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

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The Pineal Organ - Eyes of Invertebrates

Chapter 11 The Eyes of Arthropoda

This great class comprises the Crustacea, Onychophora (Peripatus), the centipedes and millipedes, insects, and arachnids. Hitherto the eyes with which we have been dealing have been chiefly of the simple type. We have now to consider, in addition to the simple eyes of the ocellar type, the laterally placed, paired, compound eyes. These attain a very high degree of differentiation and specialization in certain insects and Crustacea, and their structure has attracted the attention of some of the most distinguished pioneers in the domain of microscopical zoology. Owing to the vastness of the subject, the account which we are able to give will be only in the way of a brief sketch ; this, however, we hope will indicate the lines along which devolution and evolution of the simple median type of eye and the highly complex lateral eyes of this class have taken. Although many of these changes and developments, when considered from the standpoint of the genealogical history of eyes in general must be regarded as having occurred in types of animal which have branched off very widely from the vertebrate stem, they nevertheless help us to understand something of the history of the " pineal eye," and incidentally many interesting points of general biological interest.

Eyes of Crustaceans

One of the most interesting Orders of the Class Crustacea is that which includes the species Apus (Fig. 67) and Lepidurus (Fig. 68). In these small fresh-water animals the back is covered by a shield which resembles the dorsal shield or carapace of Limulus poly phemas, the king crab (Fig. 69). In both, median paired eyes and lateral paired eyes are present. The median paired eyes are of simple type ; the lateral eyes are compound.

In Apus the median eyes consist of 3 or 4 groups of large, clear sensory cells surrounding a mass of pigmented tissue (Figs. 247, 248, Chap. 24, pp. 350, 360) ; together they appear under a low magnification as a single black spot and hence are often described en bloc as " the median eye." The compound lateral eyes which have been described by Bernard are covered by a transparent cuticle forming the cornea, beneath which is a narrow space, the water sac, which opens on the exterior by a pore. The eye itself is composed of a large number of radially arranged units called ommatidia, each of which consists of an outer and an inner segment. The outer segment is formed of clear cells enclosing within them a conical vitreous body, the crystal cone. The inner segment is formed by a group of sensory cells enclosing a clear axial rod, the rhabdite and forming with the former a retinula. The retinula is the actual sensory part of the ommatidium, and its cells are comparable in this respect, although not in details of structure, with the sensory cells of the retina in vertebrates. The retinula; of adjacent ommatidia are separated from each other by a circumferential zone of cells, containing black pigment.

Fig. 67. — Dorsal Aspect of Apus Cancriformis, a Small Fresh- water Crustacean of Primitive Type, possessing a Single Median Eye and Paired Compound Lateral Eyes. (From Bronn's Thierreich.) abd. : abdomen. af. : caudal style. d.o. : dorsal organ. E. : paired lateral eye. e. : median eye. th.f.l. : endites of first thoracic foot.

The sensory cells of the retinulae appear to be continuous with corresponding separate fibres of the optic nerve and since each ommatidium is surrounded by pigment cells and is thus isolated from its fellows, it would be able to transmit one part only of the whole field of vision to the particular nerve fibres which are directly connected with its sensory cells. The development of the median eye of Apus is similar to that of Cyclops, in which a deeply pigmented and bilobed eye-spot is developed (see description by Patten alluded to in Chap. 24, p. 360).

In Cypris the nauplius or larval form of Lepas fascicularis (allied to Lepas anatifera, the common barnacle) (Fig. 70), both median and lateral compound eyes are present at one period of its development and both have disappeared in the fully developed adult animal. In Apus and Lepidurus both types of eye are retained in the adult animal, as in Limulus, and it appears probable that many types of Crustacea, which in the adult state are parasitic or becoming attached by a pedicle to rocks, lead a fixed existence, have descended from ancestors who possessed both median and lateral eyes and made use of them in their free-swimming adult life.

In the Cladocera, as previously mentioned, the paired lateral eyes have fused into a single median organ, as in Daphnia, Polyphemus, and Leptodora (Fig. 21, p. 27), while in Cyclops, the water-flea (Fig. 71), it is the simple, median pair of eyes which have fused into a bilobed eye-spot, no compound eyes being developed in this form.

In the parasitic Eucopepods, or fish lice, both median (simple) and lateral (compound) eyes are present. The median eyes in the adult animal are represented by a composite organ which appears as a threelobed structure in the median line, considerably behind the plane of the two compound eyes. In the centre is a Y-shaped pigmented area which separates three clear lens-like bodies. The anterior of these is probably formed by the complete fusion of two eyes, while the fusion of the hinder pair is incomplete. The arrangement is somewhat similar to the fusion of ocelli which takes place in Sagitta hexaptera (Fig. 55, p. 86), but in Sagitta the ocelli which have fused, presumably correspond to the lateral paired groups of ocelli in the larval stages of higher types. In the Phyllosoma larva {Palinurus) (Fig. 72) the median eye is bilobed, and it is probable that one of two median pairs of eyes has degenerated or completely disappeared. In this crustacean there is a very high degree of development of the stalked compound eyes and optic ganglia.

The structure of such a compound eye is well seen in Caprella acutifrons, belonging to the Order Amphipoda. If a whole specimen is viewed from above under a low power of the microscope by transmitted light from below, it presents the appearance shown in Fig. 73, and if the equator of the eye is focused each retinula is seen to be covered by a transparent corneal or cuticular facet. The central ommatidia show a star-like, five-rayed, central axis, which transmits the light upwards from below. Each of these stars corresponds to a retinula with its central rhabdite and crystal cone (see Fig. 44, showing longitudinal and transverse sections of the eye of Gammarus ornatus, the fresh-water shrimp, a nearly allied species). Surrounding the clear area are five pigment cells filled with black pigment, while the intervening spaces are occupied by interstitial tissue, and in Gammarus also by the white pigment cells already referred to (p. 65).

The compound eyes and optic ganglia of the crayfish, Astacus fluviatilis, the shrimp, Crangon, and the prawn, Palcemon, exhibit an extraordinary complexity and diversity of structure, and they afford a most valuable insight into the relation of the sensory receptive cells to those of the ganglia in the central nervous system. The early stages of development of Astacus were first studied by Reichenbach (Fig. 74), who showed that the superficial cells which will give rise to the future eye are at first indistinguishable from those of the cerebral lobe and appear to be continuous with them. Further, it is believed by Kappers and others that the cells of the cerebral lobes have been derived from the superficial epithelium, the cells of the latter having sunk down into the subepithelial tissue, where they develop dendritic- and axonal-processes, thus becoming bipolar. These cells which retain at first their direct connection with the surface are called the primary receptor cells. Their place on the surface is afterwards taken by the formation of new cells which later also send processes into the subepithelial tissue. The primary receptor cells now losing their connection with the surface are termed secondary or bipolar neurones. By a repetition of the process a third layer of cells is formed, so that eventually the retina consists of three principal layers of cells with their communicating processes (Fig. 12, Chap. 3, p. 17). Such a retina is known as a compound retina, and is comparable in this respect with the compound retina of the lateral eyes of vertebrates, which also consist essentially of three layers of sensory elements :

1. Receptor cells .... Rod- and cone-cells.

2. Secondary neurones . . . Bipolar cells.

3. Tertiary neurones . . . Ganglion cells.

There is this difference, however, between the lateral invertebrate eyes under consideration and the lateral eyes of vertebrates, namely, the retina of the invertebrate eye is upright, whereas the vertebrate retina is inverted.

Fig. 68. -Lepidurns Kirkii : semi-diagrammatic, sagittal section showing the median eye and its relation to the supra-oesophageal ganglion or brain ; also the relation of the oesophageal connectives to the gullet and of the heart and genital gland (r. ovary) to the alimentary canal. an. : anus. mth. : mouth. br. : brain. ces. c. : oesophageal connective. c. ap. : cephalic apodeme. ovd. : oviduct. cp. : carapace. ovy. : ovary. d.gl. : digestive gland. p.a.p. : post anal plate. d.m. : dorsal muscles. pcd. s. : pericardial sinus. e. : median eye. sh. gl. : shell gland. gul. : gullet. st. : stomach. ht. : heart. v.n. cd. : ventral nerve-cord. Ibr. : labrum.

Ideal Crawate : sagittal section of a typical vertebrate for comparison w

al. bl. : allantoic bladder. ms. nph. : mesonephros. an. : anus. nit. n. d. : metanephric duct. au. : auricle. nit. nph. : metanephros. b.d. : bile duct. nch. : notochord. buc. c. : buccal cavity. p.a.g. : post-anal gut. c.a. : conus arteriosus. pc. : pericardium. coel. : coelome. ph. : pharynx. orb. : cerebellum. pn. : pancreas. c.s. c. : cerebro-spinal cavity. pn. b. : pineal body. dien. : diencephalon. pn. d. : pronephric duct. g.b. : gall bladder. pn. e. : pineal sense organ. gl. : glottis. p. nph. : pronephros. gon. : gonad. pros. en. : prosencephalon. h.c. : haemal canal. pty. b. : pituitary body. i. br. a. : internal branchial aper- pty. s. : pituitary sac.

tures.

sp. c. : spinal cord. int. : intestine. spl. : spleen. Ig. : lung. st. : stomach.

lr. : liver. s.v. : sinus venosus. m.b. : midbrain. thd. : thyroid. med. obi. : medulla obloingata v. : ventricle. ms. n. d. : mesonephric duct. mth. : mouth.

(Redrawn from Parker and Haswell's Textbook of Zoology.)

Fig. 69. A — Limulus polyphemns (dorsal aspect). B — Larval form of Limulus (Trilobite stage). C — Prestwichia rotunda.

The position of the paired median and the paired lateral eyes is indicated. (After H. Woodward.)

Fig. 70. — The Left Side of Cypris, exposed by the Removal of the Left Valve of the Shell, showing the Median Eye and Appendages. (After Gerstaecker and Zenker.)

abd. : abdomen. md. mandible. ant. 1 : antennule. m.e. median eye.

ant. 2 : antenna.

mx. 1 , mx. 2 : maxillae

f.\f. 2

thoracic feet.

Fig. 71. — Dorsal View of Cyclops, a Free-swimming Eucopepod (Class Crustacea), showing a Single Median Eye, formed by Fusion of a Pair of Simple Median Eyes.

abd. : first abdominal segment. ov. :

an. : anus. th.~

ant} : antennule. th. 6 e. : median eye.

ovary.

thoracic segments, thoracic segments.

Between the receptor cells and the secondary neurones in both vertebrate and many invertebrate retinae is a plexiform layer, formed by the communicating axonal and dendritic branches of these cells, and another plexiform layer is formed between the axonal branches of the bipolar neurones and the dendritic branches of the ganglion cells. Also in invertebrates, as in vertebrates, the fibres of the optic nerve are carried inwards as a tract to the central nervous system. In invertebrates, however, the optic tract is formed by a series of ganglia, with intervening plexiform zones, the tissue of which is known as the neuropil or punctate substance. The ganglia appear as localized swellings in the stalk of the eye, which differ in number in different types of Crustacea. In Astacus, if the retinal ganglia are included, there are four ganglia ; in others there are three or two. Thus in Astacus we have a compound retina and a continuation of the nerve-chain as a tract in the form of a series of ganglia. The compound retina of crustacean eyes is, moreover, complicated by the modification of the outer ends of certain of the cells to form refractive elements, namely, the crystal-cones — cylinders or prisms which are isolated by separate pigment cells. Moreover, there is a further modification both in the retina itself, as in Palcemon (Fig. 37, p. 52), and in the optic stalk beyond the basement membrane in the form of spindle-shaped swellings which are highly refractile and are either transversely striated or show spiral markings. These were named " rhabdomes " by Grenacher, and are especially well seen in the retina of Palcemon, which besides the corneal facets or lenses and the principal crystal cones CC, which are intermediate, has distal cc and proximal refractile elements cc" . This type of eye is described as an upright compound eye, and since it consists of many retinulas or ommatidia, arranged in a radiating manner side by side, it is also a composite eye. It will be unnecessary to attempt a full description of the detailed structure of the eyes of Astacus and Palcemon, as this has been so admirably achieved by Grenacher in his great work on the Eyes of Arthropods. It is worth noting, however, with reference to Fig. n, that the smooth outer layer of the eye is continuous with the surrounding cuticle and that the cells of the retina and its basement membrane are directly continuous with the hypoderm cells and basement membrane of the stalk ; also that the optic ganglia and fibres of the optic nerve are enclosed in a loose, double-layered sheath, which will permit of free movement of the stalk without injury to the optic nerve, and that this movement can be brought about by the bands of striped muscle-fibre, some of which are seen to be inserted into the basement membrane of the skin. These movements can be made in all directions, including rotation, as evidenced by the transverse direction of some of the fibres cut in crosssection and the spiral twist of the optic nerve.

Fig. 72. — Head of Phyllosoma Larva, showing Lateral and Median Eyes

(Palinurus) (R. J. G.).

c.l.e. : compound lateral eye. f.m.e. : fused median eyes.

Fig. 73. Compound eye of Caprella acutifrons, viewed from above and seen by transmitted light under a low-power magnification of the microscope. The drawing shows how light can pass upwards through the central axes of a particular group of retinulae, towards the eye-piece of the microscope ; and it may be presumed that in the natural condition the direction of light passing in the reverse direction would be transmitted by definite groups of retinulae, whose sensory cells would be those chiefly affected. By varying the position and direction of the source of light, different groups of retinulce would be affected and different fibres of the optic nerve would carry impulses to the brain. (R. J. G.)

Fig. 74. — Early Stages in the Development of Astacus Fluviatalis, showing the Relation of the Ocelli to the Protocerebrum.

at. 1 : antennule. lab. : labrum.

at. 2 : antenna.

at. 1 g. : ganglion of antennule.

car. : fold which becomes edge of carapace.

caud.j. : forked extremity of abdomen.

mandible, first maxilla, second maxilla, ocellus.

protocerebrum. th. abd. : rudiment of abdomen. (From MacBride after Reichenbach.)

m.n. mx. 1 mx. 2 oc. pr. c.

thoraco

The Eyes of Peripatus (Onychophora)

Peripatus is a caterpillar-like animal which, although generally classed among arthropods, differs from these in certain important respects and it is regarded as intermediate in type between the Arthropoda and Annulata. The paired lateral eyes of Peripatus (Fig. 75) are very similar to those of Nereis (Fig. 63, p. 97), and they also resemble the lateral eyes of spiders. They are of the simple, upright type, there being : (1) a corneal lens, formed by cuticle and hypoderm cells ; (2) a central vitreous body ; (3) a retina consisting of an outer, clear, bacillary layer, an intermediate pigment zone, and an inner nerve-fibre layer containing the nuclei of the sensory cells. The optic nerve passes directly into the central neuropil of the supra-oesophageal ganglion or brain.

The development of Peripatus was investigated by Sedgwick, who showed that the eyes are formed at the antero-lateral ends of the two cerebral grooves, which later become closed off from the exterior and form for a time hollow outgrowths (optic stalks) from the brain (Fig. 76) ; later the cavity disappears and the optic nerve becomes solid. In Agelena (cellar spider) the posterior ends of these cerebral grooves give rise to the central or median pair of ocelli (Kishinouye).

Fig. 75. Transverse Section through the Head of Peripatus, a Primitive Type of Arthropod, showing the General Arrangement of Nerve Cells in the " Brain " or Supra-cesophageal Ganglia of Invertebrates. (R. J. G.)

The development of Peripatus, which was described by Balfour in 1 88 1, is extremely interesting from the morphological standpoint, since in many ways it resembles that of vertebrates. The importance of the comparison of invertebrate development with vertebrate development becomes still more obvious when the details of development of other arthropods such as that of certain insects, e.g. Hydrophilus, and of arachnids are taken into consideration. To do this it will be necessary to recall some well-known points of resemblance between the early stages of embryonic development of arthropods and vertebrates in order to understand the general position of the cephalic area of certain arthropod embryos with reference to the development of the brain and that of the median and lateral eyes.

In Peripatus, when the early superficial segmentation of the ovum has been completed, the central mass of yolk is covered by the blastoderm, except at one part on the ventral aspect, where an invagination of cells takes place. This is the blastopore, and the floor of the depression, which is bounded by its thickened, involuted lips, is formed by the underlying yolk. Later, the blastopore appears as an elongated groove ; the sides of the groove then join in the middle part, leaving an aperture at either end. The anterior opening becomes the mouth, the posterior the anus. On each side of the blastoporic groove a series of hollow mesodermic somites are formed, the cavities of which give rise to the internal vesicles of the nephridia and genital ducts. At the anterior end in front of the mouth is a crescentic area which gives rise to the cerebral lobes and head.

Fig. 76. — Ventral Aspect of Head of a Peripatus Embryo, showing the Cerebral Grooves and Anterior Ocelli. ant. : antenna. /. ; lip enclosing buccal cavity.

cer.gr. : cerebral groove. hoc. : lateral ocellus.

gn. : gnathite or jaw. leg 1 . : first leg.

gn. b. : swelling at base of jaw. or. p. : oral papilla.

" The anterior ocelli which were primarily posterior in position are carried forward by the forward growth of the head." — After Sedgwick, from MacBride : Textbook of Embryology, Vol. 1.

A rounded process which will give rise to the labrum is situated in front of the mouth, and it is from the base of this that the two cerebral grooves, previously mentioned, diverge towards the roots of the antennas.

In Hydrophilus (Fig. 77), before the appearance of the cerebral grooves, however, the whole area, including the blastoporic groove, somites, and cephalic region, is enveloped in an amnion fold which grows up over the ventral plate and becomes cut off from the superficial layer of the amnion fold or serosa, in the same way as in vertebrates, but on the ventral aspect of the body instead of on the dorsal aspect. Another interesting point is that in the scorpion the covering of the dorsal surface of the embryo with skin is effected by the lateral growth of the ventral plate, its right and left edges carrying with them the lines of origin of the amnion on each side, round the dorsal aspect of the ovum, until they meet in the median line. The further study of the development of arthropods, more especially of spiders, scorpions, and Limulus, emphasizes the importance of our knowledge of their relation to each other and to their remote ancestors of the Silurian epoch, such as Hemiaspis, and still further back to the ancestors of the Trilobites. The whole subject is, however, very intricate, and it will be impracticable to discuss here the problem of the origin of vertebrates without diverging from the main subject of this part of our treatise, namely an inquiry into the origin and nature of the median eyes of vertebrates.

Fig. 77. — Ventral View of Three Stages in the Development of Hydrophilus. (From Lang, after Heider.)

a. and b. : points at which the blastopore first closes.

af. : edge of the amnion fold.

am. c. : rudiment of amnion cavity commencing at caudal end of embryo.

c.l. : procephalic lobes.

g. gr. : linear groove marking the line of invagination of gastral endoderm.

hf. : head fold of amnion.

so. : somites.

It will suffice to state here that the study of the median eyes of invertebrates does throw light on this question and also that the solution of the problem is assisted by a general knowledge of the essentials of comparative embryology of invertebrates, in. addition to that of vertebrates. The net result of such a study appears to indicate that the common ancestral stock of living vertebrates and invertebrates must have lived in a very remote geological period ; and although that part of the brain which is connected with the principal sense-organs, along with the alimentary canal and sympathetic nervous system, appears to have an origin which is common to both ; the spinal cord and vertebral column of the vertebrates seem to have been evolved as an entirely new structure which has grown backward in a caudal direction from the head-end of the animal and with its somatic system of nerves and musculature has become incorporated with the primary parts which pre-existed in the pre vertebrate stock.

If this view is accepted in principle, the possibility of the derivation of the parietal sense-organs or " pineal eyes " of vertebrates from the simple eyes of invertebrates appears to be not only possible, but in our opinion very probable, and any divergence in structure between the two types of eye, are only what might be anticipated in view of the extremely long period of time which has elapsed since the vertebrate stock branched off from the common ancestral stock of the two great Classes.

The Eyes of Insects

In the more highly evolved types, such as the dragon fly, the bees, wasps, and ants, or the butterflies and moths, there are two large, upright, lateral eyes of the compound faceted type, and frequently there are also one or two pairs of median eyes of the simple, upright type. These may be fused or separate and are usually small and degenerate (Fig. 20, p. 26). In the wingless insects, Apterygota, compound eyes and ocelli are sometimes present and sometimes absent. In the Order Collembola, which includes the spring-tails, compound eyes are never present. Ocelli or simple eyes with a non-faceted cornea are sometimes placed laterally, especially in the larval condition, where they may precede or co-exist with the rudiments of the compound eyes. The facets on the surface of a compound eye are usually hexagonal, and it is stated that in the dragonfly there may be as many as 28,000. When seen in longitudinal section each ommatidium is found to consist of a cornea-lens formed by a modification of the cuticle which corresponds to one of the hexagonal facets seen on the surface ; beneath this is a crystalline cone or a group of four crystal cells. In the central axis is a clear, glass-like rod, the rhabdome ; the sensory cells of the retinula end in nerve-fibres which pierce a fenestrated basement membrane, beneath which is a plexus of nerve-fibres. The retinulae and crystal cones are surrounded and isolated from one another by pigment cells.

The simple eyes, or ocelli, have been described by Grenadier, Lowne, Gunther, and others, and their structure has already been alluded to in the description of the ocellus of a young larva of the beetle, Dytiscus (Fig. 4, Chap. I, p. 10) ; in this a biconvex " corneal-lens " is formed by a thickening of the cuticle which lies over the elongated distal ends of specially modified hypoderm cells. These converge to the central axis or potential cavity of the eye and they form a refractile vitreous body. Deep to the vitreous is the retina composed of elongated sensory cells, which are continuous laterally with the cells of the vitreous and thus with the surrounding hypoderm cells. The outer ends of the retinal cells are clear and rod-like ; in the middle portion of each cell is the nucleus and the inner end is continuous with a fibre of the optic nerve.

Fig. 78. — The Heads of Larva of Dytiscus Marginalis, showing Two Stages of Development. (After Gunther, from MacBride.)

A — Aquatic larva.

B — Larva about to pupate.

mx. 2 : second maxilla. oc. c. : rudiment of compound eye. oc. s. : simple ocelli. oc. s. 1 : lenses of the simple eyes, carried away by the loosening of the cuticle. ant. es. : es. 1 antenna.

eye-spot of larva. 1 : lens of eye-spot carried away by loosening of the cuticle.

mn. : mandible.

mx. 1 : first maxilla.

Fig. 79. — Vertical Section through Rudiment of Compound Eye — " Imaginal Disc " — of Dytiscus Marginalis, showing First Differentiation of Retinula. (After Gunther.)

ect. : unaltered ectoderm.

pr. z. : proliferating zone of cell elements at margin of disc.

ret. : retinulaj.

According to the description given by Giinther in 1912 of the development of the eyes of Dytiscus marginalis, a water-beetle, the young larva has six ocelli on each side of the head and a rudimentary eye-spot. In the early stage of development only ocelli are present ; at a later stage a crescentic area lies in front of these, which is the rudiment of the compound eye (Fig. 78). In section each ocellus is seen to be formed as a slit-like depression of the epithelium. The cells at the base of the pit end in visual rods ; each rod consists of two semicylindrical segments adherent to each other and appearing to consist of a mass of agglutinated fibrillar The cells lining the sides of the lower part of the pit bear horizontally directed rods which are similar to the rods of the larger basal cells, but are much smaller (Fig. 38, Chap. 3, p. 53). Nearer the mouth of the pit are some cells which secrete a gelatinous, non-cellular vitreous body. When the larva leaves the water and undergoes its pupal moult, a fine, pigmented line surrounds each group of ocelli. This line, which is horse-shoe shaped, afterwards thickens, and when the moult takes place the lenses of the ocelli are torn away from the deeper pigmented portions, since the lenses belong to the larval cuticle which is shed off from the deeper pigmented parts. The horse-shoe pigmented portion has now widened out and increased in breadth so as to form a crescent, the pigmented ocelli lying close behind its concave border. The cuticle covering the eye-spot is also carried forward with the lenticular parts of the ocelli. Before the final moult to form the imago is completed, the remnants of the ocelli recede from the surface, and the pigment area which is the rudiment of the compound eye, becomes circular in outline, spreading over the place which was originally occupied by the ocelli. These do not disappear completely, but remain through life as closed pigmented vesicles which are attached to the optic nerves.

Fig. 80. — Later Stage in the Development of the Compound Eye of Dytiscus Marginalis, as seen in Vertical Section. (After Gunther.)

bas. c. : basal cell.

pig

pigment cells.

b.m. : basement membrane.

ret.

• retinula.

cut. : cuticle.

rh. .

rhabdome

I.e. : lentigen cells.

vitr.

c. : crystalline cone-cell

ph. s. : vacuole or phEeosphere.

Fig. 8l — Vertical Section through a Small Part of an Adult Compound Eye of Dytiscus Marginalis. (After Gunther.) bas. c. : basal cell. pig. : pigment cells.

b.m. basement membrane.

ret. retinula.

c.l. : corneal lens.

rh. : rhabdome.

I.e. : lentigen cell.

vitr. c. : crystalline cone.

n.f. nerve-fibres.

The epithelium of the now disc-like pigmented area consists of tall, columnar cells which differentiate into basal and retinular cells from which the rhabdomes are formed ; cells which form the lenses, and crystalline cones, and inter-retinular pigment cells. At one end of the area there is a part where cells boundaries are indistinguishable and the epithelium appears to consist of a mass of protoplasm containing numerous nuclei irregularly distributed through it ; this is the proliferation zone from which for a considerable time new ommatidia are added to those which are already formed (Fig. 79). The ommatidia consist of two parts, namely, the crystalline cone and the retinula. The crystalline cone is formed by four superficial cells in which clear vesicles appear. The four cells cohere to form the cone, the vesicles in them also coalescing to form a single retractile body. The cells which will form the retinula retreat from the surface, each group of these consisting of eight cells, one central and seven peripheral. One of these peripheral cells is squeezed out from between the rest, while the central cell and the remaining six peripheral cells co-operate in forming one long visual rod or rhabdome. The lower end of the rod has the form of a flask-shaped basal cell, while its upper end thins out into a tapering process before reaching the upper limit of the retinula cells (Fig. 80).

Fig. 82. — Eye of Acilius Larva. (After Patten.)

/. ; chitinous lens.

c. . corneagen, or hypoderm cells, forming the vitreous portion of the lens, optic nerve. p. : pigment. pr. : pre-retinal layer.

o.n.

ret. : retina.

rh. : rhabdomes.

Fig. 83. — Diagram showing the Formation of an Inverted Image on the Retina of a Typical Lateral Eye of a Vertebrate.

Fig. 84. Diagram showing the course of rays of light from three points — x, y, z — through nine visual rods (supposed to be empty tubes) A to /, of a compound eye ; a to i : nerve-fibres connected with the visual rods. The arrows have been inserted to indicate that the retinal image would not be inverted as in the vertebrate eye (Fig. 83).

(Slightly modified after T. H. Huxley.)

In both ocelli and in the adult compound eyes (Fig. 81), the nerve fibres of the optic nerve originate as basal outgrowths from the retinal cells.

± (library

Fig. 85. — Diagram showing the Connections of the Ocelli and the Faceteyes with the Brain in the Honey Bee. (After Kenyon, Jonescu, and Fortuyn, From Ariens Kappers.)

The protocerebrum of insects is connected with two optic systems, the ocelli and the compound facet-eyes. Of the three ocelli the middle one is connected with the pons and central corpuscle {ex.), where it acquires relations from optic impressions from the compound eyes, and also with impressions from the deutero-cerebrum in which the olfactory antenna ends. The corpora pedunculata also receive optic impulses, but from the facet eyes only.

The same types of larval simple and adult compound eyes are found in Acilius, the large central cells of the retina, with the long rods, being especially well-developed in the larval form (Fig. 82). It was this example which Gaskell selected for comparison with the " pineal eye " of Ammocoetes. The principal objection which was raised at the time to the validity of the comparison was that the lens of the Acilius larva is formed by a thickening of the cuticle, whereas in the case of the " pineal eye " of Ammocoetes the lens is formed as a modification of the anterior or distal wall of the vesicle and is cellular in type (Figs. 13 and 14, Chap. 3, pp. 18, 19). Another objection to the theory was that the ocellus of the Acilius larva was formed as a depression of the hypoderm cells, whereas the " pineal eye " is an evagination from the roof of the thalamencephalon after the brain has been separated from the cutaneous ectoderm. The evidence in favour of and against these objections will be considered on p. 358.

It may be noted here that in the course of evolution of the inverted types of eye there has been a change not only in the position and direction of the retinal cells with respect to the incidence of light, but in the vertebrate or inverted types of eye, owing to the evolution of a single biconvex lens in a position which will cause an inverted image to be thrown upon the retina, as indicated in Fig. 83, the rays coming from the upper part of the object fall upon the lower part of the retina, whereas in the upright, aggregate type of eye, of insects and Crustacea, the retinal image is not inverted (Fig. 84), since each ommatidium can only transmit those rays of light which fall upon the surface of the eye in the direction of its longitudinal axis, and the retinal image is not inverted. A decussation of the fibres of the optic nerve, however, takes place in the chiasma externa in the higher arthropods (Fig. 85), and in the Cephalopods (Fig. 122, Chap. 12, p. 166), whereby the cortical impressions received in the optic lobe form, it may be presumed, a mental picture which is in the same position as the object viewed — this picture, it is believed, being similar as regards orientation to that formed in the visual cortex of the human brain.

In the higher Mollusca the lateral eyes resemble the vertebrate type with regard to the position of the lens and iris relative to the retina, but differ in having an upright retina and in other respects which will be mentioned in Chapter 12.

The Eyes of Spiders — Arachnida

The Class Arachnida includes, in addition to the various forms of spider, the scorpions, mites and ticks, king crabs and the extinct Eurypterida. The Class is much less homogeneous than the Insecta, and there is considerable variety in the number of eyes present in each animal and in the type of eye. As in insects, many of the spiders have both median and lateral eyes. The lateral eyes of the adult animal are, however, not compound, as in many of the insects, but are of the ocellar type, having a single, non-faceted cuticular or corneal lens. As a rule the upright eye is simple in type with respect to the retina, but the central eyes of the scorpion differ in having the sensory cells grouped together in a form resembling somewhat the retinulae of the compound eyes of insects and crustaceans . Compound eyes are also present in Limulus, in which the retinulae of the lateral pair of eyes (Fig. 86), are barrel-shaped and lie beneath papilliform downgrowths of the cutis, each of which is believed to serve as a separate lens for the retinula which lies beneath it. The central eyes of Limulus (Fig. 87) also show a grouping of the sensory cells into units which have been described as resembling retinula;, and have a marked general resemblance to those found in the central eyes of Euscorpius. These tulip-like buds (Figs. 86 and 87), however, differ markedly from the compact cylindrical retinula; of the eyes of insects and Crustacea, and although they may be regarded as intermediate in type do not indicate a close relationship. The eyes of the ordinary garden and house spiders are also very remarkable. They vary in number from one to six pairs, four pairs being present in the leaping spider, Salticus scenicus, shown in Fig. 88, in which they are seen to be situated on the carapace, the central or frontal pair being the largest. The frontal eyes of Epeira diaderma are of the simple, upright type ; whereas the posterior or lateral eyes show what is described as an " inversion " of the constituent parts of the retinal cells. The posterior eyes vary more particularly with respect to the position of the nuclei of the retinal cells, relative to the refractile rods or bacilli ; instead of being proximal or posterior to the rods, they are in front of these, as in the lateral eyes of vertebrates. This position of the nuclei of the retinal cells was termed by Graber " pr£e-bacillary," in contrast to their " post-bacillary " situation in the simple, upright eye. It must be borne in mind, however, that the retina in the inverted lateral eyes of vertebrates is compound, whereas the lateral eyes of spiders, although inverted with respect to the constituent parts of the cells, are still simple in type, i.e. the retina consists of a single layer of sensory cells. In some types there is a definite tapetal layer which reflects the light on to certain fishes and carni the rods as in the eyes of many vertebrates, e.g. vores, also the horse and ox.

Fig. 86. Part of a section through the composite lateral eyes of a king crab (Limulus), showing c. cp. : the chitinous carapace ; cp. : the ingrowing papilla-like thickenings of this which lie over the retinulas, or separate eyes ; ret. : retinula ; hyp. ; hypodermis, the cells of which are modified opposite the papillae to form the sensory cells of the retinulas ; n. : nerves of the retinulje. (After Lankester and Bourne.)

Section of one of the median eyes of the king crab, Limulus polyphemus, showing the transparent chitinous lens, which fills the optic pit and is continuous with the surrounding chitin which forms the head-shield or carapace. The optic pit is lined by a layer of columnar cells (vitreous layer), which are continuous with the surrounding hypodermal cells. Immediately beneath this layer is a loose tissue consisting of branched connective tissue cells (ct.c), some of which contain pigment, and a few rounded or oval cells of an epithelial type (ep.c). Embedded in this tissue are the retinuke (ret.), which consist of groups of five to seven large pear-shaped, epithelial cells arranged round a refractile axial rod, called the rhabdome. Each retinular cell is continuous with a nerve fibre (n.f.), which joins with similar nerve fibres to form the main optic nerve of the median eye of the same side.

The position which the refractile body (rod or cone) bears to the nucleus of the cell is probably not of so much importance with regard to the reception and transmission of impulses through the cell as one might be led to infer from the examination of specimens prepared with nuclear stains such as hsematoxylin : these give the impression of the nucleus being an opaque, dark, oval body lying in the substance of clear cytoplasm, whereas in living tissues the nuclei of cells are transparent and probably would not interfere greatly with the passage of light through them to reach a refractile body or rhabdite situated deep to or beyond the nucleus.

Fig. 88. Above — side view of two spiders, Salticus scenicus, showing the four pairs of eyes. Below — female of the same species more highly magnified, viewed from in front. (After F. P. Smith, By-paths in Nature.)

In the central eyes of the scorpion a refractile vesicle or phasosphere lies deep to the nucleus of each retinal cell, while superficial to it, that is, nearer the lens, is a well-developed rhabdite, and it is probable that rays of light traversing the cell would be refracted by both these bodies and would give rise to an impulse which would be transmitted to the nerve fibre arising from the proximal end of the cell. Since both of these bodies appear to be developed as a modification of or a secretion into the cell substance, it seems probable that either one or the other could be further developed so as to form the main refractile element or be suppressed, thus giving rise to differences in the position of the rhabdite relative to the nucleus without there being any inversion of the cell.

A true inversion of the layers of the retina, however, occurs in the early stages of development in the median eyes of the scorpion (G. H. Parker) and in the median eyes of the spider Agelena (Kishinouye). These will be described in Chap. 24, Figs. 249 and 250.

Development of the Eyes of Agelena

Agelena is a cellar-spider, the development of which has been studied by Kishinouye (1891-4). He describes the formation of a thickening in the wall of the blastoccele, namely the ventral plate, on which the first rudiments of the future organs make their appearance. The plate soon shows a series of transverse grooves, indicating a metameric segmentation. In front there is an undivided cephalic lobe, behind there is a caudal lobe, and between these is a region which is divided into five segments of which the first bears the pedipalpi, the remaining four giving origin to the walking legs. The cephalic lobe gives rise to two semi-circular lobes on each of which a crescentic groove is formed by an invagination of the ectoderm ; this resembles the grooves already described in Peripatus (Fig. 76), which are developed also in the scorpion and Astacus. The ectodermal invaginations become closed off from the surface by union of their edges, and they give rise to the greater part of the brain. The inner segments of the two crescentic tubes now unite to form the stem of a T, the transverse arms of which are formed by the outer portions of the grooves. The central eyes (Fig. 89, A, B) arise as two pits at the posterior ends of the grooves, which when united form the stem of the T and belong therefore to the hindermost segment. The lateral eyes on each side originate as a simple ring-like pit in the ectoderm (Figs. 89, 90), which becomes divided by a continuance of the process of invagination into several deeper secondary pits, each of which becomes closed off from the exterior by constriction of its opening. The floors of these pits are converted into retinulae, their component cells becoming the visual cells. Between the upper or distal ends of the visual cells refractile rods or rhabdomes are developed, while their proximal ends become continuous with the constituent fibres of the optic nerve. The roofs of the pits are formed by the overfolding of the ectoderm and form the vitreous or hypoderm cells, while over all is formed a lens-like thickening of the cuticle, which is continuous with the surrounding cuticle.

The median eyes (Figs. 89, 90, A and B) are formed also as ectodermal pits which become closed off from the surface by constriction of their openings. The opening of each vesicle is situated behind, and when closed each vesicle has an upper and a lower wall between which is a slitlike cavity. From the upper of the two layers the visual cells arise, while the lower wall gives origin to what Kishinouye describes as the " postretinal " layer (Fig. 250 B, Chap. 24, p. 362). The vitreous, as in the lateral eyes, is developed from the covering hypoderm or ectoderm cells, and a thickening of the cuticle gives rise to the superficial corneal lens. The early stage of the development of the median eyes is thus one which corresponds to the development of an inverted retina such as is found among the Mollusca, in the Scallop or Pecten ; and since the median eyes of adult spiders are as a rule of the simple upright type, it would be interesting to obtain further information with regard to the later stages of development and the structure of the adult eyes of Agelena.

Fig. 89. — Brain of Embryo of Agelena showing the Position of the Median Eyes and Lateral Vesicles. (After Kishinouye.)

A — Seen from in front. B — Seen from left side.

c. gr. : cephalic groove, now closed. ch.g.: cheliceral ganglion. l.v. : lateral eye vesicle. m.e. : median eye.

Fig. 90. — Frontal Sections through Brain of Embryo Agelena. (After Kishinouye.) c.gr. : cephalic groove.

I.e. : thickening of ectoderm, which will form the lens of the lateral eyes. l.v. : lateral vesicle. m.e. : median eye.

v.b. : vitreous body (lens of median eye). 1, 2, 3 : the three segments of the brain.

The Eyes of Hydrachnida

A description of the eyes of freshwater mites was published by P. Lang in 1905, which affords a most instructive demonstration of widely divergent developments of the eyes of closely allied species : more especially with regard to transitions between eyes which are fixed in definite positions — the cuticular lens being continuous with or connected to the adjacent cuticle — and eyes which are enclosed within a chitinous covering, within which they are capable of movements produced by muscles arising from the wall of the cavity and inserted into the eyeball ; a condition which is somewhat similar to that of the lateral eyes of vertebrates, which being enclosed and protected by the walls of an orbital cavity are capable of movements within the capsule of Tenon, which are produced by special ocular muscles.

Fig. 91. — Diagrams showing the Position and Direction of the Ocelli, or Simple Eyes, of Different Species of Hydrachnida (Fresh- water Mites). In the genus Hydrodroma a " Fifth " Median Eye is present on the Dorsal Aspect of the Head Shield. (After P. Lang.)

The eyes vary in number in different species of the Hydrachnida, which species were originally classed by Muller in three subdivisions : (a) oculis binis ; (b) oculis quator ; (c) oculis sex. Lang describes the eyes of Limnesia undulata, Curvipes carneus, and Hygrobates longipalpis as lying completely inside the cuticle. In Limnesia (Fig. 91) the eyes of the two sides are widely separated — the optic axes of the anterior pair are directed forward and laterally, while the axes of the posterior pair are directed backwards and laterally. Approximately the same directions of the optic axes are present in Diplodontus, Eylais, and Hydrodroma as in Limnesia. These four species have, therefore, fields of vision corresponding to the four quarters of the compass.

In Curvipes carneus the anterior and posterior eyes on each side are united in a double eye, of which the posterior eye on each side is the smaller. The same type of fusion, of the anterior and posterior eyes of each side, takes place sometimes in Hygrobates longipalpis.

Fig. 92. A — The dorsal-shield of Hydrodroma, seen from above, and showing the pigmented sense-organ p.s.o., called the " fifth " or " median eye ". On each side of the shield are the lateral eyes enclosed in a chitinous capsule.

B — The " median eye " highly magnified, showing seven radially arranged pigmentcells, surrounded by a circle of papillae, of the cutis.

In Eylais extendens all four eyes, which are close together near the median plane, are enclosed in a so-called " spectacle frame " ; while in the genus Hydrodroma the two eyes on each side are enclosed in a chitinous capsule, which is separated from its fellow of the opposite side by the whole width of the dorsal shield. In the centre of this shield is an organ which has been described as a " fifth median unpaired eye " (Fig. 92). It consists of a minute, disc-like body containing seven radially disposed pigmented areas in which, according to Schaub's description, there is to be seen a rhabdome formation. Lang, however, who examined specimens of the organ prepared by modern technique and under an immersion lens, expresses the opinion that the structure cannot be regarded as an eye in the strict sense of the word, and prefers to speak of it as simply a " pigmented sense-organ." Whether this remarkable organ is a vestigial eye or belongs to any other type of sense-organ, its presence in the centre of the dorsal shield in the median plane between the two pairs of laterally placed eyes is of very considerable importance, and it would be interesting to learn if further research reveals the presence of such an organ in related species.

It may be noted here that in the Linguatulida or Pentastomida, which are degenerate and parasitic Arachnida, the nervous system is greatly reduced and the organs of special sense have entirely disappeared. One species, Pentastomum tcenioides, has been found in the frontal air sinuses and maxillary antra of the dog and wolf. Its embryos, escaping and falling on grass or other herbage, are eaten by hares and rabbits, perforate the w^lls of the stomach and intestine, become encysted in the liver, and undergo a metamorphosis. If the hare or rabbit should be eaten by a dog, the young pentastoma may find their way into its frontal air sinuses or antra.

In the Tardigrada or " bear animalcules," which are soft-skinned animals about 1 mm. in length living on damp moss or in fresh or salt water, there is also a reduction of the central nervous system and the senseorgans are represented merely by a couple of eye-spots at the anterior end of the body.

The Eyes of Scorpions

These have been studied by Lankester and Bourne (1883) and J. S. Kingsley (1886). The lateral eyes are placed on the margin of the prosomatic shield in a group on each side (Fig. 93). The number of separate lenses differs in various subgenera, and each lens indicates a separate eye. In Androctonus the eyes are more numerous than in other scorpions, each lateral group in A . fanestus showing as many as five lenses — three larger and two smaller.

The lenses are of the cuticular or corneal type, and are highly convex on the inner or deep surface (Fig. 94). The hypoderm cells beneath the surrounding cuticle are continued on to the circumferential aspect of the cuticular lens, where they form the vitreous " marginal body " or the " perineural epidermic cells " ; whereas beneath the central part of the lens the hypoderm cells are specially modified to form the retina. This consists of elongated sensory cells the inner ends of which are continuous with the fibres of the optic nerve. In the body of each sensory cell deep to the large spherical nucleus is a refringent globule, called the postnuclear phgeosphere, and in relation with the outer ends of the sensory cells are elliptical refractile elements termed rhabdomes. Interneural epidermic cells and some cells of a connective tissue type are also present.

The central eyes of scorpions resemble the central pair of Limulus in having a single biconvex, cuticular lens beneath which is a continuous layer of columnar cells (Fig. 95). The retina is, however, composed of retinulae disposed in groups, as in the composite eyes of insects and crustaceans. The retinulas in the median eyes of scorpions differ, however, in being much less compact and cylindrical and less highly organized than in the higher types of insects and Crustacea. They consist of sensory cells which terminate at their proximal end in one of the fibres of the optic nerve ; at the outer ends of these cells are elliptical refractile elements, the rhabdomes, and there are also interstitial connective tissue cells containing pigment and an outer layer of epidermal cell-elements next tp the basement membrane. The eyes are generally regarded as intermediate in type between the simple, upright eye of the ocellar type, with a single cuticular lens, and the composite and compound faceted types such as are found in certain Crustacea. Lankester considered that the resemblance of the scorpion's eyes, both lateral and medial, to those of Limulus was important evidence in favour of the latter being more closely related to the arachnids than to the crustaceans.

Fig. 93. — Diagram showing the Median and Lateral eyes of a Scorpion, and their connection with the " brain," or supra-cesophageal Ganglion. Hrt. : heart. Int. : intestine. L.E. : group of lateral eyes, connected by a common optic nerve with the supra-oesophageal ganglion. M.E. : median eye, also connected with the supra-oesophageal ganglion. P. Gl. : poison gland. P l : pulmonary sacs. P 2 : pulmonary sacs. P 3 : pulmonary sacs. P 4 : pulmonary sacs.

nerv.

Fig. 94. — Section through a Lateral Eye of Euscorpius Italicus. int. : intermediate cells. lens : single cuticular lens. nerv. c. : nerve-cells. nerv.f. : nerve-fibres, of optic nerve. rhabd. : rhabdomes.

Fig. 95. — Section through Median Eye of Euscorpius.

A single lens is present in both the central and lateral eyes of Euscorpius. In the central eye the hypodermal cells are continued across the eye as a vitreous layer which lies between the retinulas and cuticular lens.

lens : cuticular lens. nerv. c. : terminal nerve cells. nerv.f. : fibres of optic nerve. pigm. : cells containing pigment. rh. : rhabdomes. vitr. : vitreous layer. (After Lankester and Bourne.)

The Eyes of Limulus Polyphemus

Limulus, or the American king crab, is the only known living representative of the extinct palaeozoic forms of the gigantic marine scorpions such as Eurypterus and Pterygotus which inhabited the sea in the Silurian period. The body consists of two parts separated by a movable articulation, namely, the cephalothorax, covered by the dorsal shield or carapace ; and the abdomen, also covered by a carapace. The body terminates in a tapering sword-like tail, which has given origin to the name " Xiphosura " applied to the Order to which these animals belong (Fig. 69). Beneath the head-shield are placed two pairs of eyes which were especially investigated by Lankester and Bourne ' in 1883, who pointed out the fundamental resemblance of the central and lateral eyes of Limulus to those of the scorpion.

Each lateral eye of Limulus is covered superficially by a continuous transparent corneal stratum, the under-surface of which presents a series of papilliform downgrowths which lie over a corresponding number of barrel-shaped retinulae (Fig. 86, p. 124). The papilliform downgrowths are believed to function as separate lenses for the separate retinulae which lie beneath them. Each of the latter consists of a group of elongated sensory cells, the inner ends of which are continuous with the constituent fibres of the retinular nerve. The papilliform downgrowths forming the separated lenses of the composite eye were believed by Lankester and Bourne to correspond to the numerous two to seven small lenses belonging to the groups of lateral eyes which are present on each side of the scorpion's head. In addition to the receptive or sensory cells of the retinulae, there are refractile elements or rhabdomes which on transverse section are seen to be arranged in a radial manner and also " intrusive " connective tissue cells containing pigment.

Each central eye consists of a single cuticular lens which is almost spherical in form and is continuous with the surrounding cuticle ; beneath this is a vitreous layer which consists of refractile columnar cells which are continuous circumferentially with the hypoderm cells beneath the general cuticle. We thus have an immovable cuticular lens of the corneal type, which is supplemented by a vitreous layer of hypoderm cells (Fig. 87). Beneath this is the receptive portion of the eye, which is composed of groups of large sensory cells, the inner ends of which are continuous with fibres of the optic nerve ; outside there are from five to seven fluted rhabdomes and between the sensory cells and around the retinulae a considerable number of " intrusive " connective tissue pigment cells. The retinulae differ from the retinulae of the lateral eyes in their more open character, as contrasted with the compact barrel-shaped retinulae of the lateral eyes. This loose character of the retinulae of the central eyes, coupled with the large amount of intrusive connective tissue, was considered by Lankester and Bourne to be an indication of loss of function and degeneration of the central eyes as compared with the lateral.

1 Lankester and Bourne, " The Minute Structure of the Lateral and the Central Eyes of Scorpio and Limulus" Q.J. Micro. Sc, 23, 1883.

The Eyes of the Extinct Eurypterida

The Eurypterida were extinct Arachnida. They were often of gigantic size and they inhabited the sea in the Silurian and early Devonian periods. Specimens of Pterygotus (Fig. 96) have been found which measure over 6 ft. in length. They agree with Limulus in many points, and more especially in the possession of a pair of both median and lateral eyes.

Fig. 96. — Pterygotus osiliensis. Dorsal View, reduced. An Extinct Arthropod, found in Upper Silurian Strata, and showing Paired Pits in the Situation of the Paired Median Eyes of Limulus and Euscorpius. (From Wood's Palaeontology, after Schmidt.)

These were situated in the dorsal shield or carapace in the same position as in the king crab, and also in the same position as in the ancient mailed fishes, the Ostracodermata. The resemblance of Limulus to the latter is striking, but there are also important differences in the general structure of the body and limbs, which will be discussed in Chapter 23, on the geological evidence of the antiquity of the pineal organ.

The structural resemblances between the Eurypterids and certain living Arachnida, the Trilobites, and Limulus may, however, be alluded to at this stage. It is obvious that in the enormous period of time which has elapsed since the extinct Eurypterids and Trilobites existed, divergent changes in structure will have been evolved. These will be most evident in the adults of animals which have descended from the original stock and there will be less difference in the larval forms ; secondly, although the larval forms may be expected to resemble the ancestral stock more closely than do the adult forms, we may also expect very considerable differences in these. Thus, the likeness of the larval form of Limulus to the Trilobites is so close that the resemblance was regarded as unquestionable evidence of the origin of Limulus from a Trilobite stock. The resemblances are chiefly in the anterior part of the body, and concern the structure and general form of the head-shield and the relative position of the senseorgans, including the median and lateral paired eyes ; on the other hand, the contrast between the appendages of the prosoma and those of the abdomen which is so marked a feature in Limulus, is absent in the Trilobite, and Limulus shows no trace of the antennae of the Trilobite. Thus, according to MacBride, the so-called trilobite larva of Limulus represents not a Trilobite but an Arachnid, which is not very unlike the adult animal, but with segments which are free from one another and without the long tail. Such Arachnids are known to have existed in the Silurian epoch (Hemiaspis), but the stock from which the Arachnids and Trilobites diverged must have been still farther back. The conclusion to be drawn from these premises is that the head region, with its important senseorgans, although it has undergone adaptational changes, has remained in its general form essentially the same since the early palaeozoic periods ; and it was thought that the Protostraca, a name suggested by Korschelt and Heider for the precursors of the Palaeostraca, were the ancestors of the Arachnids, Trilobites, and possibly also of the Ostracodermata, or ancient mailed-fishes ; further, that they possessed sense-organs, visual and static situated in the head-shield in the same relative positions to one another as in Apus, Limulus, certain insects, spiders, and vertebrates.

The Eyes of Trilobites

The existence of eyes has been demonstrated in most of the Trilobites, and they present great variations in form and size. By far the greater number are of the composite, faceted type ; they are placed laterally, and are supported by the movable cheeks ; the visual area is usually crescentic, but it may be rounded or oval in form. The adjoining part of the fixed cheek is often raised in the form of a lip or palpebral lobe. Their size varies ; they may be quite small, as in Encrinurus and Trimerocephalus, or very large as in A^glina (Fig. 97, A), in some species of which, nearly the entire area of the movable cheeks is faceted and the visual surface extends right round the outer edge of the head-shield. In one species, Remopleurides radians, the number of facets has been estimated to be 15,000. In some Trilobites the eyes appear to be absent altogether, e.g. Conocoryphe and Agnostus, and in others they are so difficult to detect that for a long time they remained unrecognized — Agraulus, Sao, Ellipsocephalus.

The paired lateral or composite eyes of Trilobites are of two types :

Holochroal (Fig. 97, A) : the visual area being covered with a continuous horny cuticle or cornea, which may be either smooth, so that externally it gives no indication of its aggregate nature, or granular on account of the unevenness produced by the facets beneath. According to von Zittel the lenses of the ommatidia are often visible by translucence.
Schizochroal (Fig. 98) : this type is limited to the single family Phacopidae. The visual area consists of small round or polygonal openings occupied by separate corneal facets between which is an interstitial test or sclera.

Fig. 97. A — JEglinaprhca. Ordovician ; Vosek, Bohemia. Glabella (gl.) large, showing a median impression (m. imp.), and paired lateral impressions (/. imp.) ; the fixed cheeks are suppressed and the facet eyes (/. ey.), which are very large, occupy nearly the whole area of the free cheeks. (After Barrande.)

B — Harpes iingitla, showing paired ocelli (oc.) on the fixed cheeks (/. ch.). The free cheeks are ventral in position and there are no facet-eyes. The cephalon, or dorsal shield, has a wide, pitted, marginal expansion (m. exp.), which is prolonged backward on each side into a pointed spine. Ordovician, Bohemia. (After Barrande, from von Zittel.)

Fig. 98. — Phacops Latifrons. Bronn. Devonian. Gerolstein, Eifel district. (From von Zittel.)

The animal is rolled up and seen from the side. The prominent compound eyes are of the schizochroal type.

But beside the faceted or lateral paired eyes of Trilobites there are indications in some of the more primitive types of eye-spots or ocelli. These are simple in type and may be paired or unpaired. The existence of such ocelli is indicated in Harpes (Fig. 97, B), and some specimens of Trinucleus (Figs. 100, 101, 102). They appear in the form of one to three simple elevations or small tubercles, on the fixed cheeks at the ends of the eye sutures, while the ordinary faceted eyes on the movable cheeks are absent. They are regarded as being correlated with the ocelli or median paired eyes which are present in many of the Crustacea and in Limulus ; and probably also those of Arachnida, Insecta, and other Classes of invertebrates.

Fig. 99. — Deiphon. Silurian. Bohemia. (After Barrande.)

Glabella globular, without lateral furrows. Free cheeks minute. Fixed cheeKs produced on either side into a long curved spine. Eyes small at base of spine.

The meaning of the impressions on the glabella of certain Trilobites is not certain, but the existence of two pairs of ocelli on the carapace of certain living crustaceans, Arachnida, and Insecta, either with or without lateral eyes, in the same relative position as in the Trilobites, suggests that there may have been two pairs of ocelli on some Trilobites and that either one or both of these pairs might in some cases have fused so as to form a single or median eye ; or that indications on the superficial surface of the glabella of either one or both pairs might have disappeared altogether. Further, that these median eyes were already degenerating in some of the earliest known examples of Trilobite and that they were being replaced by lateral paired eyes of the faceted type, the variations in size and complexity of which points to the great length of the period during which this differentiation had been taking place.

The possibility of there being more than one pair of median eyes — or even of lateral eyes — in trilobites is indicated by the distinct evidence of metamerism in the glabella and other parts of the head. There are marked variations in the size and form of the glabella in different species of Trilobite. In the typical forms, however, it is seen to be composed of five segments (Figs. 103 and 104); these are separated by four transverse or oblique grooves. The anterior segment is known as the frontal lobe and sometimes bears a single median impression, or there may be three pits, one median usually in front and two lateral. The posterior segment is known as the occipital ring and is separated by an occipital or nuchal furrow from the fourth segment.

Fig. 100. — Median Eye Tubercle in Tretaspis Seticornis (His).

A, 80 ; B, 36 ; C, D, 50. A, C, D showing five small pits on the tubercle. A-B : meraspid stage II. C-D : adult. B : lateral view of cephalon with the big median tubercle, m.e. A-B : from the Trinucleus shale Dalarne. C-D : from the upper Trinucleus limestone, jr. : pseudo-frontal lobe. Frogno Ringerike. (After L. Stormer.)

Fig. 101. — Transverse Section through the Median Eye Tubercle (m.e.) of an Adult Specimen of Tretaspis Kia^ri n.sp. (After L. Stormer.)

According to von Zittel the Trilobites differ from Limulus and all other arthropods in having compound (faceted) eyes supported on free cheekpieces, which he believed represent the pleura of a head-segment, which is lost except in some forms with stalked eyes and in the cephalic neuromeres of later forms.

Fig. 102. — Longitudinal Section through the Median Eye Tubercle (m. e) of Trinucleus Bucculentus. Adult Specimen. (After L. Stormer.)

Since the publication of von Zittel's Palceontology in 1900 important recent work, which has been carried out with the employment of modern technique, has confirmed and supplied additional evidence of the presence of median eyes in the larvae and adult specimens of trilobites. Thus Lief Stormer in 1930 demonstrated the presence of a hollow, slightly raised median-eye tubercle in several species of Trinucleidae, Fig. 100 ; he also succeeded in obtaining sections of these, including a transverse section of the median eye of Tretaspis Kiceri (a photograph of which is reproduced in Fig. 101) and a longitudinal section of the median eye of Trinucleus bucculentus (Fig. 102). In Fig. 100, which shows the median eye tubercle of Tretaspis seticornis (seen from above + 80 diameters), there are five small pits arranged like the •' of a playing-card, and a similar arrangement is seen in the meraspid stage II of Tretaspis seticornis. The explanation of the median pit has not been definitely settled, two or three alternative suggestions having been advanced. Thus in Apus, according to the accounts of Patten (191 2) and Holmgren (191 6), the median parietal eye consists of four distinct ocelli, which have migrated inward during development from the sides, and Patten states that they are enclosed in a common sac which opens to the exterior by means of a short duct or pore. Holmgren says that in the Nauplius eye there are, in his opinion, no less than five retinas. The five pits in Tretaspis, therefore, might be interpreted as representing two pairs of ocelli, one pair in front of and the other behind the central opening of the common sac. The central pit may, however, represent a pair of ocelli which have fused in the median plane, as commonly occurs in the single median eyes of insects and some crustaceans. In this connection it is interesting to recall the conditions found in the freshwater mites (Hydrachnida) (see p. 129), more particularly the eyes of Eylais extendens, in which four eyes close together near the median plane are enclosed in a dense capsule shaped like a spectacle frame ; and the genus Hydroma in which two eyes enclosed in a chitinous capsule are separated by the whole width of the dorsal shield, in the centre of which is placed the organ which has been described as the 5th median unpaired eye (Fig. 92).

Fig. 103. Dorsal aspect of a Trilobite, Dalmanites socialis, an extinct arthropod, peculiar to and characteristic of the palaeozoic rocks, the class being especially abundant from the upper Cambrian to the Carboniferous.

c. sh. : cephalic shield. m.c. : movable cheek.

e. : situation of eye. p. : pygidium.

fixed cheek. pi. : pleura. frontal suture. th. thorax, glabella. f.s. gl.

(After Gaerstaecker, redrawn from Textbook of Zoology, Parker and Haswell.)

B. Hanstrom in 1926 published a full account of the genesis of the eyes and visual centres of the Turbellaria, annelids, and arthropods, and included in the latter some important observations on the eyes of

Fig. 104. A — Dalmanites socialis 1/1. Dorsal aspect of cephalon, or head-shield, showing the glabella with single median impression ; three primary dorsal furrows and an occipital sulcus, which subdivide the glabella into five segments. There is a V-shaped fixed cheek and a triangular free cheek bearing a prominent facet eye ; the genal angles are prolonged backwards as strong, tapering spines.

B — Ordovician — Bohemia. Diagrammatic representation of similar specimen.

f.l. : frontal lobe. p. I. : palpebral fold.

f.s. : facial sulcus.

s.m. : marginal furrow.

gl. : glabella.

s.o. : occipital sulcus

g. sp. : genal spine.

1, 11, in : lateral lobes.

l.m. : marginal furrow.

1, 2, 3 : lateral furrows

oc. : compound eye.

trilobites, Xiphosura, and Euryptera, in addition to those on living representatives of the class. Commenting on the work of Barrande in 1872, he alludes to his illustration of the head of /Eglina prisca (Fig. 97, A), showing three impressions on the glabella, which were regarded by Handlirsch (1905) as indications of the existence in it of three highly developed median eyes, corresponding to the three frontal eyes of insects. This interpretation of the three impressions on the glabella of JEglina is in Hanstrom 's opinion correct. For although the eyes of the trilobites have the closest connections with the simple nauplius eyes of the crustaceans, it would not be surprising if the median eyes of /Eglina, which was distinctly a pelagic species, should have developed, as an adaptation to a pelagic mode of life, and for the completion of the dorsal field of vision in the form of three independent visual organs. The three pigmented cup-shaped ocelli of certain Copepods and Ostracoda have also developed in connection with a pelagic mode of life as three independent eyes, and in his opinion this also took place in the case of the eyes of the trilobite Tr {nucleus. Moreover, Woodward has interpreted the small depressions on the glabella of trilobites as median eyes ; and in 1897 Beecher mentions ocelli as being " rarely present," although Hanstrom thinks it probable that he alluded to the stemmata of Harpes, which were also regarded by Kishinouye (1892-3) as indications of median eyes ; but according to the more recent researches of Richter (191 5-1920 and 1922), these are not median eyes but degenerated remnants of aggregated lateral eyes. Ruedemann (19 16), referring to the median tubercle on the glabella of trilobites, writes that " In studying the structure of the tubercle it was found that the median eye presents all stages of development seen in other crustaceans from mere transparent thinner spots of the test to a lenticular body covered by a thin cornea," as in Harpes. The relation of the median eyes to the surface layers in fossil crustaceans is interesting in connection with the work carried out on living representatives of the extinct types, such as that by Lankester and Bourne on Limulus and by Margaret Robinson in 1892 on decapod arthropods, more particularly in adult specimens of Palcemon serratus, Verbius varians, and Pandalus annulicornis . Both in Limulus and in eight separate species of Candidas, paired median eyes which are present in the nauplius stage have been demonstrated as persisting in the adult animals. In these they are covered over by a dense chitinous layer, lined on its under surface by an epithelial stratum. That is to say, they are deeply buried and can have little or no function. An idea of their general appearance and relations may be obtained by reference to Fig. 105, A, B, C, which shows in A the two median eyes exposed by removing the rostrum from a fresh specimen ; C, a transverse section of the same, illustrating the structures lying superficial to the fused median eyes, namely, a chitinous layer, connective tissue and muscle, an epithelial layer and a blood sinus ; it also shows the relation of the pigment cells to the nerve-end cells and the epithelial cords by which the eyes are suspended between the chitinous roof above and the cerebral ganglia below ; B, a transverse section in front of C, shows the relations of the suspensory cords to the epithelium of the roof anterior to the median eye.

The general disposition of the parts will be realized if we quote in full Miss Robinson's description of her dissection of Pandalus : After removal of the rostrum, " the median eye can be seen as a black speck lying in the centre of the triangle formed by the brain and the stalks of the lateral eyes. The brain is covered by chitin which is lined by a thin layer of ectoderm ; if the dorsal part of this and a small part of the brain be removed, the eye can be seen lying in a blood space just dorsal to the brain. It has the appearance of a black X which is slung on to the

Fig. 105. — Paired Median Eyes in Nauplius of Decapod Carididae. (After Margaret Robinson.)

A : Median eyes of Pandalus annulicornis, viewed from dorsal aspect after removal of the rostrum. The pigment appears as an X-shaped mass between two lateral groups of nerve-end cells. B : Transverse section through region of median eyes of Virbius varians, showing ingrowth of suspensory ectodermal cords. C : Transverse section farther back than B, showing the central mass of pigment cells, with the nerve-end cells on either side. A median ventral group of nerve-end cells is also present. D : Diagrammatic horizontal section through brain of Virbius varians. The chitinous roof which covers the median eyes is not shown in the sections B and C.

ant. 1 and ant. 2 : first and second

antennas. bl. c. : blood corpuscle. bl. s. : blood space.

comm. : circumoesophageal commissures. ect. : ectoderm. end. : endoderm. g. br. : nuclei of ganglion cells of brain.

g.c. : ganglion cell.

I.e. : lateral eye.

m. : muscle.

n.c. : ectodermal cord.

op. n. : optic nerve.

p. : pigment.

p. corp. : pigment corpuscles.

p.m.e. : pigment mass of median eye.

ectoderm by two slender threads, which swell out in the concavities of the X and then narrow again as they approximate to each other.

" The X consists of two large pigment cells, and the supporting strings are formed partly of ectoderm and partly of club-shaped nerve-end cells. No trace of a refractive body could be found in the eye. On the whole the resemblance to the eye of Branchipus, as described by Claus and more recently by Patten, also in the scorpion (Chap. 24, Fig. 350, p. 362, Fig. 246, p. 349), is very close."

Although Miss Robinson alludes to the eye as a single structure, it is obvious that in doing so she was following the customary designation at the time, and that she was well aware of the bilateral nature of the eyes in the nauplius phase of development. The interesting points, apart from the question of bilaterality, in the comparison of the median eyes of Limulus and the Decapod Arthropoda with the median eyes of trilobites, is the fact that in the adult stages of both the living and in many of the extinct forms the eyes lay deeply buried beneath the surface and presumably can have had little or no function. Judging from the appearances seen in many adult specimens of trilobite, the median eyes were already buried and presumably functionless even at the remote epoch, lower Cambrian, when some of the earlier and more simple types of trilobite were alive. On the other hand, indications of the existence of median eyes are present on the surface of the glabella in certain adult forms, e.g. /Eglina prisca, as well as in the nauplius stages and afford valuable evidence of a transitional phase in the phylogeny of the median eyes, namely, an earlier stage when these eyes were superficial and presumably functional and a later stage when in the adult animal they became buried, their function having been taken over by the lateral eyes. Thus in many species, and more particularly those which are more recent and more highly evolved, no indications are present on the glabella of the frontal or median eyes. In view of the recent work by Giinther on the ontogeny of the simple and compound eyes of the water-beetle Dytiscus marginalis (p. 117), in which he has shown that the eye-spot and the six ocelli which on each side precede the development of the compound eye sink beneath the surface, their remnants, however, remaining during life as closed pigmented vesicles attached to the optic nerve, the absence of indications of their presence on the superficial surface of the glabella in some species is not surprising. The replacement of the median eyes by the compound lateral eyes involves the whole problem of the evolution of the compound eye. For since, as pointed out by MacBride, Peripatus, Myriapoda, the lower insects and the larvae of the higher insects agree in possessing only simple, pit-like ocelli, it is fairly clear that the massing together of these ocelli to form a compound eye must have occurred during the evolution of the arthropodan stock and presumably had not yet occurred in the primitive land Arthropoda. But primitive Crustacea, primitive Arachnida, and most Trilobita possess compound eyes. The Insecta, therefore, must have been derived from a primitive arthropod stock in which a compound eye had not yet been developed, and considering that a fully formed Orthopteran insect is already found in Silurian strata, this is what we might naturally expect.

Intimately associated with the question of the origin of the compound eyes in trilobites is that concerned with what are described as blind trilobites, a problem which was investigated by Cowper Reed in 1898. These he conceived as being divisible into two groups :

(1) those in which the eyes were not present, because of the low phylogenetical and morphological rank of the genera in question, as their general structure and stratigraphical appearance indicate ; and

(2) those which are genetically identical or closely allied to forms possessing eyes which are of high phylogenetic rank, and have lost their visual organs by a secondary modification, presumably as a result of adaptation to special conditions. The first group he called the " primitive group " and the second the " adaptive group." These will be described later in the section on Geological Evidence of the Presence of Median Eyes in Extinct Animals, Chapter 23. Quite apart from evidence of the existence of trilobites in such ancient geological strata as the Upper Cambrian, the extreme variability in the types of eye met with in trilobites (Figs. 97, 98, 99, 103) as well as the marked differences in their general form point to the great antiquity of these primitive arthropods and the probability not only of evolutionary changes occurring in the direction of differentiation of more complex eyes from simpler types, but also, as has been suggested by Cowper and others, of degenerative or devolutionary changes taking place in the eyes of certain species. The recognition of such degenerative changes, as we shall see later, plays an important part in the interpretation of the conditions which are found both in the median eyes of invertebrates and in the pineal system of vertebrates.

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Cite this page: Hill, M.A. (2024, April 27) Embryology Book - The Pineal Organ (1940) 11. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_11

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