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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF

   The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography
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Chapter 23 The Geological Evidence of the Existence of Median Eyes in Extinct Vertebrates

The anatomical evidence of the existence of both median and lateral eyes in the extinct order of Palaeozoic fishes known as the Ostracodermata, which lived in the Silurian and Devonian periods, is of great value not only with reference to the antiquity of the vertebrate eyes but also with respect to the constancy of the pattern formed by the relative positions of the cavities and impressions on the head-shields which lodged the sense-organs, namely : the small pineal impression in the centre ; the orbital cavities for the lateral eyes on each side ; the narial and hypophyseal apertures in front and the smooth " glabellar plate " forming the roof of the cranial cavity which contained the brain behind l (Fig. 228). Moreover, palaeontology affords very strong evidence of the bilateral origin and nature of the pineal body, e.g. the existence of two impressions placed side by side on the outer surface of the pineal plate of Pholidosteus and Rhinosteus, recorded by Stensio (Fig. 229) ; the two pits also placed side by side but on the inner surface of the pineal plate of Titanichthys, described by E. S. Woodward (Fig. 230), along with the heart-shaped foramen figured by E. S. Hill on the dorsal aspect of the skull of Dipnorhynchus (Fig. 140, Chap. 18, p. 200) and the similar heart-shaped pit on the intracranial aspect of the pineal plate of Dinichthys intermedius shown in the drawing reproduced from Adolf Heintz (Fig. 231).

All these examples tend to confirm the similar conclusions with respect to the bilateral origin of the pineal which have been founded on the comparative anatomy and comparative embryology of living species and advocated by Cameron (pp. 292, 294), Dendy (p. 245), Gaskell (p. 187), Hill (p. 218), Kingsbury (p. 215), Locy (p. 202), and others. Further, the palasontological evidence of the position and relations of the orbital cavities and pineal impressions in fossil vertebrates, more especially in the head-shields of the Ostracodermata, taken along with other evidence of a more general character seems to indicate the existence at a very early period of a common ancestral stock from which arose the prevertebrate stem of these Palaeozoic fishes and the main or parent stem from which certain extinct and living arthropods have descended. The resemblances are found chiefly in the head region and are most evident in the Eurypterida and Trilobites among the extinct classes, and in Limulus, Apus, and Lepidurus among living species. But the fundamental differences in the relative positions of the thoracic and abdominal viscera between vertebrates and invertebrates (Fig. 68, Chap, 11, p. 106) precludes the assumption of there being any close relationship between these divergent types ; and they indicate that the period during which the divergence of the vertebrate from the invertebrate stock and the primary changes in the evolution of their respective types of lateral eyes took place must have been infinitely remote and probably occurred in a very simple form of animal showing bilateral symmetry and paired ocelli of the simple upright type. Thus it may be conceived that the eyes of the prototype animal, from which by gradual differentiation the median and lateral eyes of both invertebrate and vertebrate animals have been evolved, resembled the simple paired eyes of the type found in living Planaria and Turbellaria or in the Annelid Hcemopis sanguisuga (Fig. 16, Chap. 3, p. 21). In these animals there is a bilaterally disposed nervous system ; the eyes lie on the dorsal aspect of the head, in or near the surface, and they are connected by nerve-fibres with a supra-oesophageal ganglion which represents the brain. Further it is probable that the rudimentary eyes of this prototype animal, which were at first freely exposed on the dorsal surface of the head — no carapace or head-shield as yet having been evolved — as they gradually developed into more complex organs which, with the brain and other delicate parts of the head, required protection from injury, were afforded this protection by the deposition of a chitinous shield around the ocelli and over the surrounding soft parts, leaving a transparent epithelial or horny covering for the eyes, which became transformed into either a faceted or a continuous smooth cornea. Moreover, as the type of animal became more active and left the mud or sand at the bottom to swim in the water, the more lateral eyes, being more suitably placed than the median eyes, became more highly developed ; while the latter either retained their original simple character or degenerated, and in some cases when displaced towards the median plane, by the great development of the lateral eyes, the median pair of eyes fused with each other to form a cyclops eye, as in Tabanus (Fig. 20, Chap. 3, p. 26) and in many of the Crustacea. In the stem which branched off to form the vertebrates it may be supposed that in the evolution of the lateral eyes the simple optic pit which arose as a downgrowth from the medullary plate (Figs. 5 and 6) became transformed into a stalked vesicle which afterwards came in contact with the surface layer of epithelium, as occurs in the ontogenetic development of the typical lateral eyes of vertebrates, and that this stage was followed or accompanied by inversion of the retina and the development of an epithelial type of lens from the overlying ectoderm. Further, that at an early stage in the evolution of the lateral eyes a cartilaginous or bony capsule was formed around each, and served as a special protective covering corresponding to the similar nasal and otic capsules.

When we examine the skulls of the earliest fossil fishes, such as the Anaspida of the Silurian period (Fig. 232) or that of Osteolepis (Fig. 132, Chap. 17, p. 182), a lobe or paddle-finned fish of the Devonian period, we find that the orbital cavities for the lateral eyes, are surrounded by a series of flat plates — in Pterolepis (Fig. 232) six in number on each side — and that there is a single pineal foramen in the centre between the two fused frontal bones. It seems probable, therefore, that the median eyes or their stalks were already fused, or possibly that one member of the pair had become suppressed owing to the more active growth of the other member of the pair. The median posterior part of the skull in these fishes (Fig. 132) was separated by an interval or articulation from the anterior part formed by the frontals, post-frontals, post-orbitals, and squamosals. This median part had grown backwards over the hinder part of the brain and formed the parietal, supratemporal, and occipital region of the skull, whereas the lateral horns which bounded the gap on each side gave rise to the squamosal plates and maxillae. In the course of evolution of the brain it seems that the pineal region has — relatively to the cerebral hemispheres — been gradually displaced backwards ; thus the pineal foramen in the earlier types of skull such as Osteolepis lies in the same transverse plane as the orbital cavities for the lateral eyes and between the frontal bones. In later types the foramen lies between the parietal bones, where, owing to the formation of the median anteroposterior crest formed between the two temporal muscles and running backwards to the occipital region (Figs. 205, 206, Chap. 22, pp. 300, 301, and Fig. 233), it eventually becomes obliterated. The direction of the pineal foramen or plate seems also to have changed in the course of time. Thus in the restoration of the skull of Dinichthys intermedins (Fig. 234) the plate seems to have been directed forward as well as upwards, in Osteolepis directly upwards ; while the apex of the pineal organ, inside the skull, in adult mammals is directed backwards over the quadrigeminal plate and towards the vermis of the cerebellum. The relative position of the pineal organ to the skull and the fore-brain is greatly affected in the human subject and in mammals generally by the growth of the hemispheres, which as they enlarge in a forward direction are also bent ventral ward, thus forming the primary or cephalic flexure. The pineal organ remains for a time in close relation to the membranous capsule of the brain and skin at the summit of this flexure. In this primarily superficial position the pineal organ of amphibians and reptiles appears to have attained its highest degree of development, as is evidenced by the large size of the pineal foramen in certain of the extinct amphibia, e.g. Protriton (Fig. 170), and Procolophon (Fig. 170, a), Chap. 19, p. 237. More over in the more primitive types of living reptiles, such as Sphenodon, the organ is found to be more highly differentiated than in the less primitive ; but in all living reptiles, including Sphenodon, the pineal eye frequently shows signs of degeneration, such as the development of pigment in the lens or the frequent absence of its nerve in the adult animal, and judging from the large size of the pineal canal in some extinct amphibia and reptiles the organ in living species is relatively extremely small.

1 The glabellar plate corresponds to the " dorsal electric field, a slightly depressed spear-shaped area, which Stensio considers may have lodged a dorsal median electric organ.

Fig. 228. — Restoration of Cephalic Shield of Ki^eraspis auchenaspidoides. Dorsal View. (After Stensio.)

c. : cornu. na 1 : opening of hypophyseal sac.

dsf. : dorsal electric field. na 2 : nasal opening.

d.sp. : dorsal spine. orb. : orbital opening.

/. en. : fossa circumnasalis. ps. : pectoral sinus.

Isf. : lateral electrical field. pin. : pineal foramen.

Fig. 229. — Dorsal Aspect of the Parietal Plates of : A — Pholidosteus ; B — Rhinosteus, showing Paired Pineal Pits. (After Erik-a-son Stensio.)

In Rhinosteus the left pit is completely preserved as an impression, whereas the impression of the right pit has been broken off. ( X 5 diameters.)

Fig. 230. — Pineal Plate of Titanichthys, from the Upper Devonian of Ohio, U.S.A. (inner view, one-half natural size). Paired Pineal Pits or Openings are seen in the Middle Area of Thickened Bone. (After A. S. Woodward.)

b. : radiating spicules of bone. p.p. : paired pineal impressions.

p. pi. : pineal plate.


A — Part of the roof of the skull, viewed from above.

B — The same area, seen from the inside, and showing the parietal pit on the inner aspect of the pineal plate. (After Heintz, A.)

C. : central plate. P. : parietal or pineal plate.

C.P. : central part. M.B. : median basal plate.

br. : branch of central part. Pr. O. : preorbital.

R. : rostrum.

Fig. 232. — Schematic Sketches of the Cranial Roof of the Norwegian Anaspida, showing the Pineal Plate and Foramen ; the Orbital Sceral Plates and General Arrangement of the Scales. (After Kiaer.) a. : Pterolepis. b. : Pharyngolepis. c. : Rhyncholepis.

Fig. 233. — View from above of the Skull of an Ictidosaurian Reptile, showing the median parietal crest. all trace of a parietal foramen has disappeared. there are two occipital condyles, and other mamMALIAN Characteristics. (After Broom.)

F. : frontal. L. : lacrimal. N. : nasal.

J. : jugular. Mx. : maxilla. Par. : parietal.

This difference in size of the pineal foramen suggests that in those extinct animals in which the foramen was large, the organ itself may have been not only larger but also more highly differentiated, and may even have served as a visual organ, as contrasted with a light-perceiving organ.

Fig. 234. — Restoration of Skull of Dinichthys intermedius, viewed from in Front and showing the Position of the Pineal Plate. (After Anatol Heintz.)

But in many of the fossil skulls of fishes, amphibians, and reptiles the foramen is closed or small and in some completely absent, indicating that the organ had become vestigial in these animals at a very early period of their evolution.

The pineal impression in certain of the ancient Ostracoderm fishes is, as we have already mentioned, seen only on the inner surface of the pineal plate, the outer or superficial surface in some cases being smooth (Fig. 234), while in others an external impression is also visible, but it is less deep than the pit found on the intracranial surface. These fossil markings correspond closely with the conditions which are found in many living examples of the more primitive types of cartilaginous and bony fishes, such as the spiny dogfish (Spinax niger) (Fig. 49) and the spoonbill (Polyodori) (Fig. 50, Chap. 3, p. 73, and Fig. 235). In these the pineal canal does not perforate the roof of the skull and open on the dorsal aspect, but ends blindly, and the pineal organ consists merely of the basal portion and the stalk. The distal end of the stalk is sometimes slightly expanded, but there is no differentiation of a definite eye, such as is seen in Petromyzon, and it is probable that the small pear-shaped vesicle at the distal end of the hollow stalk does not represent the parietal organ. The appearances in the fossil fishes may be accounted for by supposing that : (1) during the ontogenetic development of the cranial roof, the pineal diverticulum was not only surrounded circumferentially by the developing membrane-bone but also covered by it ; or, in other words, the pineal organ never pierced the skull (Fig. 235). Moreover, it may be inferred that the cases included in this category were : (a) those in which single or paired parietal sense-organs were developed but not fully differentiated as an eye or eyes (Fig. 177, p. 248) ; (b) those in which the regression or arrest of development was more pronounced and the pineal apparatus was represented merely by the unpaired base and stalk of the pineal organ (Fig. 49, Chap. 3, p. 73, and Fig. 153, Chap. 18, p. 219). (2) Those in which the growth of the pineal organ was more vigorous and in the development of the roof of the skull, the stalk of the organ was surrounded on all sides by the developing bone, but the pineal canal not being closed over superficially by a covering layer of bone, the terminal vesicle (or pair of vesicles) was left free to develop outside the skull in the subepidermal areolar tissue. Further, it may be inferred that the cases comprised in this second category may be subdivided into two groups : (a) those in which a more or less fully differentiated eye was developed with an optic

Fig. 235. — Lateral View of the Brain and Pineal Organ of Polyodon folium. The Cranial Capsule has been opened from One Side. (After Garman.)

Cr. : primordial cranium. Opt.

Hm. : hemisphere. Pn.

Olf. : olfactory nerve. St. :

optic nerve. end-vesicle of pineal organ, stalk of pineal organ.


nerve connecting it with the central nervous system (Fig. 134, Chap. 17, p. 188, and Figs. 183, 185, Chap. 20, pp. 259, 261), and (b) those in which the parietal sense-organ was constricted off from its stalk by the growth of the skull and was left as a vestigial cyst (or pair of such cysts) in the subepidermal extracranial areolar tissue (Figs. 162, 167). In the latter all connection with the central nervous system would have been severed and the parietal vesicle would have been quite functionless as a sensory organ. In some instances an impression or pair of impressions was left on the dorsal aspect of the pineal plate (Fig. 229) ; in others the development of the vesicles may have been less pronounced, and though they may have persisted in the adult animal — as is the case with Stieda's " frontal organ " in the frog — they left no impression on the outer surface of the skull.

On comparing the pineal pits and impressions which have been observed in the skulls of fossil Ostracoderm fishes with those in the skulls of living species, it may be concluded that certain of the more ancient extinct specimens fully confirm the evidence which has been independently obtained from both embryonic and adult specimens of the bilateral origin of the pineal body. The palaeontological evidence also points to regressive changes having already commenced in some of the most primitive and earliest known fishes, and having continued in their descendants until the present time. Nevertheless vestiges of the pineal system have persisted in nearly all types of living vertebrates, and structural changes have taken place in the organ which have suggested the occurrence of an evolutionary transformation into an organ of internal secretion. These changes are, however, much more pronounced in the embryonic stages of development and the early post-natal period of life than they are in the adult animal, in which evidence of regression is nearly always present.

Now, it is interesting in connection with the general occurrence of regressive changes which have taken place in the pineal system since the period when it appears to have attained its highest degree of development, in the extinct amphibians and reptiles of the late Palaeozoic and the Mesozoic periods, to recall that among the existing Dipnoan fishes — Lepidosiren, Protopterus, and Ceratodus — which in some respects appear to be intermediate between fishes and amphibians, their general features when compared with the early fossil fishes have remained essentially unchanged since the middle of the Devonian period, and such changes as have been observed are regressive rather than evolutionary. According to the description given of them by Smith- Woodward, " they have in the interval merely abandoned the fusiform shape which is adapted for a free-swimming life and become more or less eel-shaped in adaptation to a wriggling and grovelling existence at the bottom of the rivers into


which the last survivors retreated at the end of the Mesozoic era." In the extinct Dipnoi (Dipterus, Ctenodus, Sagenodus) found in the Old Red Sandstone (Devonian) the general form of the animal resembles the primitive fish Osteolepis (Fig. 132, Chap. 17, p. 183), and the roof of the skull consists of numerous small, flat, dermal bones most of which are paired. No parietal foramen was, however, present in the extinct Dipnoi, nor is a pineal foramen present in Ceratodus, the most primitive of the living Dipnoi (Fig. 156, Chap. 18, p. 222), and in this animal it will be seen that extensive fusion of the bones of the skull has taken place, only two elongated bones being present in the median region of the roof of the

Fig. 236. — Dorsal View of Head of Dipterus, showing Dermal Bones of Roof of Skull and Pores of the Lateral-line System in One of the Earliest Known Fishes. (After Goodrich.)

Fr. : frontal. Po. : postorbital.

//. : interfrontal. Poc. : postoccipital.

It. : intertemporal. So. : supraorbital.

Pa. : parietal. Soc. : supraoccipital.

Pf. : prefrontal. Ta. : tabular.

skull, namely the " ethmoid " — probably including the frontal, interfrontal, and prefrontal elements — and the " occipital," which is formed by the fusion of the parietals with the supraoccipital. The Dipnoi, therefore, although in other respects they constitute a very interesting primitive type of fish, are to be regarded as a degenerating side-branch and not likely to afford any clue as to the origin of the pineal organ, which even in the fossil types belonging to this order did not perforate the skull.

According to the description given by A. S. Woodward in 1922, the real links between the fishes and amphibians appear to be the paddlefinned fishes — Crossopterygyii — including Polypterus and Calamoichthys, the ancestors of which, e.g. Osteolepis (Fig. 132, Chap. 17, p. 183) and


Diplopterus, showed a pineal foramen in the same position as in the skulls of the Stegocephala (Fig. 170, Chap. 19, p. 237). Some of these also resemble the Stegocephala in the possession of a ring of sclerotic plates around the lateral eyes. The skull of one of the living representatives of the order — Polypterus — shows no pineal foramen, but there is a wellmarked pineal diverticulum in the embryo. Moreover, in the nearly related order Chondrostei — including Acipenser (sturgeon) (Figs. 147, 148, Chap. 18, p. 211) and Polyodon (spoonbill) (Fig. 235) — and in the order Holostei — which includes Lepidosteus (bony pike) and Amia calva (bow fin) — the pineal organ, although vestigial, is easily recognizable in the adult animal, and in Amia shows evidence of bilaterality (Fig. 149, Chap. 18, p. 213).

The position and relations of the pineal organ and foramen in living Crossopterygii and allied living orders, when compared with the extinct Osteolepis, appear to confirm the connection of the Crossopterygian fishes with the more ancient type of Osteichthyes, which on account of other general resemblances is considered to be close to the parent stock from which the Stegocephala and modern Amphibia have arisen. This is a point of very considerable interest, since it is in some of the Stegocephala and extinct reptiles of the Carboniferous, Permian, and Mesozoic periods that the pineal organ seems to have reached its greatest size and possibly highest degree of differentiation.

Among extinct Amphibia, the parietal foramen is found to be of very variable size. It is, relatively to the size of the skull, very large in Protriton, one of the smaller Stegocephala (Fig. 170, Chap. 19, p. 237). In this primitive animal the pineal foramen lies between the parietal bones in a transverse plane behind the orbital cavities. The lateral eyes were protected by a ring of bony plates in the sclerotic ; these are similar to those in Ichthyosaurus, and they probably indicate that either Protriton itself or its ancestors were able to dive to great depths in the water, and that they served to resist the pressure of the water on the eyeballs. In the skulls of Branchiosaurus ambly stoma and Metanerpeton (Fig. 169, A, Chap. 19, p. 236), which represent primitive examples of the Stegocephala, the pineal foramen was also large.

The pineal foramen was, relatively to the size of the skull, of moderate size in Eryops megacephalus and in the curious snake-like Dolichosoma longissimum (Fig. 169, B, Chap. 19, p. 236), found in the Permian strata of Bohemia. In the skull of this animal the frontals and parietals are fused into a single elongated plate near the hinder end of which the circular pineal foramen is conspicuous owing to its margins being slightly raised above the general surface of the skull.

In Diplocaulus magnicornis (Fig. 168, Chap. 19, p. 235) — Permian of



Texas — the foramen is very small, and it is also small in Palceogyrinus — Carboniferous — and in Trematosaurus. In the latter the foramen is situated far back in the roof of the skull behind the central point of the interparietal suture.

In the adult skulls of living Anura {Rand) and living Urodela (Molge) the pineal foramen is usually absent, although a slight depression is visible on the dorsal aspect of the skull, in the usual situation of the foramen in the skull of Cryptobranchus japonicus.

Since some of the extinct Labyrinthodonts were of gigantic size — the skull of L.Jcegeri measuring more than 3 ft. in length and 2 ft. in breadth — it may be presumed that the actual size of the pineal eye was proportionately large and also more highly differentiated than in modern Amphibia, in which the terminal vesicle is constricted off during the larval or tadpole stage of development by the growth of the skull, and since it is completely separated from its connection with the brain it can have no function as a visual organ.

It is in some of the extinct carnivorous reptiles of the Mesozoic period that the pineal foramen attained its maximum size, both relatively to the size of the skull in some of the smaller animals and actually in some of the larger types, such as the Ichthyosauri — Lias, Oolitic, Chalk (Fig. 190, Chap. 20, p. 268). It was also large in the mammal-like or Theromorph reptiles, including Titanosuchus, in which the diameter of the foramen was approximately 1 centimetre (Watson) (Fig. 237).

The foramen is more circular in the primitive flat-headed types such as Conodectes (Permian), Captorhinus, and Procolophon (Fig. 171, Chap. 19, p. 237). It is more oval in form in those types in which the cranial cavity is narrowed in association with elongation of the skull and a high degree of development of the temporal fossae with their contained masticatory muscles, as in the Anomodont reptile Dicynodon (Broom) (Fig. 205, p. 300). In Dicynodon the canines were very large, and in the dog-toothed Cynodont reptiles in which incisor and molar teeth were also present both these and the canines were well developed. Moreover, associated with this development of the teeth there was a corresponding development of the masticatory muscles, including the temporals, which grew upwards over the roof of the skull, where they became attached to a median sagittal crest, within which was the pineal canal. Finally, as the muscles increased in size and the crest became deeper and narrower, the pineal canal within it became obliterated, as in Ictidosaurus (Fig. 233, p. 332).

Having referred to the regressive changes which have occurred in the pineal system, during the period which has elapsed since the Palaeozoic era, in the sub-classes Teleostomi and Dipnoi, with special references to the supposed origin of the amphibians and reptiles from extinct repre



sentatives of the Crossopterygian fishes, it will be necessary to consider the relations of the Cyclostomata to the fossil representatives of this order, and in particular to the Anaspida, Cephalaspida, and Palaeospondylus. We have already seen that in amphibians and reptiles the pineal system attained a maximum development in the extinct Labyrinthodonts

Fig. 237. — Reconstruction of Skull of Titanosuchus, a Mammal-like Reptile of S. Africa. (After D. M. Watson.)

The specimen shows a large parietal canal for the pineal organ, " the walls of which form a special little projection raising the opening more than a centimetre above the general line of the surrounding bone."

A — Dorsal aspect of skull. B — Posterior aspect. C — Lateral aspect.

/. PAR : interparietal. SQ. : squamosal.

PC. : parietal canal. TAB. : tabulare.

PO. : post-orbital.

In the account of another specimen (Mormosaurus seeleyi) belonging to the same order, Deinocephalia, Professor Watson describes the parietal canal as " a long cylindrical tunnel " which lies in the median suture between the two parietal bones.

and in the Ichthyosauri and Plesiosauri, during the late Palaeozoic (Amphibia) and the Mesozoic periods (reptiles). Many of the extinct forms seem to have died out completely and left no representatives ; while in the modern amphibians and reptiles the parietal sense-organ is less developed and shows signs of degeneration. Similar changes appear to have occurred also both generally and in the pineal system of Cyclo


stomes, and at one time it was thought that the Cyclostomes were a degenerate form of the true fishes. Now, although the older view that the Cyclostomes are degenerate fishes has been replaced by the modern conception that the living Cyclostomes, with their fossil representatives the Ostracodermi, constitute a separate and distinct branch of the phylum Vertebrata, there is no doubt that in many respects the living Cyclostomes are less highly developed than their fossil ancestors. Thus the living lampreys and hag-fishes have no exoskeleton, no pectoral fins, and in the hag-fishes not only are the lateral eyes vestigial and sunk beneath the skin, but the static organ has only one semicircular canal, as compared with two (anterior and posterior) in some of the Ostracodermi, and according to Studnicka the pineal organ (at any rate in some specimens) is absent altogether.

In considering the markings found in the head-shields of the subclass Ostracodermata it is necessary to inquire whether there is anything which corresponds to the head-shield of these most primitive extinct vertebrates among living species of vertebrates. Now Gaskell (1908) showed that the head-shield of the larval Petromyzon closely resembles that of the extinct Palaeozoic fishes (Cephalaspidae), not only in general form (Fig. 135, Chap. 17, p. 191) but in the structure of the mucocartilage, which forms the branchial skeleton and the dorsal and ventral head-plates of the Ammoccetes larva. This is a peculiar type of embryonic cartilage which consists of fibrils whose direction is mainly at right angles to the investing layers of perichondrium ; these fibres are intersected by others which run parallel to the surfaces of the plate. At the points of intersection of the fibrils are star-shaped cells, and the spaces enclosed between the fibrils are filled with a semi-fluid mucoid material which stains a purple colour with thionin. A somewhat similar structure is seen in the head-shields of the extinct Cephalaspid fishes in which there is an appearance of spaces enclosed by osseous laminae running at right-angles to each other. These appearances suggest the existence of a fibro-cartilaginous matrix which, having become calcified, formed a hard plate, different from bone in the absence of definite Haversian systems showing concentric laminae, surrounding the vascular canals.

Since Gaskell's time the structure of the head-shields of the Cephalaspids has been studied in detail by Stensio (1927), who describes an exoskeleton consisting of superficial, middle, and basal layers (Fig. 238). The basal layer shows thin fibrous laminae enclosing numerous cellspaces, the fibres in each lamina being arranged in such a way that they are nearly at right-angles to the laminae next above and below. He differs from Gaskell, however, in that he considers the basal layer is composed of true laminated bone. He further describes minutely the relation of



the vascular canals contained in the three layers mentioned, with reference to certain polygonal areas and inter-areal grooves which are present in some of the specimens, and small pores which open on the smooth,



Fig. 238. — Two Schematic Sections through the Head of a Cephalaspid

Fish. (After Stensio.) A — Median sagittal section. B — Transverse section through the posterior part of the otic region. The exo skeleton shown with thick lines, and the perichondral-bone layers with

fine lines, cartilage dotted.

a. eff. com. : space for the arteria efferens communis.

aort. : aortic canal.

aort. gr. : aortic groove.

a. marg. : canal for marginal artery.

ch. : notochord.

c. sem. post. : posterior semicircular canal.

c.v. : cranial cavity.

d.p.r. : area which bounded the mouthcavity on the dorsal side.

d.e.s. : canal for the electric nerve to the dorsal electric field, dsf.

l.s.f. : lateral electrical field.

m. : mouth opening.

na 1 J rna 2 : nasal opening 4- the opening

of the hypophyseal sac. obr. ch. : oralobranchial chamber. oes.+tr. : opening for the oesophagus

and trachea + the truncus arteriosus

in the posterior branchial wall. pin. : pineal foramen. pr. au. : otic prominence. r. aort. : right aorta and aortic ridge. sel. : nerve canal to lateral electrical

field. vest. : vestibular division of labyrinth


shining external surface, these appear to have transmitted the external branches from a series of radiating canals originating from a subepidermal vascular plexus. The superficial layer consists of dentine — orthodentine and osteodentine — which, as the dentine canals diminish in diameter as


they course distally, acquires an enamel-like appearance. This agrees very closely with the enamel which occurs in the true fishes. Beneath the superficial layer are pulp-like cavities, from which radiating " dentinal canals " issue. From its minute structure Stensio concludes that the

i/i Apis. A fossil bee, 2/1 after Maye. Fossil


Fig. 239.

A — Xylocopa senilis. Miocene. Baden. (After Heer.)

snowing ocelli and lateral compound eyes. B — Prionomyrmex longiceps. Oligocene, Baltic amber.

ant, showing ocelli and compound eyes.

(From von Zittel.)

exoskeleton with the exception of the most superficial part of the superficial layer which was formed by the epidermis must have arisen in the corium, and also that it occupied the corium in its entire thickness. In


Fig. 240.

A — Aracheoniscns Brodei. Purbeck, Wiltshire. A small fossil crustacean similar in type to the wood lice of the living species. (After Woodward.)

B — Eosphaeroma Brougniarti. Middle Oligocene, Butte de Chaumont, near Paris. (After Quensted.) Order Edriophthalmia. Sub-order Isopoda. Both specimens show large sessile eyes on the sides of the head. (From von Zittel.)

Thyestes verrucosus, the superficial layer is incomplete, and is limited to the tubercles, the middle layer being exposed in the inter-areal grooves.

Beside the exoskeleton there was beneath it a continuous bony endoskeleton, in which was lodged the brain, sense-organs, cranial nerves and vessels, and the branchial system ; and Stensio considers that it is probable


that the exoskeleton did not give rise to the endoskeleton, as has been generally maintained, but that the exoskeleton and the endoskeleton were formed simultaneously.

Moreover, since Gaskell's time the general architecture of the Ostracoderm fishes has been studied with great exactitude by Stensio, Kaier, and other Norwegian palaeontologists, who by the use of modern methods of research have been enabled to study the markings on the interior of the shields and obtain from serial sections reconstruction models representing complete casts of the cavity of the skull (Fig. 238, A and B, p. 341). Wax models of the interior of the skull have been made in much the same

Fig. 241. — Front of Head of Porthetis spinosus, Transvaal, showing the three Frontal Ocelli, arranged in the Typical Triangular Manner, One in front, Two behind. (Redrawn from Cambridge Natural History.)

way as embryological material is reconstructed from serial sections by the wax-plate method of Born. The reconstructions made from serial sections of the fossil specimens are magnified sufficiently to allow of a detailed study being made of the main cavities which enclosed the brain and branchial system ; the minor cavities and canals which contained the cranial nerves, venous sinuses, and cerebral arteries ; the afferent and efferent vessels of the branchial apparatus ; also the orbital cavities ; labyrinth ; naso-hypophyseal canal ; pineal recess ; and the canals for the nerves and vessels leading to the impressions which Stensio describes as the median " dorsal electrical field," and the marginal or " lateral electrical fields " on the dorsal aspect of the shield, Fig. 228.

From an extensive study of a very large amount of material obtained from the Downtonian and Devonian strata of Spitzbergen and other


sources, Stensio has classed vertebrate Crania into two main branches, namely, those without jaws — Agnathi ; and those with jaws — Gnathostomi. The Agnathi comprise the Class Ostracodermi or Cyclostomata, which he subdivides into two subclasses with their respective Orders as shown in the following table :


Branch I : Agnathi

Class : Ostracodermi (Cyclostomata) Subclass A : Pteraspidomorphi Order i : Heterostraci „ 2 : Palaeospondyloidea „ 3 : Myxinoidea Subclass B : Cephalaspidomorphi Order I : Osteostraci „ 2 : Anaspida „ 3 : Petromyzontia

Branch II : Gnathostomi

Stensio concludes that the Ostracodermi constitute a group of primarily agnathous craniate vertebrates " which have nothing to do with either the Arthrodira or the Antiarchi, and are true fishes related to the Elasmobranchs."

Of the two living Orders of the Ostracodermi, the Myxinoidea (hagfish) is included in the pteraspid Subclass A and the Petromyzontia (lampreys) belong to the cephalaspid Subclass B.

In the subclass Pteraspidomorphi the rostral part of the head formed by the ethmoidal region and the common naso-hypophyseal opening lie on the ventral side of the head, close in front of the mouth, whereas in the subclass Cephalaspidomorphi the rostral part of the head is formed by the excessively developed upper lip, and is thus of visceral origin ; moreover, owing to the great development of the upper lip the nasohypophysial opening is carried upward on to the dorsal aspect of the head, where it lies immediately in front of the pineal foramen. Stensio also concludes that the hag-fishes and lampreys being persistent representatives of the extinct Ostracoderms must be a primitively low type of agnathous craniate vertebrate and not degenerate descendants of the true fishes, but it is also quite clear with regard to the skeleton that they have undergone regressive changes, and that the absence of pectoral fins, which is characteristic of the living species, is one of these secondary regressive changes.

We may add, further, that the degenerate condition of the parietal sense-organs, more especially of the left pineal eye, is an additional fact



in support of Stensio's conclusion that the Myxinoidea and Petromyzontia have undergone regressive changes in their descent from the ostracoderm ancestral stock.

Now in the Downtonian strata of Ringerike, about 35 miles northwest of Oslo, there have been found, according to the description given

Fig. 242.

-Front of Head of Copiophora cornuta (female). Locustid^e.

Demerara —

There is a high degree of development of the median frontal eye which forms a conspicuous feature of the Frons ; the other two ocelli are poorly developed and are placed one on each side of the curious frontal cone. (Redrawn from Cambridge Natural History.)

by Kaier, large quantities of the remains of fossil crustaceans belonging to the order Merostoma (e.g. small and giant Eurypterids). In close proximity to these, embedded in a stratum termed by him the " fish horizon," are found numerous examples of the Ostracodermi and more particularly of the groups Anaspids, Cephalaspidae, Ccelolepidee, the head shields of which are represented in a schematic manner in Fig. 232. Although the general exoskeleton of the Anaspidae consists mainly of


small lancet-shaped scales ; on the dorsal aspect of the head there is in the centre a well-defined median pineal plate perforated by a pineal foramen. The pineal plate lies between the orbital cavities, which are surrounded by a ring of orbital plates ; and in front is a single — naso inh.s. oc.tent. v iiUijuLmJ

^ tr. sc



Fig. 243. — The Eyes of Cardium nuticum. A — tentacles bearing eyes around siphonal opening.

B — section through eye, showing the relations of the lens, retina, rods, " choroid," non-cellular tapetum, and pigment layers. 1, 2, 3, 4, 5 : stages in the development of the eyes. c. : cornea. ch. : " choroid." ep. inv. : epithelial invagination. inh. s. : inhalant siphon. /. : lens. op. n. : optic nerve.

(After Kishinouye.) The structure and development of these eyes resemble in certain respects that of the lateral eyes of vertebrates.

hypophyseal — opening similar to that in Kaieraspis auchenaspidoides (Fig. 228). Moreover, behind the pineal plate there is an elongated oval area marked by small scales and corresponding to the glabellar plate or post-pineal area of Auchenaspis verrucosus. The Anaspida show no traces

pig. : pigment.

rd. : rods.

ret. : retina.

tap. : tapetum.

tr. sc. : triangular screen.


of a bony skeleton, although it is probable that a cartilaginous endoskeleton did in reality exist, but it has left no evidence of its presence, the cartilage not having been fully calcified and thus not being preserved. The strata, according to Kaier, appear to have been laid down rapidly at the estuaries of large rivers which in periods of floods deposited thick layers of mud, in which the fossils were imbedded and preserved in situ. The remains are thus in some cases remarkably complete and perfect, and although sometimes distorted, it is possible to reconstruct the parts and make comparisons of the different species, as shown in Fig. 244, in which it will be seen that a series or branchial apertures are present which resemble those in living species of Marsipobranchia or cyclostomes ; and also that pectoral spines were present which it is believed correspond to the pectoral fins of fishes. Moreover, since the olfactory organs of the cyclostomes, although opening on the surface by a single aperture, are,

Fig. 244. — Reconstruction of Pterolepis nitidus. (After Kiaer.) Downtonian strata. rlngerike, showing orbital scleral plates, ten branchial apertures, rudimentary pectoral fin, and hypocercal tail.

as was strongly emphasized by Gaskell, truly bilateral in nature — there being paired olfactory nerves and paired olfactory lobes of the brain both in the living and extinct species of Cyclostomata — and since there is a close correspondence in the brain, sense-organs, cranial nerves, and many other structural points between the cyclostomes and fishes, it is obvious that they must have originated from a common stock ; and the circumstance that the existence of a single nasal aperture in the class Cyclostomata has been employed as the basis of their classification as a separate branch — " Monorhina " — must not be thought to imply that they are totally different and have nothing to do with the class Pisces. Further, the presence of a pineal foramen located in the centre of a pineal plate which is situated between the orbital cavities and the proof of the bilateral nature of both the pineal and olfactory organs constitute along with other structural resemblances important evidence substantiating this conclusion.

Geological Evidence of the existence of Median Eyes in Invertebrates

In tracing the evolution of median eyes from the earliest known classes of fossil animals which show indications of such, we may consider


first that great class of extinct animals the trilobites (Figs. 97, 100, 102, Chap. 11). These vary greatly in size and form, and they range from the Cambrian to the late Carboniferous and Permian eras. According to recent estimates of geologists the Cambrian age comprises a period from 500,000,000 to 750,000,000 years ago, and the Permian, which marks the

Fig. 245. — Attempted Restoration of Brain of Kiaeraspis in dorsal view. (After Stensio.) Compare with Fig. 22, Chapter 3, p. 28.

die. : diencephalon. mec. :

ep. : epiphysis. me d. .

f. rh. : fossa rhomboidalis. met. :

hab. I. : left habenular ganglion. olf. c.

hab. r. : right habenular ganglion. sp. d.

hy. s. : hypophyseal sac. sp. v.




olfactory capsule. dorsal roots of spinal nerves, ventral roots of spinal nerves.

lat. : roots of the prootic lateral nerves. /. to X. : cranial nerves 1 to 10.

end of the Palaeozoic period, from 215,000,000 to 280,000,000 years ago. When the first trilobites existed other invertebrate phyla were abundant, and it is considered that the trilobites and kindred types were preceded by a hypothetical class for which the name Protostraca has been suggested, the term signifying the earliest type of animals possessing a shell — the



name " Palaeostraca " being reserved for the whole group of known fossils which includes such primitive types of marine arthropods as the Gigantostraca, including Eurypterus, Pterygotus (Fig. 96, Chap. 11, p. 135), and Stylonurus, sometimes spoken of as the sea scorpions, and also the Xiphosura or Merostomata, of which Limulus, the king crab, is a living example (Fig. 69, Chap. 11, p. 108).

The trilobites have affinities with both the Crustacea and the arachnids, or spiders, and are usually allocated to a separate subclass or are classified as an appendix of the Crustacea or Arachnida, the trilobites being regarded as a precursor of both. The trilobites and all the marine fossil arthropods

A — Section through the parietal eye vesicle of a scorpion (stage H), showing the approaching pallial folds previous to the union of the two retinas.

B — Section through the parietal eye of a newly born scorpion showing the parietal eye vesicles and the ventricle, V., bounded by the optic ganglia, the pallial folds and the neuromeres of the fore-brain. The ventricle extends downward and forward into the cavities of the olfactory lobes, ol. v.

at. : atrium, or cavity of parietal eye


v. : olfactory ventricle.



e. : parietal eye.

br. n. 2 : dicephalon.


e.g. : parietal eye ganglion

c.g.n. : corneagen.


pallial groove.

cor : commissure of dicephalon.

r. .


oe. : oesophagus.

s. :

median sulcus.

ol. I. : olfactory lobe.

v. .


mentioned above, include types which either in the adult or larval form show indications of (1) both median and lateral eyes, (2) only median eyes, (3) only lateral eyes. In some, known as " blind trilobites," eyes are said to be absent. It is probable, however, that in many adult specimens which have been described as blind, median eyes may have been present in the larval form which disappeared and left no traces of their existence in the adult animal. The lateral eyes of trilobites are usually faceted or aggregate eyes. These are borne on the movable or lateral cheeks and, as have been described on p. 137, are of two types : (1) holochroal, in which the visual area is covered by a smooth, continuous film



or cornea through which the lenses of the ommatidia are visible by translucence — Asaphus, Illcenus, Calymene (Fig. 97, A) ; and (2) schizochroal, in which the cornea is transected by protrusion of the sclera between the ommatidia and is limited to or present only on the surfaces of the ommatidia (Fig. 97) — Phacops, Daimanites, ? Harpes. Barrande recognized a third type which is exemplified in the genus Harpes, the eye of which is regarded as an aggregate of ocelli which are disposed in groups of two or three together, a circumstance which may be regarded as indicating a primitive stage in the evolution of compound faceted eyes in which the ommatidia are uniformly disposed, are close together and are covered



Fig. 247. — Sagittal Section through the

Apus. (After

A. : anterior.

a. rt. : anterior retina.

c. ev. : common cavity which encloses the lateral eye vesicles and into which the cavity, at., of the parietal eye vesicle previously opened, at the recess marked o. at.

m. : fold covering the lateral eyes.

Parietal Eye Vesicle of an Adult Patten.)

opening of common cavity. posterior. rt. : posterior retina. Each placode consists of a single row of large colourless columnar cells the distal ends of which are buried in a dense mass of pigment, their proximal ends being clear.

by a continuous cornea. Harpes ungula (Fig. 97, B), in which this primitive condition of the lateral eyes is met with, belongs to the Ordovician period, the age of which is estimated to be 480,000,000 to 590,000,000 years ago. The three impressions on the glabella of /Eglina prisca (Fig. 97, A), which are considered by some eminent palaeontologists to represent ocelli, are arranged in the form of a triangle with the apex forward. In Daimanites socialis (Fig. 104, A) a single impression is present. In a similar specimen of Daimanites three impressions are found, disposed in the form of a triangle with the apex behind (Fig. 104, B), while in certain examples of Trinucleidae there are five impressions on the surface of the


median-eye tubercle arranged in a manner similar to the five of a playingcard. An almost identical disposition of ocelli in a group of three with the apex forward is found in certain recent fossil and amber-preserved insects, e.g. Xylocopa senilis, Apis, a fossil bee (Fig. 239, A), and Prionomyrmex longiceps, a fossil ant (Fig. 239, B), and this characteristic arrangement is maintained in the living representatives of the same species and other insects, e.g. Porthetis spinosus (Fig. 241) and Copiophora cornuta (Fig. 242). There is considerable difference of opinion among the older writers as to whether these markings really correspond to ocelli or not, but recent workers, e.g. Stormer, employing new methods of microscopical technique, have demonstrated not only the presence, but details of the structure of the median eyes in the glabella of several Norwegian species of TrinucleidEe both in the larval and adult specimens. 1 It is with his kind permission that we have been enabled to reproduce the following photographs, which serve to illustrate some of the more important points which he has described in this most interesting group of trilobites and of the frontal ocelli of insects in general, and of some arthropods.

Fig. 100 (Chap. 11, p. 139) shows the general position on the cephalon of the median-eye tubercle of Tretaspis seticornis, in the meraspid stage II, when viewed from the side as in B and as seen from above in A ( <8o). C and D show the tubercle in adult specimens. Both in the meraspid stage and in the adult five impressions are visible on the dorsal aspect of the tubercle. The appearance of the tubercle when seen in section are well shown in Fig. 1 01, which is a transverse section through the medianeye tubercle of an adult Tretaspis Kiaeri, n. sp., and Fig. 102, which is from a photograph of a longitudinal section through the median-eye tubercle of Trinucleus bucculentus .

Stormer comes to the following conclusions :

(1) The median tubercle found on the top of the glabella in several

Trilobites must be regarded as a true median-eye.

(2) The median-eye which has been found in four species of Trinu cleidae shows a different structure to the lateral eyes.

(3) The surface of the median-eye tubercle shows five distinct im pressions, indicating four or five ocelli below.

(4) The structure resembles that of the median eyes in recent Phyllo poda (Apus, Fig. 67, Chap. 11, p. 104, and Fig. 247, and Figs. 248, 250, Chap. 24, Lepidurus, Fig. 68, A, Chap. 11, p. 106).

(5) No lens such as that in the lateral eyes has been found.

(6) The ontogeny of the Trinucleidas shows that the median eye is

highly developed in the early larval stages, when the lateral eyes are small and little developed.

1 Lief Stormer, Scandinavian Trinucleidce, with Special References to Norwegian Species and Varieties, I to III, 1939, Skrifter ut gin av det Norske Videnskaps, Akademi i Oslo.

   The Pineal Organ (1940): 1 Introduction | 2 Historical Sketch | 3 Types of Vertebrate and Invertebrate Eyes | Eyes of Invertebrates: 4 Coelenterates | 5 Flat worms | 6 Round worms | 7 Rotifers | 8 Molluscoida | 9 Echinoderms | 10 Annulata | 11 Arthropods | 12 Molluscs | 13 Eyes of Types which are intermediate between Vertebrates and Invertebrates | 14 Hemichorda | 15 Urochorda | 16 Cephalochorda | The Pineal System of Vertebrates: 17 Cyclostomes | 18 Fishes | 19 Amphibians | 20 Reptiles | 21 Birds | 22 Mammals | 23 Geological Evidence of Median Eyes in Vertebrates and Invertebrates | 24 Relation of the Median to the Lateral Eyes | The Human Pineal Organ : 25 Development and Histogenesis | 26 Structure of the Adult Organ | 27 Position and Anatomical Relations of the Adult Pineal Organ | 28 Function of the Pineal Body | 29 Pathology of Pineal Tumours | 30 Symptomatology and Diagnosis of Pineal Tumours | 31 Treatment, including the Surgical Approach to the Pineal Organ, and its Removal: Operative Technique | 32 Clinical Cases | 33 General Conclusions | Glossary | Bibliography
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