The retina of the phalangid,Opilio ravennae,with particular reference to arhabdomeric cells

Summary The retina of the phalangid, Opilio ravennae, consists of retinula cells with distal rhabdomeres, arhabdomeric cells, and sheath cells. The receptive segment of retinula cells shows a clear separation into a Proximal rhabdom, organized into distinct rhabdom units formed by three or four retinula cells, and a Distal rhabdom, consisting of an uniterrupted layer of contiguous rhabdomeres. One of the cells comprising a retinula unit, the so-called distal retinula cell (DRC), has two or three branches that pass laterally alongside the rhabdom, thereby separating the two or three principal retinula cells of a unit. The two morphologically distinct layers of the receptive segment differ with respect to the cellular origin of rhabdomeral microvilli: DRC-branches contribute very few microvilli to the proximal rhabdom and develop extremely large rhabdomeres in the distal rhabdom only, causing the rhabdom units to fuse. Principal retinula cells, on the other hand, comprise the majority of microvilli of the proximal rhabdom, but their rhabdomeres diminish in the distal rhabdom. It is argued that proximal and distal rhabdoms serve different functions in relation to the intensity of incident light.In animals fixed 4 h after sunset, pigment granules retreat from the distal two thirds of the receptive segment. A comparison of retinae of day- and night-adapted animals shows that there is a slight (approximately 15%) increase in the cross-sectional area of rhabdomeral microvilli in dark-adapted animals, which in volume corresponds to the loss of pigment granules from the receptive segment. The length of the receptive segment as well as the pattern and shape of rhabdom units, however, remain unchanged.Each retinula unit is associated with one arhabdomeric cell. Their cell bodies are located close to those of retinula cells, but are much smaller and do not contain pigment granules. The most remarkable feature is a long, slender distal dendrite that extends up to the base of the fused rhabdom where it increases in diameter and develops a number of lateral processes interdigitating with microvilli of the rhabdom. The most distal dendrite portion extends through the center of the fused rhabdom and has again a smooth outline. All dendrites end in the distal third of the proximal rhabdom and are never present in the layer of the contiguous distal rhabdom. Arhabdomeric cells are of essentially the same morphology in day- and night-adapted animals. They are interpreted as photoinsensitive secondary neurons involved in visual information-processing that channel current collected from retinula cells of the proximal rhabdom along the optic nerve. A comparison is made with morphological equivalents of these cells in other chelicerate species.     

The lateral eyes of the scorpion,Androctonus australis

Summary The dioptric apparatus of the lateral eyes of the scorpion, Androctonus austrails, consists of a cuticular lens, but lacks a vitreous body. The retina is formed by (1) retinula cells displaying a contiguous network of rhabdoms; (2) arhabdomeric cells bearing a distal dendrite that contacts retinula cells via numerous projections and ends before the rhabdomere of the retinula cells; (3) pigment cells that ensheath retinula and arhabdomeric cells with the exception of the contact regions; and (4) neurosecretory fibres possibly originating in the supraesophageal ganglion. The ratio of the number of retinula to arhabdomeric cells is determined to be close to 2 1 in the three larger anterolateral eyes, in contrast to the median eyes where the ratio is 5 1.The construction of the dioptric apparatus as well as the anatomy of the retina imply that in the lateral eyes of Androctonus australis visual acuity is reduced. A certain degree of spatial discrimination, however, may be retained by the presence of a relatively high number of arhabdomeric cells. It is suggested that the lateral eyes of A. australis mainly function as light detectors, e.g., for Zeitgeber stimuli.Supported by grant no. FL 77/8-10 from the Deutsche Forschungsgemeinschaft     

The fine structure of the lateral eyes ofNeocarus texanus Chamberlin and Mulaik, 1942 (Opilioacarida,Acari, Arachnida,Chelicerata)

Summary Neocarus texanus, a primitive mite, bears two pairs of eyes, which are principally similar in ultrastructure. Each eye is covered externally by a cuticular cornea. It is underlain by flat sheath cells which send extensive processes into the retina. The retina is composed of distal and proximal cells. The 20 distal cells of the anterior eye are inversely orientated and form 10 disc-like rhabdoms. They represent typical retinula cells. Each rhabdom encloses the dendritic process of a neuron, the perikaryon of which is located outside the retina (proximal cells). The significance of this cell is not known. The retina is underlain by a crystalline tapetum. In the posterior eye 14 retinula cells form 7 rhabdoms in an arrangement similar to the anterior eye. The eyes of one side of the body are located within a capsule of pigment cells. Together the axons of the distal and proximal cells form the two optic nerves, one on each side of the body. The optic nerves leave the eyes anteriorly and terminate in two optic neuropils located in the brain.From structural evidence it is concluded, that the resolution of the eyes must be rather low.The peculiar proximal cells have not been observed previously in Acari. They probably resemble at best the eccentric cells and arhabdomeric cells of xiphosurans, scorpions, whip-scorpions and opilionids. Also, inverse retinae and tapeta of the present type have not been found in Acari until now, but are present in other Arachnida. Thus the eyes ofNeocarus texanus evidently represent a unique type within the Acari.     

Polarization sensitivity of individual retinula cells

Summary This paper elucidates the influence of the structure of a rhabdom on the polarization sensitivity of its retinula cells. The terminology polarization sensitivity (PS) and dichroic sensitivity () needs clarification. expresses the directional property of the local microvillar medium and is independent of the gross morphology of the rhabdom. The PS of a retinula cell is that found by single cell electrophysiology and depends strongly on the gross morphology of the rhabdom. Both and PS are ratios of the effects of theE vector of linear polarized light parallel to, to that perpendicular to the microvilli. From the theoretical analysis and its correlation with experiments the following is concluded.     

Neurosecretory fibres in the median eyes of the scorpion,Androctonus australis L.

Summary The retina of the median eyes of the North African scorpion, Androctonus australis L., is supplied with numerous neurosecretory nerve fibres which establish synaptoid contacts on the retinula cells. The number of fibres or profiles of varicosities of fibre terminals associated with a retinular unit (five retinula cells with a fused rhabdom) varies between 10 and 20. Electron-opaque vesicles with a diameter of 80–100 nm are abundant within the axonal profiles. The synaptoid junctions are characterized by postsynaptic electron-dense material on the inner leaflet of the retinula cell membrane and, frequently, presynaptic submembranous dense material. Because of these ultrastructural features, the junctions observed here resemble typical interneuronal synaptic contacts. Hence this kind of neurosecretory junction appears to be unique among arthropods.It is suggested that the neurosecretory fibres within the retina represent the efferent pathways for the control of the circadian pigment movements within the retinula cells.Supported by the Deutsche Forschungsgemeinschaft (F1 77/7)     

THE MICROSTRUCTURE OF THE COMPOUND EYES OF INSECTS

The apposition eyes of two diurnal insects, Sarcophaga bullata (Diptera) and Anax junius (Odonata), have been examined with the electron microscope. In the latter case only the rhabdom is described. The rhabdom of the fly consists of a central matrix and seven rhabdomeres, one for each retinula cell. The rhabdomeres show an ordered internal structure built up of transverse tubes, hexagonal in cross-section. These slender compartments running the width of the rhabdomere are 370 A in diameter. After fixation with osmium tetroxide the walls of the compartments are more electron dense than the interiors. The retinula cells contain mitochondria, and pigment granules smaller than those found in the pigment cells. These granules tend to cluster close behind the membranes which separate the retinula cells from their rhabdomeres. The rhabdom of the dragonfly is a single structure which appears to be composed of three fused "rhabdomeres," each similar to a rhabdomere of Sarcophaga. Reasons are given for believing that the rhabdom may be the site of photoreception, as well as the organ for analyzing plane-polarized light, as suggested by other workers.     

Rhabdom breakdown in the eye of Cirolana borealis (Crustacea) caused by exposure to daylight

Summary The effect of daylight on the compound eye was investigated in the deep-water crustacean isopod Cirolana borealis Lilljeborg. The animals were captured and fixed at night (dark-exposed, i.e. not exposed to light) and day (daylight-exposed), respectively. Changes in light and darkness have an effect on the retinula cells; the ultrastructure of dark-exposed eyes is characterized by well-preserved organelles and cytoplasm. The photoreceptor membranes covering the microvilli are regularly aligned, and the outline of the villi is smooth. Electron-dense pigment granules are evenly distributed in the cytoplasm of the retinula cell outside the rhabdom. Daylight-exposed eyes differ from the dark-exposed eyes in the following aspects: (i) the microvilli are disrupted, (ii) retinula-cell pigment is found in the rhabdom, and (iii) the cytoplasm of retinula cells is vesiculated. These results are interpreted as retinal damage caused by excess exposure to light.     

The fine structure of some carabid beetle eyes,with particular reference to ciliary structures in the retinula cells

Paired centrioles and associated ciliary root material occur in all eight retinula cells in the nine species investigated. In the diurnal Notiophilus, Elaphrus and Bembidion where the distal rhabdomere of cell 7 is fused with the proximal rhabdom formed by cells 1 to 6, the roots in cells 1 to 6 extend for the entire length of the retinula. In Notiophilus their arrangement around the rhabdom suggests a complementary mechanical relationship between the six large roots and the four Semper cell processes. In five relatively nocturnal species a retinula cell column separates the distal rhabdomere from the proximal rhabdom. In cells 1 to 6 root material is associated with the distally located centrioles as follows. In Leistus roots extend into the proximal rhabdom layer. In Loricera and Agonum roots at the level of the proximal rhabdom are not continuous with the rootlets or short roots associated with the centrioles. In Pseudophonus and Feronia, and in the diurnal Cicindela, short rootlets link the centrioles. Cell movements on dark-adaptation of Notiophilus and Cicindela include shortening of the crystalline tract. In Notiophilus the entire rhabdom is apparently displaced, whereas in Cicindela the narrow distal rhabdomere becomes dissociated from the proximal rhabdom.     

The compound eye of Parnara guttata (Insecta,Lepidoptera, Hesperiidae): Fine structure of the ommatidium

Summary The fine structure of an ommatidium of a skipper butterfly, Parnara guttata, has been studied using the electron microscope. Each ommatidium has nine retinula cells, which were classified into three groups: two distal, six medial and one basal retinula cells. The rhabdomeres of the distal retinula cells are localized in the distal part of the rhabdom, while those of the six medial retinula cells appear throughout most of the rhabdom. The rhabdomere of the basal retinula cell occupies only the basal part of the rhabdom. The rhabdomeres of four medial cells are constructed of parallel microvilli, while fan-like microvilli form the rhabdomeres of other two medial retinula cells. The distal and basal retinula cells have rhabdomeres consisting of both parallel and fan-like microvilli. This is the first time the construction of the rhabdomeres of the distal and basal retinula cells has been described in such fine detail for a skipper butterfly. Nine retinula cell axons of each ommatidium extend to the first neuropile of the optic lobe, the lamina ganglionaris. No difference was found in the number of retinula cells of an ommatidium or the shape of the rhabdom between the dorsal and ventral regions of the compound eye.     

The Anostracan Rhabdom and the Basement Membrane. An Ultrastructural Study of the Artemia Compound Eye (Crustacea)

Abstract The ommatidia of the compound eyes of Artemia salina L. are normally composed of four crystalline cone cells containing glycogen. The cells are enveloped by two so-called “cellules épidermiques juxta-cristallines”. There are also six pigmented retinula cells, all contributing to the rhabdom. A peculiar feature of the Artemia crystalline cone cells is that their elongated parts, the so-called cone cell roots, widen and flatten proximally, forming interdigitating “endfeet”. The basement membrane thus consists of a cellular portion combined with the basal lamina. The main mass of the rhabdom of the Artemia eye is built up by five retinula cells, two contributing a smaller part. The microvilli are oriented in four directions, two being orthogonal. The sixth cell contributes on two small portions to the rhabdom in the distalmost and a more proximal position. The rest of it runs axon-like outside the omnatidium. Where the sixth cell wedges in, the direction of the microvilli is changed and has no orthogonal pattern. Two rhabdom types of compound eyes are distinguished: the decapod or banded or layered rhabdom: and the anostracan rhabdom with continuous rhabdomeres.     

The fine structure of the eyes of some bristly millipedes (Penicillata, Diplopoda): additional support for the homology of mandibulate ommatidia

The eyes of adult Phryssonotus platycephalus (Synxenidae) and Polyxenus lagurus (Polyxenidae) were investigated by light and electron microscopy. At each side of the head, various numbers of eye cups are situated on projections, the eye hills. The eye cups of P. platycephalus and P. lagurus are similarly structured and considered homologous sense organs. Each corneal lens is biconvex and formed by four to six pigmented corneagenous cells with their nuclei displaced towards the mid-periphery of the eye cup. The corneal surface displays a conspicuous nanostructure of fingerprint-like ridges in P. platycephalus. However, the corneal surface appears smooth in P. lagurus. In P. platycephalus. A rudimentary crystalline cone is observed in each eye cup, always produced by a constant number of three eucone cells. The crystalline cone is wedged between the corneal lens and the distal rhabdom and consists of three distinct compartments. Each cone compartment is connected to the voluminous proximal nuclear region by one elongated cytoplasmic process, which runs through the infraretinular space. A dual type retinula is always arranged in two distinct horizontal cell layers. The distal retinula contains an unfixed number of four to five cells in P. lagurus, whereas it contains five to eight cells in P. platycephalus. The distal retinula cells form a large and fused axial rhabdom. A constant number of three proximal retinula cells give rise to a small axial rhabdom, which looks more or less triangular in cross sections. The basal matrix is rather thin, inconspicuous and lines the bases of the eye cups. The ultrastructure of the eye cups of P. platycephalus resembles that observed in the ommatidia of the centipede Scutigera coleoptrata. The present study lends additional support to the homology of mandibulate ommatidia, because of the common possession of crystalline cone cells and a bilayered dual type retinula in the eye cups of P. platycephalus. Ommatidia or unicorneal eyes that include eucone cells with nuclei displaced outside the cone compartments, as found in Scutigeromorpha and Penicillata, might also be interpreted as an additional autapomorphy of the Myriapoda. The suggested homology of scutigeromorph and penicillate eyes implies that penicillate eye cups have to be considered modified, probably miniaturized ommatidia.     

Fine structure of light- and dark-adapted eyes of desert ants,Cataglyphis bicolor (Formicidae,Hymenoptera)

Among ants, Cataglyphis bicolor shows the best performance in optical orientation. Its eye is of the apposition type with a fused rhabdom. Morphological studies on the general struture of the eye as well as the effect of light have been carried out with transmission and scanning electron microscopy. An ommatidium is composed of a dioptric apparatus, consisting of a cornea, corneal process and a crystalline cone, the sensory retinula, which is made up of eight retinula cells in the distal half and of an additional ninth one in the proximal half. The ommatidia are separated from each other by two primary pigment cells, which surround the crystalline cone and an average of 12 secondary pigment cells, which reach from cornea to the basement membrane. The eye of Cataglyphis bicolor possesses a light intensity dependent adaptation mechanism, which causes a radial and distal movement of the pigment granules within the retinula cells and a dilatation of cisternae of the ER along the rhabdom. Until now, no overall order in arrangement of retinula cells or direction of microvilli has been found from ommatidium to ommatidium. Such an order, however, must exist, either on the retina or the lamina level, since we have proven the ant's capacity for polarized light analysis.     

The eye of the soldier beetle Chauliognathus pulchellus (Cantharidae).

The soldier beetle eye is unusual in having large optically isotropic corneal cones which project inwards from a thick isotropic cornea. Refraction is mainly at the corneal surface. Calculation shows that the first focal plane is near the tip of the cone, from which the optical pathway continues as a crystalline tract. At the distal end of the crystalline tract, 3 micrometer in diameter, the four cone cells enclose the proximal tip of the corneal cone; at the proximal end they enclose the distal tip of a long fused rhabdom rod. The eye is remarkable in that there are two classes of retinula cells; four cells contribute to the long thin axial rhabdom, 2 micrometer in diameter and 120 micrometer long, and the other four cells form two rounded rhabdoms, 10 x 4 micrometer in cross-section and 20 micrometer deep, which lie to one side of the optical axis. The physiological properties of individual retinula cells were measured by intracellular recording. The retinula cells are of three spectral types with peaks near 360, 450 and 520--530 nm. Except by the criterion of spectral sensitivity, the retinula cells sampled could not be sorted into more than one class. The measured value of the acceptance angle, near 3 degrees in the dark-adapted state, is consistent with the hypothesis that all sampled cells were of the anatomical type that participate in the central rhabdom rod. A calculation of the theoretical field size of individual retinula cells from measurments of refractive index and lens dimensions predicts that cells which participate in the central rhabdom will have acceptance angles near 3 degrees. The conclusion, therefore, is that only one anatomical type of cell has so far been sampled.     

The eyes of mesopelagic crustaceans: I. Gennadas sp. (Penaeidae)

Summary The eye of the deep-sea penaeid shrimp Gennadas consists of approximately 700 square ommatidia with a side length of 15 n. It is hemispherical in shape and is located at the end of a 1.5 mm long eye stalk. The cornea is extremely thin, but the crystalline cone is well-developed. A clear zone between dioptric structures and the rhabdom layer is absent. A few pigment granules are found within the basement membrane; otherwise they, too, are absent from the eye of Gennadas. The rhabdom is massive and occupies 50 % of the eye. It consists of orthogonally oriented microvilli (the latter measuring 0.07 m in diameter) and is 75 m long. In cross sections adjacent rhabdoms, all approximately 8 m in diameter, form an almost continuous sheet and leave little space for retinula cell cytoplasm. In spite of a one h exposure to light, rhabdom microvilli show no disintegration or disruption of membranes. Vesicles of various kinds, however, are present in all seven retinula cells near the basement membrane. Bundles of seven axons penetrate the basement membrane. On their way to the lamina they often combine and form larger aggregations.The authors wish to thank the director of the Meat Industry Research Institute in Hamilton and his staff for the use of their electron microscope facilities     

Der Feinbau des Auges der Mehlmotte,Ephestia kuehniella Zeller (Lepidoptera,Pyralididae)

Zusammenfassung Die Retinula im Ommatidium der Mehlmotte besteht aus einer wechselnden Anzahl (9–12, meist 11) langgestreckter, prismatischer Sinneszellen. Außerdem enthält jede Retinula nahe der Basalmembran im Zentrum zwischen diesen distalen Retinulazellen noch eine basale Retinulazelle. Die Längsachse der Retinula wird von der Achsenstruktur eingenommen, die aus Mikrovilli besteht. Ihr distaler Teil ist der Achsenfaden, der breitere, proximale Teil bildet das Rhabdom. Dieses erscheint im Querschnitt meist vierstrahlig gelappt, da seine Außenseite in Längsrichtung tief gekehlt ist. Der Rhabdomquerschnitt gliedert sich in mehrere Schöpfe parallel angeordneter Mikrovilli (Rhabdomsektoren); jeder Rhabdomsektor besteht aus 1 oder 2 Rhabdomeren. Die basale Retinulazelle entsendet einen kleinen Schopf von Mikrovilli in die proximale Spitze des Rhabdoms. Die distalen Retinulazellen setzen sich proximal in Neuriten fort, welche sich in Einkehlungen der basalen Retinulazelle bzw. der Tracheenendzelle einschmiegen. Jeweils eine Tracheole durchbricht zusammen mit dem Neuritenstrang einer Retinula die Basalmembran; sie verzweigt sich distal zu ca. 30 Tracheolen, die die Retinula umhüllen.Die Kristallkegelzellen grenzen distal an die Cornea; proximal laufen die Kristallkegelzellen eines Ommatidiums in einen gemeinsamen Fortsatz aus, der zwischen den Retinulazellen unmittelbar am Achsenfaden endet. — Nur das helladaptierte Auge wurde untersucht. Hierbei erscheint im distalen Teil der Retinula nur der Achsenfaden lichtdurchlässig, das Cytoplasma der Retinulazellen hingegen von Pigmentgrana durchsetzt und für Licht undurchlässig.

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Fine structure of the eye of the meal moth, Ephestia kuehniella Zeller (Lepidoptera, Pyralididae)

Summary In each ommatidium of the meal moth a retinula is formed from a varying number (9–12, mostly 11) of elongated, prismatic sense cells. In addition, a basal retinular cell is situated near the basement membrane in the center of the other (distal) retinular cells. The axis of the retinula is occupied by many microvilli forming the axial structure, the distal section of which is the slender axial thread. Proximally, the axial structure widens (to 8.5 m instead of 1 m in diameter) and is now called rhabdom. Cross sections of the rhabdom mostly look like a petaloid with four petals; this figure is due to longitudinal infoldings along the length of the rhabdom surface. The rhabdom cross section is subdivided into several brushes of microvilli (rhabdom sectors), each one being characterized by an approximately parallel arrangement of its microvilli. One rhabdom sector may be composed of one or two rhabdomeres respectively.The basal retinular cell participates in rhabdom formation through a small brush of microvilli at the proximal end of the rhabdom. Proximally, the distal retinular cells taper into slender neurites which are embedded in grooves at the surface of the basal retinular cell and the tracheal end cell respectively. One tracheole piercing the basement membrane together with the neurites of one retinula branches into about 30 tracheoles surrounding the retinula.The crystalline cone cells touch the cornea; proximally, their cytoplasm forms a point which eventually terminates amongst the distal tips of the retinular cells, immediately at the axial thread.—Our work was restricted to light adapted eyes; in this condition, light transmission in the distal part of the retinula seems to be blocked by retinular cell pigment except inside the axial thread.

Mit Unterstützung durch die Deutsche Forschungsgemeinschaft.     

Colour receptors in the eye of the digger wasp,Sphex cognatus Smith: Evaluation by selective adaptation

Summary Pigment granule migration in pigment cells and retinula cells of the digger wasp Sphex cognatus Smith was analysed morphologically after light adaptation to natural light, dark adaptation and after four selective chromatic adaptations in the range between 358 nm and 580 nm and used as the index of receptor cell sensitivity. The receptor region of each ommatidium consists of nine retinula cells which form a centrally located rhabdom. Two morphologically and physiologically different visual units can be described, defined by the arrangement of the rhabdomeric microvilli, the topographical relationship of the receptor cells with respect to the eye axes and the unique retinula cell screening pigmentation. These two different sets of ommatidia (type A and B) are randomly distributed in a ratio of 13 throughout the eye (Ribi, 1978b). Chromatic adaptation experiments with wavelengths of 358 nm, 443 nm, 523 nm and 580 nm and subsequent histological examination reveal two UV receptors, two blue receptors and four yellow-green receptors in type A ommatidia and two UV receptors and six green to yellow-green receptors in type B ommatidia. The pigments in cells surrounding each ommatidium (two primary pigment cells, 20 secondary pigment cells and four pigmented cone extensions) were not affected significantly by the adaptation experiments.     

Regional differences in a nematoceran retina (Insecta,Diptera)

Summary The compound eye of Psychoda cinerea comprises two types of ommatidia, arranged so as to divide the retina into distinct dorsal and ventral regions. The P-type ommatidium, in the ventral part of the eye, differs fundamentally from the other dipteran ommatidia so far described, and is regarded as a primitive ommatidium. The acone dioptric apparatus is the same in both types, with a spherical lens and four Semper cells, the processes of which expand below the rhabdom to form a ring of pigment sacs. Only the distal region of the rhabdom is surrounded by a continuous ring of screening pigment, formed by 2 primary and 12–16 secondary pigment cells. The highly pigmented retinula cells penetrate the basement membrane proximally at about the level of their nuclei; in this region they are separated from the hemolymph by glial elements. The rhabdomeres R1–6 are fused to form a tube. The two types of ommatidia are defined by the arrangement of the retinula cells R7/8: in the T type the central rhabdomeres are one below the other, in the usual tandem position, whereas in the P type only R8 is central, with R7 in the peripheral ring. In the proximal region of the retina, retinula cells with parallel microvilli in neighboring ommatidia are joined in rows by lateral processes from the R8 cells. All the rhabdomeres are short and not twisted, which suggests that the retinula cells are highly sensitive to direction of polarization. The eye can adapt by a number of retinomotor processes. These findings, together with observations of behavior, imply that the psychodids have well-developed visual abilities.     

The eyes of mesopelagic crustaceans

Summary In Streetsia challengeri left and right eyes have fused and become a single cylindrical photoreceptor, which occupies the basal half of a forward directed head projection. This unusual compound eye consists of approximately 2500 ommatidia, which are arranged in such a way that the animal has almost circumferential vision, but cannot look ahead or behind. It is thought that the eye operates on light-guide principles, and that the crystalline cones are the major dioptric component. Ommatidia in anterior-posterior rows show a greater overlap of visual fields than dorso-ventrally arranged ommatidia. Cone layer and retinula are separated by a 4 m thick screen-membrane, which contains tiny pigment granules of 0.15 m diameter. Cells of unknown function and origin, containing unusual multitubular organelles, are regularly found near the proximal ends of the crystalline cone threads. The twisted rhabdoms measure 18–20 m in diameter, and consist of microvilli 0.05 m in width, which belong to five retinula cells and which show no trace of disintegration. The position of interommatidial screening pigment, the density of retinula cell vesicles and inclusions, and the narrowness of the perirhabdomal space all suggest that the eyes have been light-adapted at the time of fixation for electron microscopy. The retinula cell nuclei lie on the proximal side of the heavily pigmented basement membrane. A tapetum or basal retinula cells are not developed. It is concluded that the eye optimally combines acuity with sensitivity, and that for distance estimation parallax may be important.Address until January 25th 1978: Scott Base, Ross Dependency, Antarctica (C/-Chief Post Office, Christchurch, New Zealand)     

The morphology of the Limulus visual system

An ocellus of the horseshoe crab, Limulus polyphemus, has been serially sectioned for light and electron microscopy, its sensory cells have been indexed, and the interconnections of a third of these traced. The ocellus contains 155 retinula cells and 26 arhabdomeric cells, which are secondary sensory neurons. Of these, 55 retinula cells constitute 7 quasi-ommatidial assemblages, each innervated by at least one and a total of 9 arhabdomeric cells. When known electrotonic coupling patterns are compared with gap-junctional connections, retinula cells sensitive to visible or ultraviolet light can be tentatively identified. Retinula cell axons contribute collaterals to a synaptic plexus, in which the arhabdomeric cells apparently do not participate.