Ultrastructure of the Protonephridial Terminal Organ in the Terrestrial Nemertean Geonemertes pelaensis (Rhynchocoela,Enopla, Hoplonemertini)

The protonephridial terminal organ of Geonemertes pelaensis consists of two cells that are equal in both size and shape and form mirror images of each other. From the perinucleate lump-shaped region basally in each cell arises a cytoplasmic column which branches at regular intervals to form 3–4 nearly circular bars. The opposed columns and alternating bars of the two cells are arranged in such a way that they form an obviously rigid, cylindrical structure which both supports and gives rise to the thin-walled weir. The fenestration of the weir consists of a single, enormously extended and sinuous cleft which represents part of the boundary between the two terminal cells. The ciliary flame comprises 92–118 cilia that originate basally in the two cells and is surrounded by a palisade of long, straight microvilli, positioned immediately within the weir's wall. The structure is very similar to that of the terminal organ of the land nemertean Pantinonemertes, except that the cytoskeleton which supports columns and bars consists of a fibro-granular substance, not, as in Pantinonemertes, of an abundance of oriented microtubules.     

Ultrastructure of the Flame Bulbs and Protonephridial Capillaries of Artioposthia sp. (Platyhelminthes,Tricladida, Geoplanidae)

The protonephridial terminal complex of Artioposthia is formed by one or two terminal cells, each with a nucleus located in the lateral wall of the flame bulb, and probably two proximal canal cells forming the wall of the protonephridial capillary. The weir is restricted to the proximal parts of the flame bulbs and consists of convoluted slits separated by thick cytoplasmic columns. Cross-striated ciliary rootlets running parallel with and obliquely or transversely to the longitudinal axis of the flame bulbs strengthen the walls of the flame bulbs and, to a lesser degree, that of the capillary. Numerous cristate mitochondria are present in the terminal and proximal canal cells. Cytoplasmic processes extend from the terminal cells into the adjacent tissue, and narrow internal leptotriches extend from the cytoplasm of the terminal cells into the lumen of the flame bulbs. The wall of the capillary contains many interconnected, liquid filled spaces that communicate with the lumen of the capillary, and two septate junctions. Phylogenetic implications of the findings are discussed.     

Ultrastructure of flame cells and protonephridial capillaries of Craspedella and Didymorchis (Plathelminthes,Rhabdocoela)

Summary The ultrastructure of the flame cells and protonephridial capillaries of the Rhabdocoela Craspedella sp. and Didymorchis sp., ectocommensals on the freshwater crayfish Cherax destructor in eastern Australia is described. The flame cells of both species have variable numbers of cilia without distinct rootlets and with decreasing numbers of axonemal tubules towards the ciliary tips. Bundles of microtubules extend from the cytoplasm adjacent to the ciliary rootlets through the ribs of the weir apparatus into the distal cytoplasmic tube, where the numbers of microtubules gradually decrease. The weir apparatus is formed by a single row of longitudinal ribs connected by a membrane. In Craspedella, but not in Didymorchis, the ribs have external branched leptotriches. Mitochondria are common in the wall of the flame cell of both species. The protonephridial capillary just above the end of the ciliary tuft narrows in both species and bends sharply in Craspedella. The lumen of the flame cell and the capillary is lined by a dark layer of cytoplasm; there is no enlargement of the surface area by microvilli or lamellae. Centrioles were seen in the capillary wall of Craspedella, and in Didymorchis the cytoplasm around the capillaries has a very loose and light appearance. The ultrastructure of the flame cells and capillaries of both species corresponds closely to that of Temnocephala sp.Abbreviations in the figures BB basal body - CE centriole - L leptotrich - M microtubules - ME membrane of weir apparatus - MI mitochondrion - PC protonephridial capillary - R rib (rod) of weir apparatus     

Ultrastructure of the protonephridial system of regenerating Stenostomum sp. (Plathelminthes,Catenulida)

Summary The ultrastructure of the flame bulbs, protonephridial capillaries and duct of fully developed and regenerating Stenostomum sp. is described. Flame bulbs are formed by a single cell whose nucleus is located basally or laterally to the weir. The weir is formed by a single row of transverse ribs connected by a thin membrane, apparently of extracellular matrix. Internal leptotriches arise from the proximal cytoplasm and extend in a (usually) single row along the weir and into the lumen of the distal cytoplasmic tube. Many or all leptotriches do not fuse with the distal cytoplasm. Two cilia are each anchored in the proximal cytoplasm by a cross-striated vertical and lateral rootlet, the latter bent forward and extending for some distance into one of the two cytoplasmic cords along the weir. Each cord contains the lateral rootlet in its proximal part, as well as many microtubules. The distal cytoplasmic tube contains two (longitudinal) junctions, i.e. lines of contact between cell processes of the same, terminal cell. Occasionally, more than two junctions were seen, apparently due to branches of the terminal cell in contact with each other. Flame bulbs join capillaries lined by several canal cells type I, containing few or no microvilli but lateral flames. Such capillaries join a duct (or ducts?) lined by canal cells type II with many long microvilli. The large protonephridial duct is lined by numerous cells with lateral flames and many long microvilli. In regenerating tissue (10.5 hours after cutting) some flame bulbs were free, i.e. not connected to capillaries, and some capillaries openly communicated with the surrounding intercellular space. In the presence of a single row of ribs in the weir, of internal leptotriches, and of vertical and lateral ciliary rootlets, the flame bulb of Stenostomum sp. resembles that of other Plathelminthes much more closely than hitherto thought. The species differs from non-catenulid plathelminths mainly in the large number of glandular cells lining the large protonephridial ducts, in the transverse orientation of the ribs in the weir and in the presence of only two cilia in the flame.     

Ultrastructure of the lycophora larva of Gyrocotyle urna (Cestoda,Gyrocotylidea)

Summary The ultrastructure of the protonephridial system of the lycophore larva of Gyrocotyle urna Grube and Wagener, 1852, is described. It consists of six terminal cells, at least two proximal canal cells, two distal canal cells and two nephridiopore cells. The terminal cells and the proximal canal cell build up the filtration weir with its two circles of weir rods. The proximal canal cell constitutes a solid, hollow cylinder without a cell gap and desmosome. The distal canal cell is characterized by a strong reduction of the canal lumen by irregularly shaped microvilli. The nephridiopore region is formed by a nephridiopore cell; its cell body is located at some distance proximally within the larva. The connection among different canal cells is brought about by septate desmosomes. Morphological, evolutionary and functional aspects of the protonephridial system within Platyhelminthes are discussed. The structure of the proximal canal cells without a desmosome is considered an autapomorphy of Cestoda.Abbreviations ci cilia of the terminal cell - Co distal canal cell - col lumen of the distal canal cell - Ep epidermis - er outer rods of the filtration weir - il inner leptotriches - ir inner rods of the filtration weir - ld lipid droplets - mt microtubule - mv microvilli - Nc nephridiopore cell - Ne neodermis anlage cells - nu nucleus - pC proximal canal cell - ro ciliary rootlets - sd septate desmosome - Tc terminal cell     

Ultrastructure of the nephridio-circulatory connections in Tubulanus annulatus (Nemertini,Anopla)

Summary The protonephridial terminal organs in the nemertean Tubulanus annulatus form an integral part of the blood vessel wall. Both endothelial and muscle-cell layers of the vessel's wall are discontinued at the site of each terminal organ. The terminal organs are usually composed of from one to three terminal cells enclosing a central lumen provided with many microvilli and separated from the blood vessel's lumen by a membranous filtration area. The latter is perforated by numerous winding clefts formed by interdigitation of minute cytoplasmic pedicels arising from processes issued by each of the involved terminal cells. Ultrafiltration of blood plasma takes place across a filtration membrane which spans the cleft system and the basal lamina of the terminal cells. Fluid is propelled into the lumen of the terminal organs through the activity of ciliary bundles, one for each terminal cell involved, perhaps supplemented by vascular turgor. All efferent conduits of the protonephridium have profuse infoldings of the luminal cell surfaces and/or numerous pinocytotic pits suggestive of reabsorption of substances from the primary urine.Abbreviations BL basal lamina - C cilium - CP coated pit - CT collecting tubule - CV inzcoated vesicle - D dictyosome - E endothelial cell - F fenestration of endothelial cell - FA filtration area - FM filtration membrane - G glycogen granule - LV lateral vessel - M mitochondrion - MC muscle cell - MV microvillus - N nucleus of terminal cell - NE nucleus of endothelial cell - NP nephridiopore - PC protonephridial capillary cell - PT protonephridial tubule - R rootlet - TC terminal cell     

Ultrastructure of protonephridia in Xenotrichula carolinensis syltensis and Chaetonotus maximus (Gastrotricha: Chaetonotida): comparative evaluation of the gastrotrich excretory organs

In an attempt to obtain detailed information on the entire protonephridial system in Gastrotricha, we have studied the protonephridial ultrastructure of two paucitubulatan species, Xenotrichula carolinensis syltensis and Chaetonotus maximus by means of complete sets of ultrathin sections. In spite of some differences in detail, the morphology of protonephridia in both examined species shows a common pattern: Both species have one pair of protonephridia that consist of a bicellular terminal organ, a voluminous, aciliar canal cell and an adjacent, aciliar nephridiopore cell. The terminal organ consists of two monociliar terminal cells each with a distal cytoplasmic lobe. These lobes interdigitate and surround cilia and microvilli of the terminal cells. Where both lobes interdigitate, a meandering cleft is formed that is covered by the filtration barrier. We here term the entire structure composite filter. The elongated, in some regions convoluted protonephridial lumen opens distally to the outside via a permanent nephridiopore. A comparison with the protonephridia of other species of the Gastrotricha allows hypothesising the following autapomorphies of the Paucitubulata: The bicellular terminal organ with a composite filter, the convoluted distal canal cell lumen and the absence of cilia, ciliary basal structures and microvilli within the canal cell. Moreover, this comparative survey could confirm important characteristics of the protonephridial system assumed for the ground pattern of Gastrotricha like, for example, the single terminal cell with one cilium surrounded by eight microvilli.     

The evolution of the protonephridial terminal organ across Rotifera with particular emphasis on Dicranophorus forcipatus,Encentrum mucronatum and Erignatha clastopis (Rotifera: Dicranophoridae)

Riemann, O. and Ahlrichs, W.H. 2009. The evolution of the protonephridial terminal organ across Rotifera with particular emphasis on Dicranophorus forcipatus, Encentrum mucronatum and Erignatha clastopis (Rotifera: Dicranophoridae). —Acta Zoologica (Stockholm) 91 : 199–211 We report on the ultrastructure of the protonephridial terminal organ in three species of dicranophorid rotifers (Dicranophorus forcipatus, Encentrum mucronatum and Erignatha clastopis). Differences between the three species relate to shape and size, the morphology of the filter region and the number of microvilli and cilia inside the terminal organ. A comparison across Rotifera indicates that the terminal organs in D. forcipatus display a number of plesiomorphic characters, but are modified in E. mucronatum and Er. clastopis. This is in accordance with the results of phylogenetic analyses suggesting a basal position of D. forcipatus compared with the more derived species E. mucronatum and Er. clastopis. Moreover, we survey available data on the terminal organ in Rotifera and discuss its evolutionary transformations. The protonephridial terminal organ in the common ancestor of Rotifera consisted of a cytoplasmic cylinder with cilia united into a vibratile flame and a single circle of circumciliary microvilli. Depending on the topology on which characters are optimized, the site of ultrafiltration was formed by longitudinal cytoplasmic columns spanned by a fine filter diaphragm or by pores in the wall of the terminal organ. In several taxa of Rotifera, the terminal organ – probably independently – lost its circumciliary microvilli.     

Ultrastructure of the flame bulbs and protonephridial capillaries of Prorhynchus (Lecithoepitheliata, Prorhynchidae, Turbellaria)

The flame bulb of Prorhynchus is formed by a single cell. Its nucleus is not located in the cytoplasm at the base of the flame. Cilia of the flame have cross-striated hollow ciliary rootlets converging towards their tips. The maximum number of cilia counted was 13. The weir consists of a single row of longitudinal ribs that contain longitudinal filaments and possess regularly arranged protrusions along their surface appearing as transverse bands in horizontal section. A 'membrane' of extracellular material extends between the ribs. and loose material fills the places between the ribs, with a denser layer midway between adjacent ribs. Distally, the ribs fuse to form a continuous tube without a junction. Small protonephridial capillaries lack junctions, larger ducts have lateral flames and patches of long microvilli. Large excretory ducts open into a ciliated and lamellated cavity which is connected by a narrow canal to the excretory pore. The terminal part of the canal close to the pore possesses many cilia and microvilli. Phylogenetic implications of the findings are discussed.     

Ultrastructure of the Protonephridium in Acteonemertes bathamae Pantin (Rhynchocoela: Enopla: Hoplonemertini)

The protonephridial system consists of terminal cell, protonephridial capillary, protonephridial tubule and efferent duct. The terminal cell is an elongated, thin-walled, fenestrated basket containing a ciliary flame circumscribed by a palisade of straight microvilli. The filtration area is confined to the terminal cell and consists of slits bridged by a filtration membrane. The cilia, as well as the microvilli, projects into the proximal bell-shaped part of the thin-walled protonephridial capillary. The terminal cells are often found in pairs connected to the same capillary, which has a very narrow lumen. The proximal part of the thick-walled, convoluted protonephridial tubule is ciliated and shows characteristic foldings of the luminal plasma membrane and numerous small vesicles in the cytoplasm. The cells of the following, non-ciliated part of the tubule have interdigitating lateral surfaces and the bases deeply invaginated to form compartments with numerous mitochondria; in the cytoplasm are many large vesicles, possibly containing lipid droplets, and small amounts of glycogen. The distal protonephridial tubule resembles various epithelia with an osmoregulatory function, including the vertebrate nephron.     

Ultrastructure of the protonephridium in Prostoma graecense (Bohmig) (Rhynchocoela, Enopla, Hoplonemertini)

Fine structure studies show that (1) the terminal cell is an elongated thin-walled and fenestrated basket with a multiciliary flame. Many short curved microvilli are confined to the cell lumen, while longer straight microvilli project from the cell's apical end into the proximal part of the protonephri-dial capillary, forming a sheath around the flame. The filtration area consists of slits between narrow cytoplasmic bars and is entirely confined to the terminal cell, which consequently is defined as a flame bulb, not closely similar to those of other phyla. (2) The protonephridial capillary is short and narrow, with few scattered cilia and luminal microvilli. (3) The coiled tubule is thick-walled, with several ciliated cells very rich in glycogen. The luminal border shows specializations probably concerned with modifying the ultrafiltrate.     

扩张莫尼茨绦虫(绦虫纲:圆叶目)的原肾管及原肾管概念的评述

本研究应用透射电子显微镜研究了扩张莫尼茨绦虫原肾管的细胞学特征 ,莫尼茨绦虫原肾管的焰茎球为一个过滤器结构 ,类似于“挡河坝”样构造 ,此构造由端细胞和近管细胞外突形成的肋条 (或称杆 )相互交错排列而成。肋条之间由细胞外物质构成的“膜”结构连接 ,过滤作用通过该“膜”发生。焰细胞与近管细胞交界处有裂缝或孔与细胞外的结缔组织 (实质组织 )相通 ;原肾管的毛细排泄管细胞质索之间没有隔状联结 ;毛细排泄管及排泄管的管腔内有大量珠状微绒毛突起以增加表面积。从扩张莫尼茨绦虫及其它一些无脊椎动物原肾管的研究结果表明 ,原原肾管概念将焰细胞作为封闭的盲端已不再合适 ,需要进行修订 ,建议修订为 :原肾管是一种焰细胞系统 ,通常由焰细胞、管细胞和肾孔细胞组成 ,焰茎球作为过滤装置与周围的结缔组织 (实质组织 )有或没有裂缝 (孔 )相通     

Ultrastructure of the protonephridia in Pycnophyes kielensis (Kinorhyncha,Homalorhagida)

Summary Pycnophyes kielensis possesses one pair of protonephridia. The single excretory organ of a female consists of 25 cells: 22 terminal cells, 2 canal cells, and 1 nephroporus cell. Generally, all cells exhibit two cilia, the only exception being the nephroporus cell, which contains a diplosome instead. The slashed peripheral cytoplasmic walls of the 22 terminal cells altogether constitute one compound filter and a common filtration area. The protonephridia discharge via cuticularized cavities and six cuticularized tubes. Two accessory cells with modified cilia penetrate the nephroporus cell. These cells are considered to be receptor cells. The protonephridium of the first juvenile stage of P. kielensis is built up of only 5 cells: 3 terminal cells, 1 canal cell, and 1 nephroporus cell. It opens to the outside via 1 cuticularized tube. The protonephridia within both the Kinorhyncha and the Bilateria are discussed. Presumably excretory organs with compound filters developed independently within Bilateria.Abbreviations bb basal body - c canal cell - ca cuticularized cavity - ci cilium - cu cuticle - d dictyosome - de desmosome - di diplosome - dl dorsal longitudinal muscle - dv dorsoventral muscle - ecm extracellular matrix - ep epidermal cell - ex excretory organ - fc filter cleft - fi filter - fm fastening muscle cell - he hemidesmosome - i intestine - if intercellular fluid - m mitochondrium - mv microvilli - n nephroporus cell - nu nucleus - r ciliary rootlet - re accessory cell (presumable receptor cell) - sj septate junction - t terminal cell - tu cuticularized tube - v vesicle - w peripheral cytoplasmic wall     

Development of the excretory system in the polyplacophoran mollusc,Lepidochitona corrugata: The protonephridium

A single pair of protonephridia is the typical larval excretory organ of molluscs. Their presence in postlarval developmental stages was discovered only recently. We found that the protonephridia of the polyplacophoran mollusc, Lepidochitona corrugata, achieve their most elaborate differentiation and become largest during the postlarval period. This study describes the protonephridia of L. corrugata using light and electron microscopy and interactive three‐dimensional visualization. We focus on the postlarval developmental period, in which the protonephridia consist of three parts: the terminal part with the ultrafiltration sites at the distal end, the voluminous protonephridial kidney, and the efferent nephroduct leading to the nephropore. The ultrafiltration sites show filtration slits between regularly arranged thin pedicles. The ciliary flame originates from both the terminal cell and the duct cells of the terminal portion. The efferent duct also shows ciliation. The most conspicuous structures, the protonephridial kidneys, are voluminous swellings composed of reabsorptive cells (“nephrocytes”). These cells exhibit strong vacuolization and an infolding system increasing the basal surface. The protonephridial kidneys, previously not reported at such a level of organization in molluscs, strikingly resemble (metanephridial) kidneys of adult molluscan excretory systems. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Investigations on the protonephridial system of post-larval Gyrocotyle urna and Amphilina foliacea (Cestoda).

The gross morphology and ultrastructure of the different parts of the protonephridial system of the monozoic tapeworms Gyrocotyle urna and Amphilina foliacea are described. The terminal cell in both species has numerous cilia which are interconnected and extend into the lumen of the first canal cell. The filtration area is built up from projections of two cells, the terminal cell and the first canal cell. The first canal cell forms a solid hollow cylinder without a cell gap and a desmosome as found in Neodermata other than cestodes and Udonella. In Gyroctyle the nucleus of the first canal cell is located in the wall cytoplasma whereas more distally located ductules of both species have subepithelial cell bodies containing the nuclei. In both taxa the protonephridial canal system is reticulate. In Amphilina the distal canals lack non-terminal ciliary flames, such ciliary tufts can be found in the larger capillaries of Gyrocotyle. The capillary cilia have rootlets and the ultrastructure of the duct wall cytoplasm containing large numbers of vesicles indicates highly active transport processes. The morphology of the protonephridial systems is discussed with regard to the evolution of Neodermata (especially of the Cestoda) and the function of the protonephridial system in cestodes as a probable organ of nutrient distribution.     

Ultrastructure of the Flame Bulbs and Protonephridial Capillaries of Microstomum sp. (Platyhelminthes,Macrostomida)

The terminal part of the protonephridia of Microstomum is formed by a branching proximal canal cell and (at least?) two terminal cells. Each weir consists of longitudinal (sometimes convoluted) ribs continuous with the cytoplasm of the terminal cell. Internal leptotriches arise from the terminal and proximal canal cells. Near the tip of the flame, the proximal canal cell tube is surrounded by the more external terminal cell and connected to it by a septate junction. Large cristate mitochondria are densely packed in the terminal and canal cells. The flame bulb of Microstomum differs markedly from that of other macrostomids (Macrostomum, Paramalostomum) examined. Phylogenetic implications are discussed.     

The Terminal Protonephridial Complex of Haplopharynx rostratus (Platyhelminthes,Haplopharyngida)

The terminal protonephridial complex of Haplopharynx rostratus consists of three terminal cells. There are no weirs consisting of ribs connected by a filtration “membrane”, but some cytoplasmic outgrowths into the lumen of the terminal cells. Excretion is by exocytotic vesicles. The terminal cells also contain Golgi complexes and large membrane-bound vacuoles containing electron-dense material. The ciliary bundles (flames) of terminal cells 2 and 3 protrude into the lumen of the centrally located terminal cell I. The complex is surrounded by a sheath containing numerous filaments. The terminal complex of H. rostratus resembles that of the macrostomid Paromalostomum proceracauda, lending support to the view that the two taxa are closely related. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

The fine structure of protonephridia in Gnathostomulida and their comparison within Bilateria

Summary The fine structure of the protonephridia of Haplognathia rosea (Filospermoidea) and Gnathostomula paradoxa (Bursovaginoidea) is described. Each protonephridium consists of three different cells: (1) a monociliated terminal cell which constitutes the filtration area, (2) a nonciliated canal cell showing a special protonephridial outlet system, and (3) an intraepidermal cell — the nephroporus cell — constituting the nephroporus. The protonephridia are arranged serially. There is no canal system connecting the protonephridial units.Protonephridial characters in other Bilateria are considered. The pattern of characters in the protonephridia in the last common gnathostomulid stem species and presumed apomorphies in the protonephridia of the Gnathostomulida investigated are discussed.Abbreviations used in figures ac acessory centriole - AC additional epidermal cell - bb basal body - bl basal lamina - bm bundle of microvilli - c cilium - cc cilium duct cell - cd cilium duct - cr ciliary rootlet - crs structures resembling ciliary rootlets - di diplosome - ds desmosome - dy dictyosome - f filtration area - g granules - m mitochondrium - mv microvillus - n nucleus - NC nephroporus cell - np nephroporus - oc outlet canal - TC terminal cell - tl tubules of lacunar system     

Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

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