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MANY of the Annelids possess relatively large nerve fibres in their central nervous system, and those of the earthworm among the Oligochaetes, and of Halla, Arenicola, Clymenella and Axiothea among the Polychaetes, have been studied. The giant fibres in these forms arise either from a single nerve cell in the case of Halla1, or each fibre from numerous nerve cells in the case of the other forms mentioned2. According to several writers3, the single nerve fibre in the ventral nerve cord of the Sabellid, Myxicola infundibulum, is exceptionally large and we have, therefore, prepared serial sections of this species.

The snook Centropomus undecimalis is a nocturnal fish inhabiting turbid estuaries, bays, and inshore waters. Its retina is duplex, containing rods and short single, long single, and double cones. Cones are arranged in a square mosaic; the rods are extremely numerous and outer segments are short and staggered in depth. Ellipsoid mitochondria undergo morphological alterations along the vitreo–scleral axis including the appearance of elongate cristae and the development of electron-dense matrices which nearly obscure the cristae. Peripheral mitochondria give off tubular elements in the calycal processes. Pigment epithelial cells have long processes which extend vitread to the cone ellipsoids and contain tapetal spheres (0.3 to 0.5 μm). The processes form a diffusely reflecting tapetum about 80 μm thick. In the scotopic eye the tapetum is uncovered when the retinal pigment retreats into the cell bases. The reflecting material is a triglyceride, largely glyceryl tridocosahexaenoate. Its high refractive index (n = 1.50) and the close packing of the spheres cause light to be backscattered. Reflection from the white lipid tapeta is about 50%; quantal catch in the retina because of the tapetum is enhanced by a factor of about 1.5.

Eyeshine in fishes is reviewed. It originates in several ways. Many benthic fishes have pupils that are iridescent at certain angles of vie w; the color is caused by reflection of light at the surfaces of thin films in the cornea. Several kinds of cellular or extracellular features are involved. Reflection of light from the retinal surface is usually dull. In Photonectes there is bright reflection from part of the retinal surface; it comes from rows of thin cytoplasmic lamellae within Müller fiber cells. Many fishes have retinal tapeta lucida: these are diffuse reflectors made up of densely packed spherules, or specular reflectors made up of layers of thin films. Tapeta cellulosa in the inner chorioid also are specular reflectors containing thin films. Often, these tapeta can be covered over by migratory pigment in the photopic eye. Eyeshine in the light-adapted eye of several groups (scorpion fishes, etc.) comes from a reflex of light at the stratum argenteum. It is made possible because all the retinal pigment enters the cell processes, and there is no dark pigment in the chorioid.

The outer chorioid (suprachorioid) of the eye of the toadfish Opsanus is a yellow argenteum which consists of two thin sheets. The inner one contains thin flat crystals of uric acid. The uric acid content is about 40 μg/cm2 of chorioid surface. The outer sheet contains yellow, birefringent granules, as yet uncharacterized. In conjunction with certain features of the retina, the yellow chorioid is responsible for eyeshine in the photopic eye.

Guanine was found to be the main component in extracts of argentea of redfish Sciaenops ocellata (Sciaenidae) and chorioids of sheepshead Archosargus probatocephalus (Sparidae). The argenteum of the redfish contained 1.55 mg guanine/cm2 surface area of chorioid. Hypoxanthine content was about [Formula: see text] that of guanine. The purines occurred as bisymmetrical hexagonal crystals ranging in shape from needles to platelets.

Toxicity of 4 components of petroleum oils to the marine amphipod Elasmopus pectenicrus (Bate) has been assessed. Two ephemeral aromatic hydrocarbons, naphthalene (A) and 1, 2, 4-trimethylbenzene (B) were more toxic than two persistent aromatics, o-cresol (C) and o-toluidine (D). The acute toxicity concentrations obtained for individual aromatic compounds were always greater than the actual concentrations found in the water-soluble fractions (WSF) of fuel oils. Results from mixtures of 2 or more components indicated that the LC50 levels were primarily determined by the more toxic substances, A and B. Naphthalene and 1, 2, 4-trimethylbenzene became more toxic to the E. pectenicrus when present in a mixture of more than 2 components, and the toxicity increased with increasing numbers of components present. Synergistic effects, therefore, possibly occur in the whole WSF. No antagonistic effects were observed among the 4 petroleum aromatics.

In the eye of the Goldeye the photoreceptors are arranged in bundles and the pigment epithelium contains a massive reflector or tapetum lucidum. Photoreceptor bundles are arranged in parallel rows, the bundles alternating in position from row to row. Each bundle contains about 60 photoreceptors, of which 30 or so are cones. Rod outer segments lie in the scleral half of the outer retinal region of the light-adapted eye. Processes of the pigment epithelium cells extend vitread almost to the external limiting membrane; they envelop the bundles of rods and cones, and a ring of four processes surrounds each bundle. A process contains two kinds of reflecting crystals (composed of uric acid). A large part of the epithelium cell is packed with small disc-shaped crystals (crystallites) enclosed in thin membranes; the tip of the process, in the region of the photoreceptor bundle, contains orderly arrays of small rod-shaped crystals (rodlets). It is suggested that the crystallites form a diffuse reflector backscattering light into the rods; and that the rodlets reflect light regularly from their surfaces into the photoreceptor bundles. In the light-adapted state, rods are enveloped by pigment and crystallites. The organization is compared with that of other fishes that have photoreceptors in bundles (grouped retinae) and tapeta lucida.

Sperm and eggs of sand dollars, Melitta quinquiesperforata (Leske), were subjected to two petroleum oils, and effects determined. The oils chosen were Kuwait crude and No. 2 fuel oil, supplied by the American Petroleum Institute. Water-soluble extracts (WSF) from oil-sea water mixes were prepared and the major aromatic components in the WSF of the fuel oil were identified. WSF of No. 2 fuel oil depressed respiration, mobility of sperm, interfered with fertilization and cleavage, and retarded larval development. The effects were detectable at dilutions of 4% and less (about 0.6 ppm of WSF). Kuwait crude was much less toxic. There was no effect on water permeability of the egg membrane. Results are compared with similar studies on other marine organisms.

The eyes of characins (Characidae), pike, pickerel (Esocidae) and wrasses (Labridae) have a red pigment epithelium. There is no melanin, the epithelial cells containing red particles (erythrosomes). These are tiny cylinders in the cell processes, and spherules in the cell bases. In characins and pike, the red cylinders participate in retinomotor activity. Cylinders are birefringent and act as waveguides; they transmit long wavelengths (> 550 nm). The red pigment has been extracted and purified. It appears to be the same in the three families. Ultraviolet, visible and infra-red spectra are presented.

Eyes of cusk-eels Ophidion welshi and Lepophidium graellsi contain mostly short rods, arranged in five or six regular rows. In O. welshi, cones are single and double, whereas cones are lacking in L. graellsi. The pigment epithelium contains a lipid tapetum lucidum. In L. graellsi the tapetal spheres are 465 nm in diameter; in O. welshi most tapetal spheres have diameters of 187 nm; others, less frequent, are 86 nm. The tapetum of L. graellsi is white, that of O. welshi is blue; they reflect diffusely and reflectance at λmax is about 0.60. Increased tonicity causes the tapetum of O. welshi to turn darker blue and reduced tonicity causes it to become white. The lipid of the tapetum was examined by thin-layer and gas–liquid chromatography; it is a triglyceride containing mostly (88%) docosahexaenoic acid. The visual pigments are rhodopsins; L. graellsi λmax 494, O. welshi λmax 500 nm. Coastal waters where cusk-eels live were fairly turbid: maximal transmission of daylight occurred at 585 nm when the extinction coefficient was 0.6. Reference is made to the secretive and nocturnal habits of cusk-eels, and the functioning of their unusual eyes.

The light-adapted eye of the toadfish (Opsanus, Batrachoididae) exhibits eyeshine. The chorioid is yellow and has a yellow stratum argenteum. This is a bilaminate tissue, consisting of an inner iridescent sheet containing pentagonal crystals, and an outer sheet containing birefringent yellow granules. There is no melanocyte layer in the chorioid. In the photopic eye the retinal pigment migrates into the cell processes and surrounds the cone outer segments. Spaces between the cones and processes are pigment free, and part of the light incident upon the retina is transmitted through it to the argenteum and reflected by the latter into the retina. The pertinent tissues are illustrated, and spectrum curves for transmission and reflection are presented.

The structure of the retina and tapetum lucidum of the teleost Howella is described. The tapetum is a diffuse reflector composed of lipid spheres. The photoreceptor layer of the retina contains only rods; in section the rods appear segmented, perhaps arranged in stacks but their precise arrangement remains to be determined.

The tapetum lucidum of the alligator gar Lepisosteus was shown by t.l.c. to contain a new phenolic amino acid, which is apparently a major constituent of the reflecting material. It was isolated in a yield of 0.5 mg/eye and its physical and chemical characteristics, especially reductive hydrolysis with hydriodic acid giving dopa (3,4-dihydroxyphenylalanine) and cysteine, suggested that it might to SS-dicysteinyldopa. Tyrosinase oxidation of L-dopa in the presence of an excess of L-cysteine yielded, in addition to known 5- and 2-S-cysteinyldopa, the same amino acid as that isolated from the eye of the gar, thus confirming the gross structure. The position of the two cysteine residues was established by the fact that tyrosinase oxidation of catechol and cyteine gave 3-S-cysteinylcatechol and 3,6-SS-dicysteinylcatechol. The natural amino acid is therefore formulated as 3-(2,5-SS-dicysteinyl-3,4-dihydroxyphenyl)alanine (2,5-SS-dicysteinyldopa), which may be formed by two consecutive additions of cysteine, first to dopaquinone and then to 5-S-cysteinyldopaquinone. The enzymic synthesis of 2,5-SS-dicysteinyldopa in vitro suggests that it may also be involved in the biosynthesis of phaeomelanin.

SUMMARYIn the common sole Solea solea retinomotor changes take place. The retinal pigment usually expands in the light and cones shorten; reverse changes take place in darkness.

Luminescence is very common among marine animals, and many species possess highly developed photophores or light-emitting organs. It is probable, therefore, that luminescence plays an important part in the economy of their lives. A few determinations of the spectral composition and intensity of light emitted by marine animals are available (Coblentz & Hughes, 1926; Eymers & van Schouwenburg, 1937; Clarke & Backus, 1956; Kampa & Boden, 1957; Nicol, 1957b, c, 1958a, b). More data of this kind are desirable in order to estimate the visual efficiency of luminescence, distances at which luminescence can be perceived, the contribution it makes to general back-ground illumination, etc. With such information it should be possible to discuss more profitably such biological problems as the role of luminescence in intraspecific signalling, sex recognition, swarming, and attraction or repulsion between species. As a contribution to this field I have measured the intensities of light emitted by some pelagic species of animals.

Luminescence has been studied in the sea pansy Renilla kôllikeri, by means of photo-electric recording, and the mode of nervous regulation investigated. The luminescent response is under control of a non-polarized nerve net and is subject to facilitation which occurs terminally, at neuro-photocyte junctions. Facilitation is analysed in detail, and a facilitation-decay curve presented (Fig. 3). Between successive bursts of stimuli, facilitation may persist for some 10 min. Certain response parameters were measured. Latent period is o-12 sec. ; the response lasts 1 sec., and maximal intensity is reached in 0-22 sec. Conduction speed is 9 cm./sec. at 16-17° C. The refractory period of the response, as determined by high frequency stimulation, is o-2 sec. It is affected by fatigue and increases under repetitive stimulation. Under repetitive and prolonged stimulation, the animal passes into a hyper-excitatory state and luminous waves continue to arise long after stimulation has ceased. Conditions affecting this post-stimulatory discharge are examined. Comparison of facilitatory processes in quick muscular responses of sea anemones and luminescent responses of sea pens shows that they are essentially similar. From visual observations it appears that facilitation operates by recruitment of photocytes. Luminescence is of sporadic occurrence among coelenterates, and has been independently evolved on many occasions. It is suggested that its mode of regulation has been determined by the characteristics of the nerve net primarily concerned with control of muscles.

The problem of how a fish can make itself invisible in the natural light-conditions in an aquatic environment is discussed with particular reference to the silvery surfaces of fish.In fish which we have examined, the silvery surfaces are of two types: (1) an argenteum which consists of long thin crystals of guanine whose reflecting surfaces are approximately parallel with the surface of the fish; (2) layers of guanine crystals lying either on the inner surfaces of the scales or in the subdermis—these crystals are not, in general, orientated with their reflecting surfaces parallel with the surfaces of the fish, and are much broader than those of the argenteum.Methods are described by which the orientation of the crystal planes with respect to the planes of the scales on which they lie can be determined.The orientation of the crystals of type 2 in different parts of the body is described for the horse mackerel, Trachurus trachurus (L.), and for the bleak, Alburnus alburnus (L.).For the bleak it is shown that although the planes of the crystals are often very much inclined with respect to the planes of the scales, the long axes of the crystals are always approximately parallel with the planes of scales. The inclination of the crystals, therefore, is away from the scales across their short axes.Measurements of the light transmitted by silvery scales of the bleak show that they reflect light strongly when this falls obliquely on the crystals which they contain and that they are most transparent to light which strikes the scales in a direction perpendicular to the reflecting planes of the crystals.

Some preliminary observations on luminescence in polynoid worms have been presented in a previous paper (Nicol, 1953). These animals produce light in scales (elytra) which cover the dorsal surface of the body. The source of the light lies in a layer of unicellular epithelium on the lower surface of the scale. Histologically, this tissue consists of columnar cells (photocytes), characterized by the presence of coarse eosinophilic granules in the cytoplasm. The nervous supply of the elytrum derives from a nerve trunk which ascends the stalk or elytrophore and proceeds to a ganglion in the centre of the scale. From this ganglion nerves radiate peripherally and proceed to the photocytes, and to sensory receptors on the dorsal surface and at the margin of the scale (Bonhomme, 1942).

A REFLECTING layer or tapetum lucidum occurs in the eyes of many fishes; in teleosts, it lies in the pigment epithelium1. The reflecting material is usually considered to be guanine, but in weakfishes or seatrouts (Sciaenidae) it is a lipid2,3. We have been able to characterize the lipid material.

The light energy emitted in a flash by single elytra of two polynoid worms has been measured, viz. Acholoë astericola and Lagisca extenuata. Maximal emission occurs at 515 mμ. Mean values for light intensity per flash from 1 elytrum are: Lagisca, 1·8 × 10−10 μJ/cm2 receptor surface at I m ; Acholoë, I·II × 10−8 μJ/cm2 receptor surface at I m. The light emitted in I flash by a single photocyte of Lagisca is 0·12 × 10−12 μJ/cm2 receptor surface at I m. It is estimated that the light from I elytrum of Lagisca could be seen by the dark-adapted human eye at 1·2 m in air.

The reflecting layer or chorioidal tapetum in the eye of many selachians was examined to determine, especially, whether it is a set feature or whether it can be occluded.Many selachians have a tapetum under the entire fundus of the eye. It consists of highly reflecting cells, towards the outer ends of which there is a layer of melanophores. In active pelagic sharks of the neritic zone (e.g. Squalus, Mustelus), the melanophores send out pigment over the reflecting cells so as to conceal them when the fish is illuminated; the pigment retreats in darkness. In benthic species of the neritic zone, notably Scyliorhinus, the fundus has a black ventral area; a tapetum occurs elsewhere and is not occlusible. The eyes of rays have a black ventral field, and pigment migration over the tapetum is slight. The tapetum is not occlusible in deep-sea squaloids and a chimaerid Hydrolagus affinis.Reflectivity of the dark-adapted tapetum of Squalus acanthias amounts to about 85%. Full expansion of pigment in the light and retreat in darkness occupy about 2 hr in this fish.Pieces of tapeta, freed of retinae, were mounted in Ringer solution, observed, and photographed by oblique illumination from above. The tapetal pigment migrates inwards over the reflecting plates in such preparations of tapeta from fishes that have occlusible tapeta.The suggestion is offered that the tapetum is concealed, either by closure of the pupil or by migration of pigment, so as to avoid displaying eye-shine. Deep-sea squaloids, living in dimly lit environments, do neither.

The piddock, Pholas dactylus L., gives off a luminous secretion when irritated. The luminous glands which produce the secretion are two longitudinal stripes in the exhalant siphon, a pair of triangular organs in the mantle cavity near the base of the siphon, and a stripe around the ventral rim of the mantle (Panceri, 1872).The histology of the light-organs has been described several times. A light-organ is covered by a simple columnar ciliated epithelium, below which are many glandular cells, which discharge through the surface epithelium. The outer part of the glandular layer consists of a mass of large mucus cells. Deeper lies a second glandular region containing large cells with long necks that extend to the external epithelial surface. Dubois (1892, 1914, 1928) believed that the photogenic tissue was made up of two kinds of secretory cells; these were the superficial ciliated cells, which possessed glandular bases (fixed secretory cells); and deeper lying glandular cells derived from clasmatocytes (migratory secretory cells). Rawitz (1891) clearly distinguished a mucous from an underlying photogenic layer. The latter, according to Förster (1914), contains pyriform cells with long necks. He believed that he could distinguish a secretory cycle in the photogenic cells. Exhausted cells at the beginning of the cycle possessed an alveolar cytoplasm; granules began to appear in the cytoplasm; the granules increased in number and stained intensely with iron haematoxylin. Those photogenic cells which were filled with granules were in the active secretory state. Transitional stages between the inactive (or depleted) cells and the active (granular) cells were rare.

A survey has been made of reflecting layers in the integument of selected fishes, including silvery sea lamprey and various silvery and partially translucent teleosts. Lamprey and juvenile rockling have only a stratum argenteum. Other silvery teleosts have a layer of oriented reflecting platelets lying outside the stratum argenteum; these lie more or less parallel to the vertical to the surface of the water.In dace, herring, and salmon parr the platelets on the upper flanks are tipped slightly upwards; but in some pelagic fishes, e.g. the mackerel and gar-fish, the platelets here are tipped downwards. Smelt, sand smelt, half-beaks, found in shallow or coastal waters, are partially translucent, partially silvery. The implications of these arrangements of reflecting layers are discussed. Both in clear oceanic waters, away from the surface, and in shallow or turbid rivers or ponds, light is distributed almost symmetrically about the vertical to the surface, and the greatest intensity is directed downwards. Under these conditions the reflecting layers diminish thevisibility of the fish from most fields of view because they reflect light approximately equal to the background light against which the fish is seen. On the lower flanks, when the surface is sloping, the reflected light tends to be spread and to beless intense than the incident light. Thin fishes, dark above with vertically reflecting sides, are effectively camouflaged except for the ventral extremity. The lower flanks of tapering or rounded fishes are well camouflaged by reflexion from below and behind because the platelets slope inwards towards the tail, and the projected area of incidence relative to reflexion is thereby increased.

The spectral composition of the light of Chaetopterus variopedatus has been measured by means of spectral niters and multiplier phototube. Spectral emission extends from about 405 to 605 mμ, with a maximum at about 465 mμ. The spectral curve of Chaetopterus light is compared with a human visibility curve (scotopic vision), a visibility curve for Limulus, and an absorption curve for fish visual purple. Luminous efficiencies, based on these curves, are calculated.

A study of the physical basis of the coloration of the shiny integument of the chimaeroid Hydrolagus colliei (ratfish) is presented. The metallic sheen is caused by reflection of light from platelets in the skin that are oriented mostly vertically. They lie in a layer of oriented reflecting cells in the dermis. Additionally, there is a stratum argenteum in the subdermis and in the peritoneum. The oriented reflecting layer is 4 to 11 cells deep; the cells contain about 12 flat crystals regularly stacked above each other. Each crystal lies in a crystal chamber, delimited by a chamber membrane; the chamber is enclosed in a crystal sac bounded by a unit membrane. Cells of the stratum argenteum also contain crystals in crystal sacs. Estimates are made of crystal thickness from interference colors, and of spacing from electron micrographs. The superposed thin lamellae are about one-quarter wavelength thick; the organization is one that could produce high reflectivity by constructive interference. There is a medley of colors, but the overall hue is unsaturated and gold.

Six species of luminescent polynoids of the Plymouth fauna have been studied, namely Lagisca extenuata, Gattyana cirrosa, Harmothoë liinulata, Polynoë scolopendrina, Acholoë astericola and Malmgrenia castanea. Their scales are luminescent, and the light is produced by granular eosinophilic photocytes, which form a unicellular layer on the lower surface of the scale. The nervous supply of the elytrum is described, and the luminescent response is shown to be under nervous control. Luminescent responses from all six species have been recorded by the use of a photomultiplier cell and oscilloscope. The normal response has been found to consist of a series of rhythmic flashes, from 9 to 1 per sec, lasting up to 1 min. Some characteristics of the luminescent responses are given, and the part they may play in the normal life of the animal is discussed.

The spectral composition of the light of four species of polynoid worms has been measured, viz. Harmothoë longisetis, Gattyana cirrosa, Polynoë scolopendrina and Lagisca extenuata. The method involved the use of coloured spectral filters and two multiplier phototubes. The spectral emission curves of the four species are similar. They are asymmetrical in shape, prolonged towards longer wavelengths, with maxima at about 515 mμ (510–520 mμ.). Values for luminous efficiency are calculated (ratio of luminous flux to radiant flux); for human scotopic vision (λmax at 505 mμ), efficiency is 61%.

Elytra of polynoids show two kinds of luminescent responses, viz. a quick flash (duration ca. 100 msec) and prolonged glow (duration 0·25–10 sec). Quick flashes are produced by nervous excitation; prolonged glow responses accompany or follow fast repetitive flashing, or are elicited by strong electrical stimuli. Anaesthetized isolated elytra still give the prolonged glow response when electrically stimulated. Facilitation is also evident in consecutive glow responses. From these various observations it is concluded that the prolonged glow response can be produced by fast repetitive nervous excitation or by direct electrical excitation of the photocytes. The occurrence of facilitation in glow responses of anaesthetized elytra raises the problem whether the same phenomenon in consecutive normal flashes may be due to intracellular changes taking place within the photocytes.

A comparative study of the following luminous copepods was undertaken: Metridia lucens, M. longa, M. princeps, Pleuromamma robusta, P. xiphias, Heterorhabdus norvegicus, H. robustus, Heterostylites longicornis, Lucicutia grandis, Hemirhabdus grimaldii, Disseta palumboi, Euaugaptilus magnus and Centraugaptilus horridus. Flashes produced by electrical stimulation (a.c. or condenser shocks) and mechanical stimulation were recorded photoelectrically. Flashes lasted from 2 to 37 sec. Latencies of some species (Metridiidae), following electrical stimulation, were very short, 7–9 msec. Intensities ranged from 0·02 × 10–5 to 14·4 × 10–5μW/s cm2 of receptor surface at 15 cm distance (0.0045 × 10–2 to 3·24 × 10–2μW/Cm2 at 1 cm) (10–20°C). Luminous glands of Metridiidae, Lucicutiidae and Augaptilidae are autofluorescent; the location of the luminous glands in these families and in Heterorhabdidae is described. Two kinds of glandular cells (types 1 and 2) occur in the luminescent areas. The cells are large saccular structures containing granular or homogeneous material, and are distinguished by staining peculiarities. Cell types 1 and 2 open through common pores, and may be the source of luminous reactants. Some measurements of luminescence in other pelagic animals are presented, for comparison with copepods, viz. Aequorea macrodactyla, Aeginura grimaldii (Hydromedusae), Periphylla periphylla (Scyphomedusa), Meganyctiphanes norvegica and Acanthephyra pelagica (Crustacea), Myctophum punctatum (Teleostei). Our present knowledge regarding luminescence among copepods is reviewed.

1. Luminescent responses in the sea pansy Renilla kollikeri take the form of light waves which run over the entire surface of the rachis. As other workers have shown, the responses are controlled by an unpolarized nerve net, and are subject to facilitation. Records obtained by photoelectric techniques are furnished to show certain features of the response. 2. At moderate rates of electrical stimulation (1 per sec.), 2-3 shocks are required to evoke a response. At slower rates, more stimuli are necessary owing to decay of facilitator. At frequencies above 3 per sec., the response rate is slower than the stimulation frequency, owing to refractoriness in the conducting mechanism (c. 0.2 sec.). 3. Summation occurs at frequencies above 1 per sec. This results largely from fusion of the more persistent glowing of autozooids. Siphonozooids are responsible for the brief flashes. 4. Maximal estimates of latent period and flash-duration are of the order of 0.5 and 0.9 sec. respectively. 5. With repeated stimulation, the response intensity is subject to decay. Experiments suggest that this is a consequence of exhaustion of photogenic material. 6. On mechanical stimulation, luminescent waves are evoked at first; but with continued stimulation, refractoriness sets in, and responses become localized to the area of stimulation. Subsequent electrical stimulation reveals that the transmitter mechanism is fatigued, and excitability gradually returns over the course of the next 30-60 mm.

This histological study is concerned with demonstrating the existence of retinomotor changes in flatfishes. In three species, selected as representatives (sole, Solea solea; plaice, Pleuronectes platessa; and merry sole, Microstomus kitt), it is shown that normal radial displacements of rods, cones, and retinal pigment take place in expected directions in light and darkness, with exceptions which may be important but whose implications have still to be revealed. In the plaice the cones of the dorsal retina are permanently retracted against the basement membrane, and mobile elsewhere. The eyes of flatfishes, therefore, conform to those of other teleosts in photomechan-ical responses to illumination.

A sea anemone, normally a passive-looking animal, reacts to suitable food-stuffs by a series of fairly complicated activities. When its tentacles encounter solid food there is, first of all, a discharge of cnidae, which poison living prey and adhere to the food mass. These cnidae are independent effectors, responding directly to external excitation (Pantin, 1942). Next, the tentacles clasp the food and bend towards the mouth; they push the food into the mouth, and the pharynx draws it down into the coelenteric cavity where it is digested (Pantin & Pantin, 1943).

Direct observations on the reflecting layers in the tapetum of the spur dog Squalus acanthias confirm the orientations of the tapetal reflecting cells which were previously found histologically (Denton & Nicol, 1964). Surface views of the tapetum show the outlines of the exposed surface of the tapetal plates as ‘polygons’. The long axes of these polygons are parallel with the plane of the tissue on which they lie and the polygons are only inclined to this plane across their short axes. The angle to which they are inclined increases as we move towards the periphery of the eye.

Bony fishes belonging to 75 families were examined for ocular tapeta lucida. The results are collated with published records, and tapeta are shown to occur in 28 families of teleostomes (Holostei and Teleostei) listed in Table 2. Except in the bigeyes Priacanthidae, they are diffuse reflectors located in the pigment epithelium. Based on chemical composition, several types can be distinguished, lipid, guanine (purine), and some others, colored yellow or light tan, of unknown composition. In several tapeta minutely examined, the reflecting material occurs as platelets (guanine type) or as minute tapetal spheres (lipid and others). The tapetum of the bigeye lies in the chorioid, it is a specular reflector containing guanine; as such it is the solitary known exemplar among teleosts.

The luminescent responses of Hydromedusae have been investigated by means of photoelectric recording. Animals examined included Aequorea , Halistaura , Phialidium and Stomotoca ; the first proved most suitable for experimentation. Light appears in a ring around the periphery of the umbrella. In Aequorea the luminous points correspond to yellow-green fluorescent masses in the marginal canal. The photogenic tissue in histological preparations appears as oval masses of eosinophilic cells bulging into the cavity of the marginal canal. Luminescence is intracellular. With carefully localized mechanical and electrical stimulation, light appears in one or a few restricted loci; there is no spread of luminescence around the margin. The luminescent response shows facilitation, summation at higher frequencies, and fatigue under continued stimulation. Each flash in Aequorea lasts 0⋅3 to 1⋅5 s; maximal intensity is reached in 0⋅1 s (14 to 16°C). It is concluded that the responses observed involve either direct excitation of the photocytes, or stimulation through local nervous pathways.

A method is described for measuring, for various angles of incidence, the polarization produced by reflexions from natural surfaces.This method was tested by measurements on matallic surfaces of known properties and then used to study reflexions from the silvery gill cover of the bleakAlburnus alburnus(L.).Althouth the reflecting surfaces of the gill cover are crystals of a dielectric guanine, there is no polarizing angle, and the reflecting properties of the gill cover resemble those of a sputtered aluminium mirrorThe biological significance of these results is briefly discussed.

Eyes of gars (Lepisosteidae) have a yellow tapetum lucidum containing an as yet uncharacterized yellow pigment. They also contain fairly high levels of riboflavin, about 14 mg/100 g wet weight, which lies in tissues external to the retina and tapetum and which is, therefore, not a tapetal pigment.

When stimulated in some suitable mannerChaetopterussecretes a luminescent material into the surrounding sea water. The luminescent secretion is discharged fromglands which are widely dispersed over the surface of the animal; the most conspicuous are two glandular patches on the dorsal surface of segment XII. Secretory materialis forced out of the cells by some contractile process; discharge is not merely theresult of secretion pressure. Under repetitive stimulation the intensity of the luminescent response decreases owing to the intervention of fatigue. Fatigue has been interpreted as a gradual exhaustion of luminescent material in the glandular cells (Nicol, 1952b,c).The present investigation seeks to analyse the onset and progress of fatigue in greater detail.

This research has been concerned with the movement of pigment in the occlusible tapetum lucidum of the spur dog Squalus acanthias. The pigment expands and covers the reflecting plates of the tapetum in light, and retreats and uncovers the plates in darkness. Movement of pigment in one eye occurs independently of that in the other. Normal movement of pigment is not affected by cutting the optic nerves and oculomotor nerves, by sectioning the cord immediately behind the medulla, nor by excising the pituitary and pineal glands. It is concluded that the process is not under nervous or hormonal control. It occurs normally in excised opened eyes that are kept in oxygenated Ringer solution, but not under partial or fully anoxic conditions. The pigment cells of the chorioid seem to be behaving as independent effectors, sensitive to light. The pigment expands in tapeta freed of retina, in light or darkness. No explanation is yet available for the latter anomalous result. Measurements were made of the reflectivity of isolated tapeta, and curves are presented showing the progressive decrease of reflectivity accompanying expansion of pigment, and changes in the colour composition of the reflected light.

Noctiluca miliarisSuriray is a well-known luminescent organism, and its light emission has been studied repeatedly. Hitherto, however, no attempt has been made to record the luminescent flashes of this species, either under normal conditions, or after experimental treatment. With the sensitive photo-electric recording methods now available it has become possible to subject the luminescent flashes ofNoctilucato experimental analysis, and with this end in view the present study ofNoctilucawas undertaken.

Pennatula phosphoreaLinn, is a well-known luminescent animal, which was studied by that pioneer investigator, Panceri (1871,1872a,b). He found that the animal luminesces when excited, and that the light emanates from the zooids. From the region stimulated luminous waves proceed, with measurable velocity, to the extremities of the animal. Since waves can be initiated in either direction, up or down the colony, it follows that the transmission system is non-polarized. Recent studies on other species of pennatulids have shown that luminescence is under control of a nerve net (Nicol, 1955a,b,c; Daven-port & Nicol, 1956). Harvey's book onBioluminescence(1952) gives a review of pertinent literature, plus a bibliography.

Many stomiatoid fishes possess a peculiar light-organ below and behind the eye, as well as other kinds of photophores. This light-organ, of diagnostic importance, is termed the subocular, postocular or cheek-organ. Stomiatoid fishes, suborder Stomiatoidei, form a suborder of the Isospondyli. Subocularorgans are found in the following groups

Tapeta were examined of light- and dark-adapted dogfish.Scyliorhinus canicula. The tapetum in the eye of this fish is fixed; pigment processes in the tapetum do not move when a fish is changed from dark to light and vice versa. A black ventral field and a bright dorsal fundus are always present.

InBranchiomma vesiculosumdecrease in light intensity causes the animal to contract and withdraw into its tube. A decrease, never an increase, in illumination is the effective stimulus. When repeatedly stimulated the animals quickly become adapted to intensity changes and no longer respond.The minimal effective intensity change has been determined for a range of intensities. ΔI/I is found to increase in low light intensities, and to be fairly constant in intensities over 50 lux. The animals are least sensitive to red light; sensitivity increases in shorter wave-lengths.

Relative spectral emission curves are available for the lights of many invertebrate animals, but none exists for fish. It seems likely that many bony fish use their photophores to signal to one another. Information about the spectral composition of fish luminescence is desirable to permit comparisons with the spectral sensitivities of fish eyes, to enable calculations to be made of luminous intensities, and to allow estimates to be made of the rate of attenuation of such lights in sea water. To further these ends, measurements were made of the spectral composition of the luminescence of the lantern-fish,Myctophum punctatum.