Investigating the developmental onset of regenerative potential in the annelid Capitella teleta

An animal's ability to regrow lost tissues or structures can vary greatly during its life cycle. The annelid Capitella teleta exhibits posterior, but not anterior, regeneration as juveniles and adults. In contrast, embryos display only limited replacement of specific tissues. To investigate when during development individuals of C. teleta become capable of regeneration, we assessed the extent to which larvae can regenerate. We hypothesized that larvae exhibit intermediate regeneration potential and demonstrate some features of juvenile regeneration, but do not successfully replace all lost structures. Both anterior and posterior regeneration potential of larvae were evaluated following amputation. We used several methods to analyze wound sites: EdU incorporation to assess cell proliferation; in situ hybridization to assess stem cell and differentiation marker expression; immunohistochemistry and phalloidin staining to determine presence of neurites and muscle fibers, respectively; and observation to assess re-epithelialization and determine regrowth of structures. Wound healing occurred within 6 h of amputation for both anterior and posterior amputations. Cell proliferation at both wound sites was observed for up to 7 days following amputation. In addition, the stem cell marker vasa was expressed at anterior and posterior wound sites. However, growth of new tissue was observed only in posterior amputations. Neurites from the ventral nerve cord were also observed at posterior wound sites. De novo ash expression in the ectoderm of anterior wound sites indicated neuronal cell specification, although the absence of elav expression indicated an inability to progress to neuronal differentiation. In rare instances, cilia and eyes re-formed. Both amputations induced expanded expression of the myogenesis gene MyoD in preexisting tissues. Our results indicate that amputated larvae complete early, but not late, stages of regeneration, which indicates a gradual acquisition of regenerative ability in C. teleta. Furthermore, amputated larvae can metamorphose into burrowing juveniles, including those missing brain and anterior sensory structures. To our knowledge, this is the first study to assess regenerative potential of annelid larvae.     

Effects of maternal investment on larvae and juveniles of the annelid Capitella teleta determined by experimental reduction of embryo energy content

Experimental manipulations of the energy content of marine invertebrate embryos have been useful in testing key assumptions of life history theory, especially those concerning relationships between egg size, length of the planktonic period, and juvenile size and quality. However, methods for such “allometric engineering” experiments have been available for only a limited set of taxa (those with regulative early development, e.g., cnidarians and echinoderms). Here, we describe a method for the reduction of embryo energy content in the spirally cleaving embryos of a marine annelid, Capitella teleta, by targeted deletion of endodermal precursor cells. Embryos of C. teleta in which up to three cells (the macromeres 3A, 3B, and 3C) were deleted formed morphologically normal lecithotrophic larvae that were much smaller than larvae developing from control embryos. Experimental larvae metamorphosed at high rates, forming juveniles that were smaller than control juveniles. Juveniles derived from treated embryos had functional midguts, ingested and digested food, and grew into sexually mature adults. These results are consistent with those from previous allometric engineering studies of echinoid echinoderms, which suggest that in facultatively planktotrophic or lecithotrophic species, little maternally derived energy is used for construction of the larval body; instead, the majority is allocated to the formation of a large, high‐quality juvenile. Cleavage programs are highly conserved among divergent spiralian taxa (e.g., molluscs, nemerteans, and platyhelminths), so this method will likely be applicable to a diverse set of embryos. Similar experiments carried out in these diverse taxa will be extremely useful for evaluating inferences on relationships between egg size, length of the planktonic period, and juvenile size and quality previously based only on experiments on echinoid echinoderms.     

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.     

Development of pigment-cup eyes in the polychaete Platynereis dumerilii and evolutionary conservation of larval eyes in Bilateria

The role of Pax6 in eye development in insects and vertebrates supports the view that their eyes evolved from simple pigment-cup ocelli present in their last common ancestors (Urbilateria). The cerebral eyes in errant polychaetes represent prototype invertebrate pigment-cup ocelli and thus resemble the presumed ancestral eyes. We have analysed expression of conserved eye specification genes in the early development of larval and adult pigment-cup eyes in Platynereis dumerilii (Polychaeta, Annelida, Lophotrochozoa). Both larval and adult eyes form in close vicinity of the optic anlagen on both sides of the developing brain ganglia. While pax6 is expressed in the larval, but not in the developing, adult eyes, expression of six1/2 from trochophora stages onwards specifically outlines the optic anlagen and thus covers both the developing larval and adult eyes. Using Platynereis rhabdomeric opsin as differentiation marker, we show that the first pair of adult eye photoreceptor cells is detected within bilateral clusters that transitorily express ath, the Platynereis atonal orthologue, thus resembling proneural sensory clusters. Our data indicate that--similar to insects, but different from the vertebrates--polychaete six1/2 expression outlines the entire visual system from early developmental stages onwards and ath-positive clusters generate the first photoreceptor cells to appear. We propose that pax6-, six1/2- and ath-positive larval eyes, as found in today's trochophora, were present already in Urbilateria.     

Sipunculid‐like ocellar tubes in a polychaete,Fauveliopsis cf. adriatica (Annelida,Fauveliopsidae): implications for eye evolution

Abstract. A retractable head region somewhat resembling the introvert of sipunculans is a characteristic feature of members of the annelid taxon Fauveliopsidae. The morphology of fauvelopsids is not well known, and additional data might help to resolve their relationships with other annelids and sipunculans. Ultrastructural investigations of the anterior end of adults of Fauveliopsis cf. adriatica revealed peculiar brain and sensory structures. From the neuropil of the brain, two pairs of lobes mainly composed of neuronal somata extend posteriorly into the peristomium and the following segment. The nuchal organs are embedded in the median pair of lobes, as are additional photoreceptor‐like sensory structures, the ocellar tubes, which are found at the bases of epidermal follicles that extend deeply into the brain. The retractor muscles of the prostomium are attached to the apices of these follicles, which are lined by tendon and supportive cells. The lumen of each follicle is completely filled with cuticular material that forms a rod. Monociliary sensory cells are present all along the length of each follicle; their cilia extend into the cuticle, and are oriented parallel to the longitudinal axis of the tube. Basally, each follicle forms an ovoid extension that is devoid of cuticular material and densely filled with numerous sensory processes—microvilli and cilia—of bipolar sensory cells. The terminal end of the 40‐μm‐deep follicle is formed by two conspicuous cells that contain numerous densely packed vesicles that resemble pigment granules. The ocellar tubes of fauveliopsids are strikingly similar to the ocular tubes of sipunculids. These similarities may reflect common ancestry or may represent convergent evolution; both alternatives are partially supported by previous morphological and molecular studies.     

武汉亮斑扁角水虻幼虫肠道微生物对其成虫产卵行为的影响

研究武汉亮斑扁角水虻幼虫肠道微生物对武汉亮斑水虻成虫产卵行为的影响。首先使用LB(Luria-Bertani)培养基对武汉亮斑水虻幼虫肠道微生物进行分离纯培养,筛选得到23株肠道微生物。以灭菌的LB液体培养基为阴性对照,使用已证实引诱产卵效果好的来自武汉亮斑水虻卵表的属于戈登氏菌属的细菌(Gordonia sihwensis)FE06为阳性对照,利用筛选到的武汉亮斑水虻幼虫肠道微生物摇瓶发酵液对武汉亮斑水虻成虫进行产卵行为实验。发现有5株武汉亮斑水虻幼虫肠道微生物(BSFLG01、BSFLG03、BSFLG04、BSFLG05、BSFLG06)对武汉亮斑水虻产卵有显著的积极促进作用,其中3株(BSFLG03、BSFLG05、BSFLG06)对水虻成虫产卵的引诱作用明显强于所设立的阳性对照FE06;菌株BSFLG07为一种毕赤酵母菌(Pichia kudriavzevii),对武汉亮斑水虻产卵有消极影响。经菌落培养特征、菌体形态比较、16SrDNA或18SrDNA序列测定和发育树比对分析,鉴定出这5种具有显著引诱武汉亮斑水虻产卵作用的微生物分别为:枯草芽胞杆菌(Bacillus subtilis)BSFLG01、一种沙雷氏菌(Serratiasp.)BSFLG03、库德毕赤酵母(Pichia kudriavzevii)BSFLG04、皱落假丝酵母(Candida rugosas)BSFLG05、粘质沙雷氏菌(Serratia marcescens)BSFLG06。     

Ultrastructure of pigmented eyes in Dorvilleidae (Annelida,Errantia, Eunicida) and their importance for understanding the evolution of eyes in polychaetes

Among polychaetes, the errant forms are the only group known so far possessing true multicellular eyes in adults which are preceded by bicellular larval eyes in many species. Most likely, two pairs of such eyes showing a specific structure belong to the ground pattern of Errantia = Aciculata. However, these eyes have primarily been investigated in only two subgroups of Errantia, but data on the third main taxon, Eunicida, are available for only two taxa. In the present investigation, the eyes in two additional species of Eunicida, the dorvilleids Protodorvillea kefersteini and Schistomeringos neglecta, were studied. In P. kefersteini, usually described as possessing one pair of small eyes, two pairs could be detected, whereas in S. neglecta only one pair was found. Each eye is made up of rhabdomeric photoreceptor cells, pigment cells and unpigmented supportive cells. Lenses or vitreous bodies are absent. From their structure most likely all eyes represent adult eyes and even the small anterior eyes in P. kefersteini structurally resemble miniaturized adult eyes. Neither persisting larval eyes nor unpigmented rhabdomeric ocelli were found in the two species. The observations in Dorvilleidae confirm the hypothesis of a common origin of adult eyes in Errantia.     

The epidemiology and public health importance of toxocariasis: A zoonosis of global importance

Toxocariasis, caused by infection with larvae of Toxocara canis, and to a lesser extent by Toxocara cati and other ascaridoid species, manifests in humans in a range of clinical syndromes. These include visceral and ocular larva migrans, neurotoxocariasis and covert or common toxocariasis. Toxocara canis is one of the most widespread public health and economically important zoonotic parasitic infections humans share with dogs, cats and wild canids, particularly foxes. This neglected disease has been shown through seroprevalence studies to be especially prevalent among children from socio-economically disadvantaged populations both in the tropics and sub-tropics and in industrialised nations. Human infection occurs by the accidental ingestion of embryonated eggs or larvae from a range of wild and domestic paratenic hosts. Most infections remain asymptomatic. Clinically overt infections may go undiagnosed, as diagnostic tests are expensive and can require serological, molecular and/or imaging tests, which may not be affordable or available. Treatment in humans varies according to symptoms and location of the larvae. Anthelmintics, including albendazole, thiabendazole and mebendazole may be given together with anti-inflammatory corticosteroids. The development of molecular tools should lead to new and improved strategies for the treatment, diagnosis and control of toxocariasis and the role of other ascaridoid species in the epidemiology of Toxocara spp. Molecular technologies may also help to reveal the public health importance of T. canis, providing new evidence to support the implementation of national control initiatives which have yet to be developed for Toxocara spp. A number of countries have implemented reproductive control programs in owned and stray dogs to reduce the number of young dogs in the population. These programs would positively impact upon T. canis transmission since the parasite is most fecund and prevalent in puppies. Other control measures for T. canis include the regular and frequent anthelmintic treatment of dogs and cats, starting at an early age, education and enforcement of laws for the disposal of canine faeces, dog legislation and personal hygiene. The existence of wild definitive and paratenic hosts complicates the control of T. canis. Increasing human and dog populations, population movements and climate change will all serve to increase the importance of this zoonosis. This review examines the transmission, diagnosis and clinical syndromes of toxocariasis, its public health importance, epidemiology, control and current research needs.     

Noninvasive two-photon imaging reveals retinyl ester storage structures in the eye

Visual sensation in vertebrates is triggered when light strikes retinal photoreceptor cells causing photoisomerization of the rhodopsin chromophore 11-cis-retinal to all-trans-retinal. The regeneration of preillumination conditions of the photoreceptor cells requires formation of 11-cis-retinal in the adjacent retinal pigment epithelium (RPE). Using the intrinsic fluorescence of all-trans-retinyl esters, noninvasive two-photon microscopy revealed previously uncharacterized structures (6.9 +/- 1.1 microm in length and 0.8 +/- 0.2 microm in diameter) distinct from other cellular organelles, termed the retinyl ester storage particles (RESTs), or retinosomes. These structures form autonomous all-trans-retinyl ester-rich intracellular compartments distinct from other organelles and colocalize with adipose differentiation-related protein. As demonstrated by in vivo experiments using wild-type mice, the RESTs participate in 11-cis-retinal formation. RESTs accumulate in Rpe65-/- mice incapable of carrying out the enzymatic isomerization, and correspondingly, are absent in the eyes of Lrat-/- mice deficient in retinyl ester synthesis. These results indicate that RESTs located close to the RPE plasma membrane are essential components in 11-cis-retinal production.     

The effects of 20-hydroxyecdysone on the differentiationin vitro of cells from the eye imaginal disc fromDrosophila melanogaster

We have examined the effects of the insect ecdysteroid, 20-hydroxyecdysone, on the differentiation of neuronal and non-neuronal elements in the developing adult visual system, usingin vitro methods inDrosophila. We examined the differentiation of early neuronal markers in the presence and absence of 1 μg/ml 20-hydroxyecdysone. Immunoreactivity to 22C10, a marker of an early neuronal antigen, as well as to the photoreceptor-specific antibody 24B10, suggests that differentiation of neuronal and photoreceptor antigens does not require 20-hydroxyecdysone. In eye-discs cultured from animals 5 hours after the white prepupa (P+5), ommochrome pigmentation first appeared after 2 days in 1 μg/ml 20-hydroxyecdysone, but cultures lacked pigmentation without 20-hydroxyecdysone. Our culture conditions failed to support the formation of the second screening pigment, drosopterins, even with 20-hydroxyecdysone. Eye discs from P+5 also formed lenses and interommatidial bristles in culture when 20-hydroxyecdysone was added but not in cultures devoid of the hormone. The differentiation of synaptotagmin and the elongation of extending photoreceptor neurites from eye disc fragments both occur in the absence of 20-hydroxyecdysone in cultures, but adding the hormone increased average neurite length. The threshold for enhanced neurite length was less than 125 ng/ml 20-hydroxyecdysone. Eye-disc cultures also developed immunoreactivity to histamine, the photoreceptor transmitter, from synthesis not re-uptake, in both the presence and in the absence of 20-hydroxyecdysone. These findings suggest that photoreceptor axons may be able to release transmitterin vivo both when they grow into the optic lobe and during the subsequent events in synapse formation.     

The effects of Mn2+ and Ca2+ on the prolonged depolarising after-potential in barnacle photoreceptor

We have studied the effects on the PDA of modifying intracellular and extracellular concentrations of Ca²⁺ and Mn²⁺. The effect of decreased Ca²⁺ concentration or addition of EGTA is mainly an increase in the PDA amplitude and length. Raising Ca²⁺ concentration using ruthenium red or high external Ca²⁺ has the opposite effect. The effect of Mn²⁺ is much more striking: In the presence of 50–100 mM Mn²⁺ the PDA is initially greatly depressed but can rise slowly for up to 20 or 30 s (in the dark) until it approaches its original amplitude and time course. Bridge measurements showed that the depression of the PDA corresponds to a depressed conductance and so is not due to an increase in K⁺ conductance. The Mn²⁺ effect is potentiated by decreased Ca²⁺. Appropriate stimulation suppresses the rising PDA as promptly as it does a normal PDA, suggesting that if lateral diffusion is the source of the slow rise, the PDA and PDA-depressing processes must be spatially linked. The action of the anti-PDA is apparently prolonged by both Ca²⁺ and Mn²⁺.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Embryonic, larval and juvenile development of the roughskin sculpin,Trachidermus fasciatus (Scorpaeniformes: Cottidae)

Embryonic, larval and juvenile development of the catadromous roughskin sculpin,Trachidermus fasciatus, were described using eggs spawned in an aquarium. The eggs, measuring 1.98–2.21 mm in diameter, were light reddish-yellow and had many oil globules, 0.05–0.18 mm in diameter. Hatching occurred 30 days after spawning at 2.3–11.3°C. The newly-hatched larvae, measuring 6.9–7.3 mm BL, had a single oil globule, 9–10+25–26=34–36 myomeres and 6 or 7 large stellate melanophores dorsally along the gut. The yolk was almost resorbed, number of pectoral-fin rays attained 16–17, and two parietal, one nuchal and four preopercular spines were formed, 5 days after hatching, at 8.2–8.4 mm BL. The oil globule disappeared, and one supracleithral spine was formed, 11 days after hatching, at 8.9–9.5 mm BL. Notochord flexion began 15 days after hatching, at 9.7–10.3 mm BL. A posttemporal spine was formed 20 days after hatching, at 10.7–10.9 mm BL. The first dorsal fin spines (VII–VIII), second dorsal fin and anal fin rays (18–19, 16–18, respectively) appeared 23 days after hatching, at 12.0–13.7 mm BL. The pelvic fin spine and rays (I, 4) were formed and black bands on the head and sides of the body began to develop 27 days after hatching, at 13.8–15.8 mm BL. Newly-hatched larvae swam just below the surface in the aquaria. Preflexion larvae (8.9–9.5 mm BL), in which the oil globule had disappeared, swam in the middle layer, while juveniles (13.8–15.8 mm BL) began swimming on the bottom of the aquaria. Swimming behavior observed in the aquaria suggested that the fish started to change to a demersal existence at the juvenile stage.     

Growth and morphological development of larval and juvenileepinephelus bruneus (perciformes: Serranidae)

The growth and morphological development of larval and juvenileEpinephelus bruneus were examined in a hatchery-reared series. Average body length (BL) of newly-hatched larvae was 1.99 mm, the larvae growing to an average of 3.96 mm by day 10, 6.97 mm by day 20, 12.8 mm by day 30, 22.1 mm by day 40 and 24.7 mm by day 45 after hatching. Newly-hatched larvae had many mucous cells in the entire body epidermis. By about 4 mm BL, the larvae had developed pigment patterns peculiar to epinepheline fishes, including melanophores on the dorsal part of the gut, on the tips of the second dorsal and pelvic fin spines, and in a cluster on the ventral surface of the tail. Spinelets on the second dorsal and pelvic fin spines, the preopercular angle spine and the supraocular spine, had started to develop by about 6 mm BL. The notochord tip was in the process of flexion in larvae of 6–8 mm BL, by which time major spines, pigments and jaw teeth had started to appear. Fin ray counts had attained the adult complement at 10 mm BL. After larvae reached 17 mm BL, elements of juvenile coloration in the form of more or less densely-pigmented patches started to appear on the body. Squamation started at 20 mm BL. Major head spines had disappeared or became relatively smaller and lost their serrations by 20–25 mm BL.     

Growth of larval and juvenile perch: the importance of diet and fish density

In August, growth rate of young–of–the–year (YOY) Perch In lake enclosures could be explained by both YOY density and mean cladoceran biomass, suggesting that in a lake where YOY perch are dominant, growth may be density dependent in late summer and mediated through top–down control on daphnid biomass. In June, growth rate of YOY perch could not be fully explained by YOY density or by mean cladoceran biomass, suggesting that growth and survival during the first part of the summer is negatively affected by a diet of Bosmina and cyclopoid copepods only. The experiments also suggest why YOY perch have a slow growth and a low abundance in eutrophic lakes where small zooplankton dominate. The June experiment also indicated that growth of late larval or early juvenile perch improved when a larger cladoceran became available and was included in the diet.     

Ultrastructure of the larval eye of the scorpionfly Panorpa dubia (mecoptera: Panorpidae) with implications for the evolutionary origin of holometabolous larvae

The evolutionary origin of holometabolous larvae is a long‐standing and controversial issue. The Mecoptera are unique in Holometabola for their larvae possessing a pair of compound eyes instead of stemmata. The ultrastructure of the larval eyes of the scorpionfly Panorpa dubia Chou and Wang, 1981 was investigated using transmission electron microscopy. Each ommatidium possesses a cornea, a tetrapartite eucone crystalline cone, eight retinula cells, two primary pigment cells, and an undetermined number of secondary pigment cells. The rhabdomeres of the eight retinula cells form a centrally‐fused, tiered rhabdom of four distal and four proximal retinula cells. The rhabdomeres of the four distal retinula cells extend distally into a funnel shape around the basal surface of the crystalline cone. Based on the similarity of the larval eyes of Panorpidae to the eyes of the hemimetabolous insects and the difference from the stemmata of the holometabolous larvae, the evolutionary origin of the holometabolous larvae is briefly discussed. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Chiton myogenesis: perspectives for the development and evolution of larval and adult muscle systems in molluscs.

We investigated muscle development in two chiton species, Mopalia muscosa and Chiton olivaceus, from embryo hatching until 10 days after metamorphosis. The anlagen of the dorsal longitudinal rectus muscle and a larval prototroch muscle ring are the first detectable muscle structures in the early trochophore-like larva. Slightly later, a ventrolaterally situated pair of longitudinal muscles appears, which persists through metamorphosis. In addition, the anlagen of the putative dorsoventral shell musculature and the first fibers of a muscular grid, which is restricted to the pretrochal region and consists of outer ring and inner diagonal muscle fibers, are generated. Subsequently, transversal muscle fibers form underneath each future shell plate and the ventrolateral enrolling muscle is established. At metamorphic competence, the dorsoventral shell musculature consists of numerous serially repeated, intercrossing muscle fibers. Their concentration into seven (and later eight) functional shell plate muscle bundles starts after the completion of metamorphosis. The larval prototroch ring and the pretrochal muscle grid are lost at metamorphosis. The structure of the apical grid and its atrophy during metamorphosis suggests ontogenetic repetition of (parts of) the original body-wall musculature of a proposed worm-shaped molluscan ancestor. Moreover, our data show that the "segmented" character of the polyplacophoran shell musculature is a secondary condition, thus contradicting earlier theories that regarded the Polyplacophora (and thus the entire phylum Mollusca) as primarily eumetameric (annelid-like). Instead, we propose an unsegmented trochozoan ancestor at the base of molluscan evolution.     

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.