Comparative ultrastructural studies of the epidermis, sperm and nerve fibres of Cleistogamia longicirrus and Seritia stichopi (RhabdocoeIa, UmagiIIidae)

The epidermis of Cleistogamia longicirrus consists of columnar, interdigitated cells with apical mitochondria, short microvilli and cilia which have a single horizontal rootlet anchored in the adjacent cytoplasm by transverse bars and a thin side branch. Cells are held together by desmosomes and intermediary junctions. Epidermal cilia have a terminal, electron dense rod between the central microtubules and doublet 1 and a terminal plate; doublets 1 and 6–9 lose one of their microtubules and gradually all doublets lose one microtubule and peripheral doublets disappear. Spermatozoa have a single closed peripheral row of microtubules, numerous electron dense granules and mitochondria and axonemes are of the 9 +"1" type and free for most of their length. Nerve fibres have microtubules and some nerve fibres are surrounded by lamellae and have invaginations of the fibre wall. The epidermis of Seritia stichopi resembles that of Cleistogamia , but cilia have a single horizontal rootlet anchored by transverse bars in the cytoplasm, without a side branch. Spermatozoa also are similar to those of Cleistogamia . Certain ultrastructural similarities between some Umagillidae and Neodermata are apparently due to convergent evolution and do not allow the conclusion that Umagillidae and Neodermata are particularly closely related.     

Ultrastructural study of the epidermis in two free-living Platyhelminthes, Mesostoma viaregginum and M. productum (Rhabdocoela, Typhloplanida)

Abstract. The epidermis of the free-living typhloplanids Mesostoma viaregginum and M. productum (Mesostominae) is described. In both species, the epidermis has polarized cells with nuclei located at the basal part of the cell, whereas mitochondria are in the apical one. The epidermis is entirely covered by microvilli and locomotory cilia anchored in the cytoplasm by vertical and horizontal rootlets. Rootlets exhibit distinct length and periodic structure in the two species. Furthermore, in each species vertical and horizontal rootlets possess different periodic structure. The pattern of termination of microtubules in epidermal cilia is described for the first time in the Typhloplanida; central microtubules shift along one axonemal side, doublets 1 and 6–9 lose their microtubule B, and gradually peripheral doublets become singlets. Finally, an electron-dense material caps the tip of the cilia. This pattern of termination closely resembles that of Temnocephalida, Kalytorhynchia, and Dalyelliida examined so far, but differences exist.     

Ultrastructure of the sense receptors of adult Multicotyle purvisi (Trematoda, Aspidogastrea)

The following presumptive sense receptors of adult Multicotyle purvisi from the intestine of freshwater turtles in Malaya are described by transmission electron microscopy: disc-like receptor with many electron-dense collars and modified ciliary rootlet forming a 'disc'; non-ciliate receptor with long rootlet; non-ciliate receptor with branching rootlet and dense mass of irregularly arranged microtubules; non-ciliate receptor with rootlet fanning out from basal body, cross-striated in its upper and with electron-dense structures in its lower part; uniciliate receptor with thick layer of cytoplasm around axoneme; receptor with short cilium, at base of deep invagination of tegument; receptor with short cilium terminating in an electron-dense apical cap; and uniciliate receptor with long cilium. In addition, there may be a small non-ciliate receptor with a long ciliary rootlet at the base of the thick dorsal tegument, and uniciliate receptors differing from the uniciliate receptor with long cilium in the number of electron-dense collars and the length of the cilium and ciliary rootlet. Implications of the findings for the phylogeny of the parasitic Platyhelminthes and for evolutionary trends within that group arc discussed. The considerable degree of divergence of receptor types between the species of one family is attributed to the archaic nature of the group.     

Light- and electron-microscopical study of Pelomyxa flava sp. n. (archamoebae, pelobiontida)

Using light and electron microscopy, the morphology of a new species of pelobionts Pelomyxa flava was studied. The coverings of P. flava are represented by plasma membrane bearing the thick layer of weakly structured glycocalyx on its outer surface. Numerous flagella are often located on the tops of short conical pseudopodia. Kinetosomes of flagella reach a length of 0.9 microm and are hollow with a pronounced central filament. Rootlet system is represented by three groups of microtubules: the radial, basal and microtubules of lateral root. The transition zone is short and does not exceed the level of cell surface; the axoneme is characterized by an unstable set of microtubules. Trophic stages of P. flava life cycle are presented by binuclear cells; plasmotomy is performed at the tetranuclear stage. Nuclei have a granular structure. Fibrillar nuclear bodies were revealed in karyoplasm. The nuclei shell has a complex organization. On its surface, the outer membrane has a layer of electron-dense material which contacts with short microtubules, located in a row at the surface of the nuclear envelope. The bubbles and cisterns of endoplasmic reticulum, which are the derivatives of the nuclear envelope, are located outward from the microtubules. The presence of structural and digestive vacuoles and grains of glycogen was noticed in P. flava endoplasm. Three types of prokaryotic cytobionts were revealed. Large multi-membranous organelles reaching 5 pm in diameter were described for the first time. We discuss morphology and biology features of P. flava in comparison with the previously studied Pelomyxa species.     

Ultrastructural studies of Austramphilina elongata (Cestoda,Amphilinidea)

Summary The fine structure of larval Austramphilina elongata is described using serial semithin and ultrathin sections. Densely packed germ cells with many ribosomes and mitochondria and with large Golgi complexes fill the middle third of the body. Some necrotic nuclei were observed near the anterior end. The neodermis consists of a subepidermal syncytium connected to pericarya in the parenchyma by means of cytoplasmic processes containing peripheral microtubules; electron-dense ovoid bodies condense in these processes. Myoblasts are connected to muscle fibres by means of cytoplasmic connections rich in mitochondria. Twelve (exceptionally eleven) type I gland cells containing large secretory granules and extensive granular endoplasmic reticulum are located in the dorso-posterior part of the body; they open through 12 (or 11) discrete ducts into an anterior invagination of the tegument which is covered by epidermis and not connected to the outside. Ten type II gland cells containing elongate secretory granules with regularly arranged longitudinal microtubules are located ventral to the type I cell bodies; they open on a ventral papilla a short distance behind the anterior end. Ten type III gland cells containing irregularly round-oval secretory granules with coiled microtubules are located anterior and ventral to the type I gland cells; they open through five discrete ventro-anterior openings on each side of the body. Ducts of all gland cells have mitochondria and microtubules. The spermatozoon has a basic pattern of two axonemes, each with a single central filament, a mitochondrion (mitochondria), and a row of surface microtubules interrupted by the axonemes. In the tips of epidermal cilia, doublet 1 and doublets adjacent to it lose their microtubules B first and close in on the central pair of filaments in a spiral fashion, enclosing an electron-dense rod. Presence of a neodermis and ultrastructure of the spermatozoon support the validity of the taxa Neodermata Ehlers and Trepaxonemata Ehlers and are strong evidence against a phylogenetic relationship of the cestodarians — cestodes with the Acoelomorpha; this is also indicated by the ultrastructure of sense receptors and epidermal ciliary rootlets.     

Two ciliate sense receptors in the larva of Austramphilina elongata Johnston, 1931 (Amphilinidea)

Summary The ultrastructure of a uniciliate and a quadruciliate receptor in the anterior end of the larva of Austramphilina elongata is described on the basis of serial sections. The uniciliate receptor has numerous branched and interconnected microvilli at its surface, several rings forming the electron dense collar, and cross-striated rootlets diverging from the basal body of the cilium. The quadruciliate receptor has four short club-shaped sensory cilia and a single electron-dense collar.Abbreviations used in figures ec electron-dense collar - ep epidermis - m microvilli - nt neurotubules - pe process of electron-dense collar - r rootlet of cilium - sc sensory cilium - sd septate desmosome     

Light and electron microscopic study of <Emphasis Type="Italic">Pelomyxa flava sp.n.</Emphasis> (archamoebae,pelobiontida)

The morphology of a new species of pelobionts Pelomyxa flava was studied by light and electron microscopy. The envelopes of P. flava are consist of a plasma membrane with a thick layer of weakly structured glycocalyx on its outer surface. Numerous flagella are often located at the apices of short conical pseudopodia. Kinetosomes of flagella reach length of 0.9 μm and are hollow with a pronounced central filament. The rootlet system is represented by three groups of microtubules: the radial, basal, and the microtubules of lateral root. The transitory zone is short and does not reach beyond the level of the cell surface; the axoneme is characterized by an unstable set of microtubules. Trophic stages of the P. flava life cycle are represented by binuclear cells; plasmotomy is performed at the tetranuclear stage. Nuclei have a granular structure. Fibrillar nuclear bodies are revealed in the karyoplasm. The nuclear envelope has a complex organization; on its surface, the outer membrane has a layer of electron-dense material that contacts with short microtubules located single-row at the surface of the nuclear envelope. Vesicles and cisterns of endoplasmic reticulum are located away from microtubules and are derivatives of the nuclear envelope. In the P. flava endoplasm, the presence of structural and digestive vacuoles and glycogen granules was found. Three types of prokaryotic cytobionts were revealed. Large multimembranous organelles reaching 5 μm in diameter were described for the first time. Peculiarities of the morphology and biology of P. flava compared to the previously studied Pelomyxa species are discussed.     

Light and electron microscopic study of <Emphasis Type="Italic">Pelomyxa stagnalis</Emphasis> sp. n. (Archamoebae,pelobiontida)

The structure of a new pelomyxa species was investigated on the level fo light and electron microscopy. The length of locomotive forms of Pelomyxa stagnalis reaches 800 μm. The thin layer of amorphous glycocalyx is located on the cell surface. Numerous nonfunctioning flagellae are revealed predominantly in the uroidal zone. The axoneme has a nonstable set of microtubules. No additional structures are present in the transition zone. The length of P. stagnalis flagella kinetosomes does not exceed 150 nm. Fifteen to twenty microtubules extend from the side surface of each kinetosome at a small angle to the cell surface. One of main components of the P. stagnalis cytoplasm are structural vacuoles. Glycogen bodies in cells are surrounded by flattened ER cisterns, which are often filled with electron-dense material. Cells of P. stagnalis were found to contain two species of prokaryote endobionts that differ in the peculiarities of their fine structure. The number of nuclei in cells of the P. stagnalis adult individuals can reach 50 or more. The nuclei are surrounded by a bilayer envelope formed by the multilaminar layer and by the outer layer composed of vesicles often filled with an electron-dense material. The nucleolus is usually single and is located in the center of the nucleus. In nuclei, predominantly in connection with nucleoli, bodies are formed that are formed by interlacing electron-dense strands.     

A comparative study on the fine structure of developing spermatozoa in the isopod,Oniscus asellus,and the amphipod,Orchestoidea sp.

Summary Developing spermatids and mature spermatozoa from the isopod, Oniscus asellus and the amphipod, Orchestoidea sp. have been examined with the light microscope and the electron microscope and have been found to have similar morphologies. As spermiogenesis proceeds the nucleus migrates to one pole of the spermatid at which point an acrosome, contiguous rod, and cross-striated tail develop. The acrosomal vesicle elongates to a cone-shaped, mature acrosome lying at the apex of a cross-striated tail and nucleus which are situated at approximate forty-five degrees to each other. The cross-striated tail originates as an evagination of the spermatid plasma membrane near the acrosomal vesicle. The tail eventually grows to lengths of four to five hundred microns. The mature, tail-like appendage is cross-striated at major 750 to 800 Å, and minor 125 to 150 Å, periodicities. When observed in vitro, mature sperm of both species appear non-motile.Possible homologies of this unusual spermatozoon with other types of spermatozoa are made and it is concluded that: 1) isopod and amphipod spermatozoa should be classified as non-flagellate; 2) the cross-striated tail, previously thought to be a flagellum, is a non-motile structure associated in development and possible function with the acrosome; and 3) the rodlike structure contiguous with the acrosome is similar to perforatoria described in some vertebrate sperm.Supported by U.S.P.H.S. Grant No. NB-06285 and Training Grant No. 5-Tl-GM-202. — The author wishes to express his grateful appreciation for the technical assistance given by Miss Ann Barnett during the course of this investigation.     

Sperm structure of Trichoptera. III. Hydropsychidae,polycentropodidae and philopotamidae (Annulipalpia)

Spermatozoa from 9 species belonging to 3 families of trichopteran suborder Annulipalpia (Philopotamidae : Philopotamus ludificatus, P. montanus, Wormaldia occipitalis, W. copiosa; Polycentropodidae: Plectrocnemia geniculata, Polycentropus mortoni, P. irroratus, Cyrnus trimaculatus; Hydropsychidae: Hydropsyche pellucidula) were studied by light, scanning, and transmission electron microscopy. The absence of axonemal dynein arms and a consequent sperm immotility are characteristic of the species studied. The presence of 7, rather than 2, central microtubules is shared by Polycentropodidae and some Philopotamidae. A greater variability in the sperm axoneme was found within Philopotamidae than in the other families. The species of the genus Wormaldia have an axial vesicle rather than microtubules and their axonemes have a 9 + 9 + 0 or 13 + 13 + 0 pattern. Hydropsychidae have spermatozoa provided with numerous finger-like appendages containing microtubular doublets. The progressive loss of sperm flagellum and motility within the group is discussed.     

Ultrastructure of spermiogenesis, spermatozoa and the tegument in Atriaster sp. (Platyhelminthes, Monogenea, Polyopisthocotylea, Microcotylidae)

Atriaster (Atriaster) sp., a gill parasite in the fish Diplodus cervinus caught near Dakar, Senegal, was studied by transmission electron microscopy. Spermiogenesis is similar to the general scheme in the Cercomeridae and particularly the polyopisthocotylean monogeneans, but shows two remarkable characteristics: (a) the zone of differentiation for each spermatid is separated from the common spermatid cytoplasmic mass by a region without longitudinal microtubules but with many mitochondria, herein termed the pre-zone of differentiation; (b) transverse sections of nuclei in late spermatids are polygonal with concave sides. These two characteristics are provisionally interpreted as autapomorphies, since observations in other species are lacking. The outer layer of the tegument is similar to that of other polyopisthocotylean monogeneans, and has microvilli and three kinds of cytoplasmic inclusions: granules with fibrous contents, granules with homogeneous contents, and elongate electron-dense bodies.     

Ultrastructure of sperm and spermatogenesis of Lobatostoma manteri (Trematoda, Aspidogastrea)

Mature sperm has two axonemes of the 9 + '1' pattern incorporated in the sperm body, a row of peripheral microtubules interrupted along part of the sperm by the axonemes, some microtubules in the interior of the sperm and a long lateral extension (lobe) of the sperm body, an elongate nucleus and mitochondrion, and many dense rod-like structures. A supporting rod extends underneath a specialized region consisting of alternating thin and thick transverse rows of irregular dense patches, and with surface ridges around (all or) most of the surface of the sperm. Primary spermatocytes in the prophase of the first meiotic division have synaptonemal complex(es), and are rich in mitochondria. In early spermiogenesis, mitochondria are arranged around the surface of the nucleus, a dense layer appears at one pole of the nucleus, close to an apposed dense layer at the cell membrane in which a row of microtubules develops. The intercentriolar (= central) body develops close to the nucleus. The fully developed intercentriolar body has a regular striation and is located perpendicular and close to the surface of the nucleus. Two flagella extend into the space surrounding the outgoing median process, their basal bodies are located perpendicular to the intercentriolar body and their cross-striated rootlets extend along the surface of the rounded nucleus. At a later stage, rootlets and flagella become more parallel with the intercentriolar body, the nucleus and the fused mitochondria migrate into the median process, and the flagella become incorporated into the median process (= sperm body). The outgrowing spermatozoa are connected to the cytoplasm of the cytophore by dense arching membranes. Finally, rootlets of flagella are resorbed and the spermatozoa are pinched off close to the basal bodies. Two species (Lobatostoma and Multicotyle) of the same family differ strongly in the type of spermiogenesis, although their mature sperm is of the same basic type, i.e. spermiogenesis is not necessarily more useful for phylogenetic considerations than sperm structure.     

Sperm ultrastructure in the inseminating Macropsobrycon uruguayanae (Teleostei: Characidae: Cheirodontinae)

Macropsobrycon uruguayanae is a small, inseminating characid (tetra) of the tribe Compsurini. Although spermatozoa can be found within the ovarian cavity close to oocytes, the exact moment of fertilization has not yet been determined. Spermatozoa have moderately elongate nuclei with electron-dense chromatin. During spermiogenesis, nuclear rotation takes place. Elongate mitochondria with lamellar cristae are found posterior to the nucleus. Centrioles are parallel to one another with the proximal centriole slightly anterior to the longer distal one. The anterior tip of the proximal centriole is located within a shallow nuclear fossa. Electron-dense spurs are associated within the anterior and posterior ends of the distal centriole. Striated centriolar rootlets radiate both anteriorly and posteriorly from the distal centriole. Nine longitudinal accessory microtubules surround the axoneme in the proximal flagellum. The flagellum has a typical 9 + 2 axoneme with no intratubular differentiation. Atypical spermatozoa are also found in the testicular lumen. These cells resemble spermatozoa in most aspects, except that their nuclei are variable in shape, with the granular chromatin less electron-dense than that seen in spermatozoa. The origin and function of these cells could not be determined. The specializations seen in the spermatozoa are discussed as possible adaptations related to the habit of insemination.     

Ultrastructure of the spermatozoon of Anomotaenia quelea (Mettrick, 1961) (Cestoda,Cyclophyllidea, Dilepididae), an intestinal parasite of Quelea quelea (Aves,Ploceidae) in Senegal

The mature spermatozoon of Anomotaenia quelea exhibits an apical cone of electron-dense material and two helicoidal crest-like bodies. The apical cone near its base is surrounded by a lucent cytoplasm and a spiraled layer of cortical microtubules. The crest-like bodies are of different lengths, spiraled and make an angle of 30–40° to the hypothetical spermatozoon axis. The axoneme is of the 9 + ‘1’ trepaxonematan pattern and is surrounded by a periaxonemal sheath of electron-dense material. The cytoplasm contains in regions III and IV numerous electron-dense granules situated between the periaxonemal sheath and the cortical microtubules. The posterior extremity of the spermatozoon of A. quelea exhibits a nucleus and a disorganized axoneme and cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in a Dilepididae. Similarly, two crest-like bodies have not been observed before in a dilepidid cestode.     

Ultrastructural studies of the commensal suctorian,Choanophrya infundibulifera hartog

Summary The feeding tentacles of Choanophrya contain a central canal lined by microtubules. Only one tentacle develops during metamorphosis of the embryo into the adult, but others develop at intervals throughout adult life. Each tentacle forms adjacent to a solitary, subcortical kinetosome which lies parallel to the body surface, lacks accessory elements and never develops a cilium. Small condensations of electron-dense material and short bundles of microtubules form adjacent to the cartwheel region of the kinetosome. Initially these bundles are orientated randomly but later they become radially arranged and curved into prolamellae around a disc-shaped condensation centre, to form a paddlewheel-like tentacle primordium 0.8–1.1 m in diameter. The condensation centre consists of alternating concentric electron-dense and electron-transparent zones, and lies with its axis perpendicular to both the kinetosome and the cortex. The microtubules in each prolamella increase in number and pairs of short tip microtubules develop between adjacent prolamellae. Subsequently the developing lamellae become enclosed by a cylinder of ring microtubules. Once all the microtubule components of the tentacle primordium are established it increases in length by addition of material to the basal ends of the microtubules to form a short microtubule canal. As the canal elongates the epiplasm above it disappears and the pellicle membranes become uplifted around the protruding tentacle. An epiplasmic collar differentiates around the growing tentacle whilst spheroid vesicles and solenocysts begin to accumulate in the surrounding cytoplasm.This investigation was supported by the J.S. Dunkerley Fellowship in Protozoology, awarded by the University of Manchester.     

Ultrastructural study of the male gamete of Glossobothrium sp. (Cestoda: Bothriocephalidea: Triaenophoridae) a parasite of Schedophilus velaini (Perciformes: Centrolophidae) in Senegal

This paper describes the ultrastructure of the male gamete of Glossobothrium sp. (Bothriocephalidea: Triaenophoridae). The mature spermatozoon of Glossobothrium sp. is filiform and possesses two axonemes, a single helicoidal crested body, a parallel nucleus, parallel cortical microtubules and granules of glycogen. In Glossobothrium sp. we describe for first time a 200-250 nm thick crest-like body in the Bothriocephalidean. The anterior part of the spermatozoon exhibits a ring of 27 electron-dense cortical microtubules encircling the first axoneme. This structure persists until the appearance of the second axoneme. When the ring of electron-dense cortical microtubules disappears, the spermatozoon exhibits two bundles of thin cortical microtubules. The posterior part of the spermatozoon contains the posterior extremity of the second axoneme, the posterior extremity of the nucleus and few cortical microtubules. Soon nucleus disappears and the axoneme is disorganized. Thus the posterior extremity of the spermatozoon of Glossobothrium sp. exhibits only singlets produced by the disorganization of the doublets of the second axoneme and few cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in the Triaenophoridae.     

Ultrastructure of the spermatozoon of Anomotaenia quelea (Mettrick, 1961) (Cestoda,Cyclophyllidea, Dilepididae), an intestinal parasite of Quelea quelea (Aves,Ploceidae) in Senegal

The mature spermatozoon of Anomotaenia quelea exhibits an apical cone of electron-dense material and two helicoidal crest-like bodies. The apical cone near its base is surrounded by a lucent cytoplasm and a spiraled layer of cortical microtubules. The crest-like bodies are of different lengths, spiraled and make an angle of 30–40° to the hypothetical spermatozoon axis. The axoneme is of the 9 + ‘1’ trepaxonematan pattern and is surrounded by a periaxonemal sheath of electron-dense material. The cytoplasm contains in regions III and IV numerous electron-dense granules situated between the periaxonemal sheath and the cortical microtubules. The posterior extremity of the spermatozoon of A. quelea exhibits a nucleus and a disorganized axoneme and cortical microtubules. This type of posterior extremity of the mature spermatozoon has never been described previously in a Dilepididae. Similarly, two crest-like bodies have not been observed before in a dilepidid cestode.     

The formation of glandular secretion in larval Austramphilina elongata (Amphilinidea)

The ultrastructure of three types of gland cells of embryos and free-swimming larvae of Austramphilina elongata is described. Type I gland cells contain large, more or less round electron-dense granules which are formed by numerous Golgi complexes. Type II gland cells contain thread-like, membrane-bound secretory granules with longitudinally arranged microtubules inside the granules; secretory droplets are produced by Golgi complexes and the microtubules apparently condense in the cytoplasm or in the droplets. Type III gland cells contain irregular-ovoid membrane-bound granules with coiled up microtubules which have an electron-dense core; the granules are formed by secretionderived from Golgi complexes and the microtubules aggregate around and migrate into the secretion; microtubules are at first hollow and the early secretory granules have a central electron-dense region.     

Structure, Cytochemistry, and Locomotion of Haemogregarina sp. from Rana berlandieri

Intra- and extracellular gametocytes of Haemogregarina sp. from Rana berlandieri were studied by light and electron microscopy. Locomotion in free gametocytes appears to be related to series of horizontal “peristaltic” waves of constriction, passing from anterior to posterior along the body. Intracellular gametocytes lie within a vacuole in the erythrocyte cytoplasm. The pellicle of the parasite consists of a trilaminar plasmalemma and an inner electron dense layer, beneath which lies a ring of 80 microtubules. The inner dense layer becomes thickened and modified in the apical region, to form a cap-like structure. The gametocytes contain a prominent nucleus, several mitochondria, and many granular inclusions. One type of inclusion consists of elliptical, electron-dense, profeinaceous bodies scattered throughout the cytoplasm, while other inclusions are larger and electron-opaque, polysaccharide in nature, and occur predominantly in the pre- and post-nuclear regions. In the electron microscope, pronounced pellicular folds were observed in longitudinally sectioned extracellular gametocytes. These folds are thought to represent the waves of constriction seen in motile specimens by light microscopy. The mechanism of movement of the parasite is discussed and compared with that in haemosporidian ookinetes, as well as in gregarines.     

Sensory cells in the sucker of Craspedella pedum (Plathelminthes, Temnocephalida): in vivo staining with DiO and SEM and TEM observations

 Two pairs of identified sensory neurons innervating the sucker of Craspedella pedum (designated ADS1, ADS2) were found to accumulate DiO (3,3′-dioctadecyloxacarbocyanine perchlorate) in vivo. The number, position and morphology of these neurons do not change throughout the postembryonic period of life. The axons of the ADS cells run forward within the ventral cords and their dendrites are parallel. They enter the sucker, ramify and terminate in numerous sensory endings in a wide peripheral zone of the disc. SEM reveals a single type of sensilla: small spot-like structures with several short cilia. They are scattered within the zone accommodating the openings of the adhesive glands and their distribution corresponds to that of the stained terminals. TEM observations (including about 20 full reconstructions from serial ultrathin sections) show five types of sensory endings on the disc with the following structures: (1) a short cilium and thin rootlet, (2) aciliary with a normal rootlet and a club-shaped apical portion, (3) aciliary with a club-shaped apical portion and a body similar to the apical part of the rootlet, (4) aciliary with large apical granule and (5) aciliary with small apical granules. Type 2–5 receptors form a morphological series suggesting that they are stages of formation of the common type 4 receptor. Not fully formed type 1 receptors have been found within the epidermis in adult animals. This suggests that, although ADS perikarya persist throughout the life of the animal, the nerve endings they form might be constantly renewed. Judging from the morphological and behavioural data, the functions of the ADS neurons might include: (1) monitoring of the close contact between the surface of the sucker and the substratum prior to adhesion and (2) checking the viscosity of the adhesive secretion prior to release of the sucker. Accepted: 16 December 1997