Alterations of the eyes of Carinaria lamarcki (Gastropoda, Heteropoda) during the long pelagic cycle

 The eyes of different larval stages of Carinaria lamarcki were examined ultrastructurally. In all larval stages the eyes consist of a cornea, a lens and an everse retina. The photoreceptors in young larvae are exclusively of the ciliary type. In old larvae, however, two types of photoreceptors are present and the retina is composed of two segments: a posterior segment with altered ciliary photoreceptors (=type I sensory cells) and an anterior segment with what are presumably rhabdomeric photoreceptors (=type II sensory cells). The anterior retina is interpreted here as an accelerted character. Furthermore, the arrangement of the pigment granules changes during the long larval development being cup shaped in young larvae versus ribbon shaped in old larvae. The findings allow for the conclusions that: (a) the ciliary photoreceptors are correlated with the long larval period of Heteropoda and that (b) the eyes are altered continuously during the larval cycle. Accepted: 6 July 1998     

The ciliary photoreceptor in the teleplanic veliger larvae of Smaragdia sp. and Strombus sp. (Mollusca,Gastropoda)

The cerebrally innervated eyes of the veliger larvae of Smaragdia sp. and Strombus sp. are composed of a lens, a cornea, and an everse retina. The retina contains two different types of cells, ciliary sensory cells and supportive cells which bear one or two cilia. It is suggested that: (a) the ciliary photoreceptors of these teleplanic veliger larvae are correlated with a long pelagic life in the ocean, which can last up to twelve months, and (b) that structural details of the photoreceptors can change during ontogenesis (ciliary vs rhabdomeric). Furthermore, the cilia of the supportive cells apparently tranport lens material and thus play an important role in lens formation. A decomposition mechanism of pigment granules is examined.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - ecm extracellular matrix - gr electron-dense granules - l lens - lb lamellar body - mp membranous pieces - mt microtubules - mv microvilli - n nucleus - oc optic cavity - on optic nerve - pg pigment granule - sc sensory cell - sj septate junction - spc supportive cell - v vesicles     

Alterations of the eyes during ontogenesis inAporrhais pespelecani (Mollusca,Caenogastropoda)

The cerebrally innervated eyes of metamorphically competent larvae, newly metamorphosed larvae, and adults ofAporrhais pespelecani are ultrastructurally investigated and compared. The eyes are composed of a lens, a cornea, and an everse retina. In adults, a humour is located behind the lens. The retina consists of two different types of cells: sensory cells and supportive cells. The present study confirms earlier results and demonstrates that the distal part of the sensory cells is altered during ontogenesis. In metamorphically competent larvae, the sensory cells are exclusively ciliary. In newly metamorphosed larvae and in adults, however, the sensory cells are of the mixed type, bearing both cilia and microvilli. Furthermore, the findings confirm that both the supportive and corneal cells, as well as the distal supportive cell processes which are restricted to the eyes of adults are involved in lens formation.     

The importance of larval eyes in the polychaete Capitella teleta: effects of larval eye deletion on formation of the adult eye

In many marine invertebrates with biphasic life cycles, juvenile/adult traits begin to develop before metamorphosis. For structures that are present at multiple developmental stages, but have distinct larval and adult forms, it is unclear whether larval and adult structures have shared or distinct developmental origins. In this study, we examine the relationship between the larval and adult eyes in the polychaete Capitella teleta. In addition, we describe a novel marker for larval and juvenile photoreceptor cells. Infrared laser deletion of individual micromeres in early embryos suggests that the same micromeres at the eight‐cell stage that are specified to generate the larval eyes also form the adult eyes. Direct deletion of the larval eye, including the pigment cell and the corresponding photoreceptor cell, resulted in a lack of shading pigment cells in juveniles and adults, demonstrating that this structure does not regenerate. However, a sensory photoreceptor cell was present in juveniles following direct larval eye deletions, indicating that larval and adult photoreceptors are separate cells. We propose that the formation of the adult eye in juveniles of C. teleta requires the presence of the pigment cell of the larval eye, but the adult photoreceptor is either recruited from adjacent neural tissue or arises de novo after metamorphosis. These results are different from the development and spatial orientation of larval and adult eyes found in other polychaetes, in which two scenarios have been proposed: larval eyes persist and function as adult eyes; or, distinct pigmented adult eyes begin developing separately from larval eyes prior to metamorphosis.     

The ultrastructure of the eyes in the veliger-larvae of Aporrhais sp. and Bittium reticulatum (Mollusca,Caenogastropoda)

Summary The cerebrally innervated larval eyes of Aporrhais sp. and Bittium reticulatum are investigated by means of transmission electron microscopy. Each organ consists of a pigmented cup containing an acellular lens. The cornea overlaps the anterior portion of the eye. The retina is composed of sensory cells and supportive cells. The sensory cells of Aporrhais sp. bear one cilium and in Bittium reticulatum two cilia, the ciliary membrane being folded into numerous finger-shaped evaginations. The supportive cells contain the pigment granules and most of them bear one or two cilia, the plasmalemma of which is likewise folded. It is supposed that: (a) these cilia have a transportive function for lens material and (b) that the ciliary photoreceptor of Aporrhais sp. and Bittium reticulatum is a functional adaptation to a relatively long larval period.Abbreviations bb basal body - bp basal plate - c cilium - cc corneal cell - cm ciliary membranes - cw ciliary whorl - gd Golgi dictyosomes - gm granular material - l lens - m mitochondrion - mt microtubules - mv microvilli - mvb multivesicular body - n nucleus - pb pigment border - pg pigment granule - rer rough endoplasmic reticulum - sc sensory cell - sj septate junctions - spc supportive cell     

Photoreceptors of cubozoan jellyfish

The anatomically sophisticated visual system of the cubozoan jellyfish Carybdea marsupialis is described. Individual cubomedusae have eight complex eyes, each with a cornea, lens, and retina of ciliated photoreceptor cells, eight slit ocelli, and eight dimple ocelli. The photoreceptor cells of the complex eyes are bipolar and resemble vertebrate rod cells. Each photoreceptor has an outer cylindrical light-receptive segment that projects into a vitreous space that separates the lens and the retina, an inner segment rich in pigment granules, and a basal region housing the nucleus. The outer segment is a modified cilium with a 9 + 2 arrangement of microtubules plus stacks of membrane. These stacks of membrane form numerous discs that are oriented transversely to the long axis of the cell. The outer segment is connected to the inner segment by a slender stalk. The basal end of each photoreceptor forms an axon that projects into an underlying layer of interneurons. Each ocellus is composed of ciliated photoreceptor cells containing pigment granules. Rhodopsin-like and opsin-like proteins are found in the membrane stacks of the outer segments of the photoreceptors of the complex eyes. An ultraviolet-sensing opsin-like protein is present in the inner segments and basal regions of some of the photoreceptors of the complex eyes. Rhodopsin-like proteins are also detected in the photoreceptors of the slit ocelli. The cellular lens, composed of crystallin proteins, shows a paucity of organelles and a high concentration of homogeneous cytoplasm. Neurons expressing RFamide (Arg-Phe-amide) comprise a subset of interneurons found beneath the retinas of the complex eyes. RFamide-positive fibers extend from these neurons into the stalks of the rhopalia, eventually entering into the subumbrellar nerve ring. Vision may play a role in the navigation, feeding, and reproduction of the cubomedusae.     

First evidence of unpigmented rhabdomeric photoreceptors in a Microstomum species (Plathelminthes, Macrostomorpha, Microstomidae) and ultrastructure of photoreceptor-like ciliary aggregations

Microstomum spiculifer possesses a pair of intracerebral photoreceptors each consisting of a single rhabdomeric sensory cell and two cup or mantle cells. The mantle cells are devoid of pigment. In addition, four so-called ciliary aggregations, presumed to have a light-sensing function, are present. Each ciliary aggregation represents a specialized cell with an internal cavity filled with axonemes of modified cilia. Rhabdomeric photoreceptors consisting of one to three sensory cells and a single pigmented or unpigmented mantle cell are widespread within taxa of the Plathelminthes Rhabditophora. On the contrary, the existence of two mantle cells forming the eye cup is only known for M. spiculifer and a few other species of the Macrostomida. Therefore, at least two hypotheses are possible: (1) two cup cells are a basic characteristic of the Rhabditophora and a reduction from two to one cup cell has occurred secondarily or (2) the stem species of the Rhabditophora possessed rhabdomeric eyes with one cup cell, and two mantle cells have evolved within the Macrostomorpha. The existence of ciliary aggregates has been documented for several taxa of the Plathelminthes Rhabditophora. From their distribution it can not be concluded whether these differentiations are either a basic feature of the Rhabditophora or have evolved several times convergently. Accepted: 26 September 1999     

Development of the eye in the turbot Psetta maxima (Teleosti) from hatching through metamorphosis

Development of the eyes during the larval and metamorphic stages of the turbot Psetta maxima (Teleosti) was studied using microscopy. Events during differentiation of both eyes occur simultaneously, and no differences between he migrating and no-migrating eye were observed during metamorphosis. At hatching, the eyes are rudimentary, consisting of a neuroepithelial optic cup and a small lens. During larval development, major changes occur in the lens and retina, in which cones are the only photoreceptors. The appearance of rods is delayed until metamorphosis. The outer ocular layers (sclera and choroid) arise during larval development as thin connective layers with little differentiation. These layers undergo important changes just before and during metamorphosis. These results indicate that development of the individual components of the eye occurs at different times. Those of ectodermal origin appear early, providing a simple visual organ during larval life. By metamorphosis, the eye shows adult characteristics, including two types of photoreceptors, a rich choroid vascular supply and ocular structures involved in protecting, shaping, and moving the eye. J Morphol 233:31–42, 1997. © 1997 Wiley-Liss, Inc.     

Fine structure of the eyes of Pseudoceros canadensis (Turbellaria,Polycladida)

Summary The cerebral and epidermal ocelli of the Müller's larva and the cerebral and tentacular eyes of the adult turbellarian Pseudoceros canadensis were studied by electron microscopy. The right cerebral ocellus of the larva consists of one cup-shaped pigmented cell and three sensory cells that bear microvilli. The left cerebral eye of the larva has the above named cells plus a sensory cell with many cilia. Evolutionary significance is attributed to the presence of both ciliary and microvillar photoreceptors in an eye of a flatworm. The one epidermal ocellus of the larva is composed of two cells: a cup-shaped pigmented one bearing flattened cilia, the presumed photoreceptors, and a cell above the cup that adds a few nonciliary lamellae to the stack of ciliary ones from the pigmented cell. The adult eyes contain only microvillar receptors; cilia were not observed.     

Retinal ultrastructure may mediate polarization sensitivity in larvae of the Sunburst diving beetle, Thermonectus marmoratus (Coleoptera: Dytiscidae)

A number of invertebrates are known to be sensitive to the polarization of light and use this trait in orientation, communication, or prey detection. In these animals polarization sensitivity tends to originate in rhabdomeric photoreceptors that are more or less uniformly straight and parallel. Typically, polarization sensitivity is based on paired sets of photoreceptors with orthogonal orientation of their rhabdomeres. Sunburst diving beetle larvae are active swimmers and highly visual hunters which could potentially profit from polarization sensitivity. These larvae, like those of most Dytiscids, have a cluster of six lens eyes or stemmata (designated E1 through E6) on each side of the head capsule. We examined the ultrastructure of the photoreceptor cells of the principal eyes (E1 and E2) of first instar larvae to determine whether their rhabdomeric organization could support polarization sensitivity. A detailed electron microscopical study shows that the proximal retinas of E1 and E2 are in fact composed of photoreceptors with predominantly parallel microvilli and that neighboring rhabdomeres are oriented approximately perpendicularly to one another. A similar organization is observed in the medial retina of E1, but not in the distal retinas of E1&2. Our findings suggest that T. marmoratus larvae might be able to analyze polarized light. If so, this could be used by freshly hatched larvae to find water or within the water to break the camouflage of common prey items such as mosquito larvae. Physiological and behavioral tests are planned to determine whether larvae of T. marmoratus can actually detect and exploit polarization signals.     

Ultrastructure of prostomial photoreceptors in four marine polychaete species (annelida)

The photoreceptors of four polychaete species were investigated by transmission electron microscopy: Eteone longa and Anaitides mucosa (Phyllodocidae), Scolelepis squamata (Spionidae), and Heteromastus filiformis (Capitellidae). Four different types of light-sensitive organs could be distinguished: 1) a simple, unpigmented rhabdomeric type; 2) a simple ocellus composed of a sensory and a pigmented cell; 3) complex eyes with a lens consisting of secretory granules; 4) a simple, unpigmented type with modified cilia. In spite of its simpler organization the fourth type is listed last, because its function as a photoreceptor seems dubious. The first type (unpigmented rhabdomeric receptor) occurs in all four species investigated. It is the only type of photoreceptor in Heteromastus. Additionally, the two phyllodocids Eteone and Anaitides possess another kind of receptor (type 4) in close proximity to the type 1 receptor. Simple ocelli (type 2) are found in Scolelepis. A pair of complex eyes (type 3) is present in both Eteone and Anaitides, but they show important differences in the two species. First, the eyes in Eteone exhibit ciliary rudiments within the sensory processes, but such rudiments are absent in the eyes of Anaitides. Secondly, the sensory cells in Anaitides possess pigment granules, whereas in Eteone they do not. Thirdly, the lens in Eteone is composed of secretion granules of equal electron density, whereas in Anaitides the lens granules show increased electron density centrally. Lens material appears to be secreted from a single corneal cell in Eteone, and from several corneal cells in Anaitides. In both species these corneal cells are located distally outside the lens.     

Behavioural and anatomical measures of visual acuity in first-feeding Yellowtail Kingfish (<Emphasis Type="Italic">Seriola lalandi</Emphasis>) larvae

Ontogenetic change in the visual acuity of Seriola lalandi larvae was measured using both behavioural and anatomical techniques. Visual acuity improved over early development (day 4 to day 7 post-hatch), although for all three larval ages examined estimates of anatomical acuity were consistently lower (higher acuity) than estimates of behavioural acuity. At hatching the eyes of larval kingfish were characterized by an undifferentiated retina surrounding a spherical lens, by day 4 post-hatch the eyes appeared to be functional, the retina was fully pigmented and the optic nerve had made contact with the optic tectum. Analysis of prey search behaviour indicated that larvae employ a saltatory type search behaviour in which brief stationary periods are interspersed with repositioning movements. The mean reactive angle increased between day 4 and day 7 post-hatch indicating that the horizontal visual field was expanding with development, thereby increasing the search area of larvae. Pre-strike distances of early larvae were substantially less than one body length, being constantly around a 1/3 of a body length for all larval ages examined.     

Vertebrate features revealed in the rudimentary eye of the Pacific hagfish (Eptatretus stoutii)

Hagfish eyes are markedly basic compared to the eyes of other vertebrates, lacking a pigmented epithelium, a lens and a retinal architecture built of three cell layers: the photoreceptors, interneurons and ganglion cells. Concomitant with hagfish belonging to the earliest-branching vertebrate group (the jawless Agnathans), this lack of derived characters has prompted competing interpretations that hagfish eyes represent either a transitional form in the early evolution of vertebrate vision, or a regression from a previously elaborate organ. Here, we show the hagfish retina is not extensively degenerating during its ontogeny, but instead grows throughout life via a recognizable PAX6+ ciliary marginal zone. The retina has a distinct layer of photoreceptor cells that appear to homogeneously express a single opsin of the RH1 rod opsin class. The epithelium that encompasses these photoreceptors is striking because it lacks the melanin pigment that is universally associated with animal vision; notwithstanding, we suggest this epithelium is a homologue of gnathosome retinal pigment epithelium (RPE) based on its robust expression of RPE65 and its engulfment of photoreceptor outer segments. We infer that the hagfish retina is not entirely rudimentary in its wiring, despite lacking a morphologically distinct layer of interneurons: multiple populations of cells exist in the hagfish inner retina and subsets of these express markers of vertebrate retinal interneurons. Overall, these data clarify Agnathan retinal homologies, reveal characters that now appear to be ubiquitous across the eyes of vertebrates, and refine interpretations of early vertebrate visual system evolution.     

Development of sensory organs in larvae of African catfish Clarias gariepinus

African catfish Clarias gariepinus hatched with morphologically immature features; however, sensory organs developed rapidly with fish growth. Although the eyes of newly hatched larvae were immature without pigment, in 2 day‐old larvae, the retina of the eyes had already developed except for the rod cells. No free neuromasts were observed in newly hatched larvae. In 1 day‐old larvae, however, free neuromasts were observed on the head and trunk. Free neuromasts increased with larval growth. Newly hatched larvae had simple round‐shaped otic vesicles; however, all sensory epithelia of the inner ear were observed until the larvae were 3 days old. Two day‐old larvae swam horizontally, had sharp teeth, commenced ingesting rotifers and also artificial feed (small‐size pellets) under both light and dark conditions; by then the larvae already had many taste buds. Three day‐old larvae showed negative phototaxis and cannibalism by eating their conspecifics. Most of the free neuromasts observed in this study had the peculiar feature of many microvilli around the sensory cells on the apical surface. Detected free neuromasts as ordinary type lateral‐line organs were not observed in previous reports in teleosts. In 10 day‐old larvae, there were two lines of free neuromasts on the flank and lower edge of the trunk; presumptive canal neuromasts were oval shaped and had begun to sink under the skin. The direction of maximum sensitivity of the neuromasts was parallel with the longitudinal axis of their elliptical apical surface.     

The filter apparatus of Rana temporaria and Bufo bufo larvae (Amphibia,Anura)

Summary In larvae of Rana temporaria and Bufo bufo the location of filter apparatus within the larval organization, the arrangement of the morphological parts as branchial food trap, ventral velum, and filter rows, as well as their surface anatomy, are similar to that of other species of Orton's larval type IV. The means by which mucous with its entrapped food particles is transported from the filter rows to the esophagus is finally resolved. The dorsally positioned ciliary cushion extends far ventrally between the filter plates. From their contact with the filter rows, the cilia transport the mucous to Kratochwill's caudally positioned Flimmerrinne and from there to the esophagus. The original chordate principle of mucous entrapment and ciliary transport is thus retained by these anuran larvae. The only modification specific to the latter is the division into a ventral filter apparatus, whose epithelia serve for mucus entrapment, and a dorsal ciliary area.Six different types of cell may be distinguished ultrastructurally: (1) The ubiquitous squamous epithelium with merocrine extrusions; (2) the large supporting cells of the filter rows and of the ventral velum; (3) the ciliary cells of the ciliary cushion; (4) three different types of mucous producing secretory cells: (a) A type of cell similar to the goblet cell is found in the ciliary cushion (merocrine extrusion); (b) The secretory pits of the ventral velum and the secretory ridges have similar bottle-shaped merocrine secretory cells; (c) The merocrine apical cells of the filter rows are the final kind. It is evident that the ciliary cushion epithelium resembles that of both the manicotto glandulare of anuran larvae and the trachea and bronchus of Mammalia.Supported by the Deutsche Forschungsgemeinschaft-DFG     

Structure and development of the larval visual system in embryos of Lytta viridana leconte (coleoptera,meloidae)

At hatching (252–264 hr. at 25 ± 0.5°C), the visual system in larvae of Lytta viridana consists of paired stemmata, stemmatal nerves, optic neuropiles, and inner and outer imaginal optic lobe anlagen. It originates between 64 and 72 hr. with invagination of an optic lobe primordium in the side of each protocephalic lobe. These primordia later differentiate into protocerebral ganglion cells and the imaginal optic lobe anlagen. Each stemma arises at 72 hr. from epidermis below and behind the optic lobe invagination and subsequently becomes cupshaped, closes over, and differentiates. At hatching, it consists of a planoconvex corneal lens, a corneagenous layer, and an everse retina of numerous, pigmented retinular cells, each with a terminal rhabdomere. Between 96 and 104 hr, proximal ends of the retinular cells grow posteromedially into a transverse, horizontal fold in the posterior wall of each optic lobe invagination and along its length to the protocerebral neuropile, which they contact by 112 hr. As the brain withdraws posteriorly within the head, these axons elongate correspondingly. Sheath cells of stemmata and stemmatal nerves descend either from protocerebral perineurium or the optic lobe primordia. Structure and development of the larval visual system in L. viridana are compared with those of other insects and its various components are shown to be homologous throughout the Insecta. However, the stemmata of this insect more closely resemble the atypical imaginal eyes of male scale insects than the photoreceptors of other holometabolous larvae–a similarity arising through convergence.     

Two visual systems in one eyestalk: The unusual optic lobe metamorphosis in the stomatopod Alima pacifica

The compound eyes of adult stomatopod crustaceans have two to six ommatidial rows at the equator, called the midband, that are often specialized for color and polarization vision. Beneath the retina, this midband specialization is represented as enlarged optic lobe lamina cartridges and a hernia‐like expansion in the medulla. We studied how the optic lobe transforms from the larvae, which possess typical crustacean larval compound eyes without a specialized midband, through metamorphosis into the adults with the midband in a two midband‐row species Alima pacifica. Using histological staining, immunolabeling, and 3D reconstruction, we show that the last‐stage stomatopod larvae possess double‐retina eyes, in which the developing adult visual system forms adjacent to, but separate from, the larval visual system. Beneath the two retinas, the optic lobe also contains two sets of optic neuropils, comprising of a larval lamina, medulla, and lobula, as well as an adult lamina, medulla, and lobula. The larval eye and all larval optic neuropils degenerate and disappear approximately a week after metamorphosis. In stomatopods, the unique adult visual system and all optic neuropils develop alongside the larval system in the eyestalk of last‐stage larvae, where two visual systems and two independent visual processing pathways coexist. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 3–14, 2018     

The larval ocelli of Golfingia misakiana (Sipuncula, Golfingiidae) and of a pelagosphera of another unidentified species

 The inverse cerebral ocelli of the pelagosphera larva of Golfingia misakiana and of another unidentified larva are composed of two or three sensory cells and one supportive pigmented cell. The sensory cells bear an array of microvilli as well as a single cilium with poor undulation of its membrane; the photoreceptive organelles are regarded as the rhabdomeric type. A striking feature of these cells is the cores, which extend within the microvilli from the tip into the midregion of the cell. It is suggested that these structures are identical with the submicrovillar cisternae found in the cerebral inverse eyes of larvae of Polychaeta. The findings allow the conclusion that in the pelagosphera of the Sipuncula, contrary to the teleplanic veliger larvae of Gastropoda, a lengthy pelagic cycle is not correlated with the development of a ciliary photoreceptor. Additionally, it is assumed that the pigment cup ocelli in larvae of Sipuncula are homologous with the cerebral inverted pigment cup ocelli of larvae of Polychaeta. Accepted: 19 March 1997     

Pigmented eyes,photoreceptor‐like sense organs and central nervous system in the polychaete Scoloplos armiger (Orbiniidae,Annelida) and their phylogenetic importance

The phylogenetic position of Orbiniidae within Annelida is unresolved. Conflicting hypotheses place them either in a basal taxon Scolecida, close to Spionida, or in a basal position in Aciculata. Because Aciculata have a specific type of eye, the photoreceptive organs in the orbiniid Scoloplos armiger were investigated to test these phylogenetic hypotheses. Two different types of prostomial photoreceptor‐like sense organs were found in juveniles and one additional in subadults. In juveniles there are four ciliary photoreceptor‐like phaosomes with unbranched cilia and two pigmented eyes. The paired pigmented eyes lie beside the brain above the circumoesophageal connectives. Each consists of one pigmented cell, one unpigmented supportive cell and three everse rhabdomeric sensory cells with vestigial cilia. During development the number of phaosomes increases considerably and numerous unpigmented sense organs appear consisting of one rhabdomeric photoreceptor cell and one supportive cell. The development and morphology of the pigmented eyes of S. armiger suggest that they represent miniaturized eyes of the phyllodocidan type of adult eye rather than persisting larval eyes resulting in small inverse eyes typical of Scolecida. Moreover, the structure of the brain indicates a loss of the palps. Hence, a closer relationship of Orbiniidae to Phyllodocida is indicated. Due to a still extensive lack of ultrastructural data among polychaetes this conclusion cannot be corroborated by considering the structure of the unpigmented ciliary and rhabdomeric photoreceptor‐like sense organs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

The fine structure of the lens and photoreceptors of Nereis virens

Summary The cell membrane in the distal region of the photoreceptors of Nereis virens is formed into large numbers of tubular microvilli which interdigitate with those of neighbouring photoreceptors. No traces of cilia or basal bodies have been found, but a prominent axial filament with a repeat pattern of 660 Å extends from the upper to the lower parts of the cell. Large numbers of mitochondria are found in the outer segment of the photoreceptors around the bases of the microvilli. As it passes through the pigment layer the photoreceptor is invaded by finger-like intrusions of the pigment cells which serve to prevent light entering the eye from behind. The lens consists of fine filaments or rod-shaped bodies secreted by the pigment cells, which pass to the lens up narrow pillars between the outer segments of the photoreceptors. Attention is drawn to the evolutionary parallel between the shadow reflex of Branchiomma and the off responses of the photoreceptors in the distal retina of Pecten, both animals being exceptional within their phyla in having photoreceptors of the ciliary type.