hypodermal cells where they cover the retinal layer. The lower limb of the S-shaped bend or post-retinal layer, may become condensed so as to form a protective capsule (Fig. 251) or it may become pigmented. Further, as described by Parker in the later stages of development of the median eyes of scorpions (and spiders), the reversal of the cell-elements caused by the infolding of the retina is corrected by migration of the nerve-fibres to the deep surface of the retina and the development of secondary rods at the primarily inner ends of the cells, which as a result of the inversion are now superficial.
We shall see later that the same processes which occur in the development of certain invertebrate eyes also take place in the evolution and development of vertebrate eyes. With certain modifications as regards details, there are to be noted a similar infolding of the ectoderm and inclusion of the optic plates within the neural tube ; there is also a similar differentiation of the retinal ectoderm into dioptric, receptive, and pigment cells (Fig. 256 ; and Fig. 10, Chap. 3, p. 15). There is, moreover, the same migration of cell-elements or their nuclei within the retina, and a similar connection of the visual receptive cells with the cortex of the brain by means of nerve ganglia and intervening plexiform bands or tracts of nerve-fibres.
In a work limited co the study of the morphology of the pineal organ it would be inexpedient to do more than allude to the controversial points which have been raised in the past by such authors as Patten and Gaskell in relation to the important bearing which it has on the whole problem of the evolution of vertebrates. But in this discussion on the relation of the median and lateral eyes of vertebrates and invertebrates, it is necessary that the reader should have a concise statement of these views in order that he may judge for himself the principles that are concerned in coming to a decision on this complex biological problem, and which we shall endeavour to summarize in our concluding chapter.
We propose in the first place to give a summary in his own words of some of the more important views of Patten on the parietal eye of vertebrates, which, as we have already stated, he regards as homologous with the median eyes of the Entomostraca.
1. "In vertebrates we recognize as belonging to the forebrain the median or parietal eyes, the lateral eyes, and the olfactory organs. At an early embryonic period they lie on the outer margins of the open neural plate in similar positions to the ones they occupy in arthropods.
2. " The parietal eye : there are probably two pairs of ocellar placodes that for a short time occupy this marginal position (Fig. 257). Later they are caught in the pallial overgrowth and carried on the inner limb of the closing neural crests to the median line. There they form a group of one, two, or three placodes lying in the membranous roof of the brain. During or after the closing of the cerebral vesicle the brain roof is evaginated at the place where the ocelli are located, thus forming a sac or tube in the blind end of which the ocellar placodes lie. The development is essentially like that of the parietal eye of Limulus and the scorpion. This fact demonstrates that the parietal eye of crustaceans and arachnids is a true cerebral eye in the vertebrate sense, and is identical with the eye of vertebrates.
Fig. 256. — Section through a Developing Lateral Eye of Ammoccetes, showing the inversion of the optic cup ; and the lens vesicle, superFICIAL to which is a Thick Layer of Mesenchyme. (After Beard.)
/. ves. : lens vesicle. sens. l.r. : sensory layer of retina.
mes. : mesodermal layer of cornea. vitr. ch. : vitreous chamber.
pig. l.r. : pigment layer of retina.
3. " The lateral eyes of vertebrates : these represent the compound or convex eyes of arthropods that have been transferred to the interior of the cerebral vesicle. In the arthropods the lateral eyes lie near the margin of the cephalic lobes on the outer edge of a deep ganglionic infolding.
In invertebrates they are seen in a very similar position on the lateral margin of the open medullary plate. Later they are swept into the infolding brain, turning the retinas inside out. They then grow out laterally on the end of membranous tubes in much the same manner as the median eyes. In arthropods the lateral eyes usually have a crescentic or kidney-shaped outline ; in vertebrates this shape is retained, giving the retinas their characteristic crescentic outline during the early stages. When the two limbs of the crescent unite a circular retina is produced, giving rise to the choroid fissure and the centrally located optic nerve, that, together with the inverted rods and cones, have long been such inexplicable features of the lateral eyes of vertebrates.
Fig. 257. — Embryo Spider. (After Patten.)
Showing the primary position of the olfactory placodes, olf., the parietal eye placodes, pa.e 1 , and pa.e 2 , and the lateral eye placodes, le. These are situated on the edge of the neural crest, n. cr.
ch. : cheliceral ganglion. op. 1. : optic lobe.
le. g. : lateral eye ganglion. ro. : rostrum.
m. ch. : median chord. st. : stomodceum. ol. I. : olfactory lobe.
4. " The parietal eye : all vertebrates possess remnants more or less distinct of a median or parietal eye, which in some forms contains true retinal cells and visual rods, and is connected by several (? four) distinct nerves with as many ganglia.
5. " There is but one median or parietal eye, consisting, however, of several parts.
6. " The eye proper consists of three or four sensory placodes, each one representing the retina of a simple ocellus of the arthropod type. The placodes form the walls of a sac on the end of a membranous tube projecting from the roof of the ' tween-brain.'
7. " The placodes have a paired arrangement and probably represent two pairs of ocelli, located originally in the ectoderm just outside the lateral margins of the open medullary plate.
8. " They were ultimately forced into or carried into the brain chamber by the same forces which produced the brain infolding. The placodes are carried on the crest of the brain, infolding towards the median line, meanwhile shifting from the outer to the inner limb of the fold (Fig. 246, p. 349). When the crests unite, the four placodes form a compact group on the membranous roof of the brain. At that point a tubular outgrowth of varying length is formed, which has a vesicle or dilatation at its distal end, in the walls of which the placodes lie (Figs. 247^
Fig. 258. — Embryo Spider seen from the Side, showing the Position of the Placodes which will give rise to the Median and the Lateral Eyes. (After Patten.)
abd. ap. : 2, 3, 4 : abdominal appendages.
an. pi. : anal plate.
ch. : chelicera.
c. nv. : cephalic navel (do. dorsal organ).
co. : commissure.
ht. : heart.
/. 1, 2, 3, 4 : legs. Lb.: lung book. I.e.: lateral eyes. m. e. : median eyes. ped. : pedipalp. st. : stomodseum.
p. 350, and 248, p. 360). This vesicle with its four placodes is the parietal eye. The primary vesicle may now be constricted, forming two unpaired lobes, or the lobes may separate, forming two separate sacs, a larger anterior and outer one, the ' ectoparietal eye,' containing the two most highly developed placodes, and an inner posterior one or ' entoparietal ' eye containing the remaining two placodes completely united into one organ and with greatly reduced structural details.
9. " The membranous tube or epiphysis may disappear in whole or in part, leaving the terminal eye-sacs either isolated or united by distinct nerves with the parietal eye-ganglia or habenular ganglia (Fig. 250, p. 362).
10. " The parietal eye of vertebrates is homologous with the parietal eye of such arthropods as Limulus, scorpions, spiders, phyllopods, copepods, trilobites, and merostomes, but not with the frontal stemmata or other ocelli of insects.
11. "In the arthropods various stages in the evolution of a cerebral eye are shown in detail, from functionless eyes on the outer margin of the cephalic lobes to a median group of ocelli enclosed within a tubular outgrowth of the brain roof.
12. " The most primitive type of a parietal eye is shown in the nauplii of phyllopods and Entomostraca, where the eye is a pear-shaped sac opening by a median pore or tube on the outer surface of the head (Fig. 247, p. 350). In the higher arachnids the process of forming an eye vesicle is merged with the process of forming a cerebral vesicle, the external opening of the forebrain vesicle and that of the parietal tube forming a common opening or neuropore.
Fig. 259. — Anterior and Ventral Aspect of an Embryo Spider, showing the Position of the Median (Parietal) Eyes and the Lateral Eyes. (After Patten.)
chl. : chelicera. ped. : pedipalp.
I.e. : lateral eyes. ro. & st. : rostrum and stomodseum.
m.e. : median eye. 1, 2, 3, 4 : first to fourth legs.
13. " The parietal eye of arthropods is an important visual organ until the lateral eyes which represent a later product are fully developed. It may then diminish in size and in activity, but it rarely if ever wholly disappears.
14. " During the evolution of vertebrates from arachnids there was a considerable period during which the lateral eyes were adjusting themselves to their new position inside the brain chamber, when they were in functional abeyance. At this period ancestral vertebrates were monoculate, that is, they were dependent solely on the parietal eye, which had come to them from their arachnid ancestors as an efficient and completely formed organ. When the lateral eyes again became functional the parietal eye began to decrease in size and effectiveness.
15. " The parietal eye is the only one now present in tunicates (Fig. 260). In the oldest ostracoderms, like Pteraspis, Cyathaspis, Palceaspis, the lateral eyes are absent or at least do not reach the surface of the head ; the functional one being the parietal eye, which is of unusual size.
1 6. " In the lampreys we see the same conditions, the parietal eye being well developed in the larvae, while the lateral eyes are deeply buried in the tissues of the head and are useless. During the transformation the lateral eyes again become functional and the parietal eye begins to atrophy, finally losing many of its structural details and its function, although still retaining very nearly its original form."
From this brief summary of some of the more important observations and conclusions relating to the morphology of the parietal eye of vertebrates and of the parietal eyes of the Entomostraca, it will be obvious that
Fig. 260. — Ventral Cord and Ocelli of Adult Cyclosalpa. Seen from neural surface. (after patten.)
A. : anterior end. n.e. 2 : nerve of posterior ocellus.
e 1 , e- : anterior and posterior ocelli. P. : posterior end.
me. c. : mesocephalon. pr. c. : procephalon.
Patten believed not only that there was a close connection between the arthropod stock and the Ostracodermi but that the vertebrate phylum actually originated from arachnids (p. 378, para 14). Further, it is clear that he believed the evidence afforded by his observations on the parietal eyes in the two groups strongly supported this conclusion which he had come to from his extensive study of eyes in general and of other systems and organs in various classes of invertebrate animals, of which we may specially mention that of the eyes of Acilius, Vespa, Pecten, and Area. This material, taken along with that afforded by other pioneer workers such as Reichenbach, Huxley, Grenacher, Bertkau, and Lowne, and more recent writers on the Palaeozoic fishes and merostomata, provides a basis for more accurate generalizations on the main principles which are concerned in the evolution of the different types of eye, and of evolution in general, than it was at the close of the nineteenth century. It is, we believe, now more generally appreciated that many of the striking similarities which undoubtedly exist in certain of the more highly differentiated animals which belong to otherwise dissimilar or divergent classes are not necessarily to be regarded as evidence of a close relationship between these classes ; and that if the environmental conditions remain similar, functioning organs which are well adapted to their environment and requirements may continue with little change for immensely long periods of time. While, on the other hand, differences such as exist between the more highly differentiated types of the principal phyla and classes also require an immensely long period of time to evolve. Moreover, the differences which constitute class distinctions were in many cases already established in the earliest known fossil representatives of the classes concerned, e.g. to take a concrete instance, between the Merostomes and Ostracodermi. This indicates that the period in which the hypothetical common ancestral stock existed must have extended many ages further back than that of which we have definite geological evidence.
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