Showing papers in "Parasitology in 1969"

Sense organs of monogenean skin parasites ending in a typical cilium.

Abstract: Single receptors seen with the electron microscope to consist of a terminal cilium embedded in a nerve bulb have been found in the skin parasitic monogenean Gyrodactylus sp., adult and larval Entobdella soleae, Leptocotyle minor (adult) and in the endoparasitic juvenile of Amphibdella flaviolineata. Their distribution in Gyrodactylus sp. has been mapped using phase-contrast microscopy, and staining with the indoxyl acetate method for non-specific esterases revealed a nervous connexion between the lateral ventral nerve cord and the ‘tangoreceptor’. The general relationships of the nervous system in Gyrodactylus were investigated by staining with the thiocholine method for cholinesterase. The ‘sensory’ neurone contains vesicles, microtubules and mitochondria and the whole nerve bulb is sealed into the epidermis by means of septate desmosomes. The basal body of the cilium is not greatly modified and there is no obvious rootlet system. The terminal cilium of single receptors in Entobdella soleae has a 9 + 2 structure. A compound sense organ from the head of Entobdella soleae has also been described. The significance of the fibre arrangement in the terminal cilia and the possible roles of these sense organs have been discussed. I should like to thank Dr D. L. Lee for training in electron microscope techniques and for his encouragement, Dr P. Tate for his continued interest and the staff of the Marine Biological Laboratory, Plymouth, especially Mr G. Best and Mr J. E. Green, for their assistance. This work was conducted during tenure of a Science Research Council Fellowship and the Tucker-Price Research Fellowship of Girton College.

Observations on the fine structural changes associated with schizogony and gametogony in Eimeria tenella.

Abstract: Changes associated with schizogony and gametogony in Eimeria tenella have been studied by means of electron microscopy.In infections of E. tenella both small and large schizonts are formed, and in both cases merozoites are developed by a surface ‘budding’ mechanism. The final severance of the merozoite from the schizont appears to give rise to the dorsal pore of the merozoite. Newly formed merozoites contain structures which are absent in the mature organism.Early in microgametogony the nucleus of the microgametocyte divides repeatedly to produce a number of smaller nuclei which migrate peripherally in the cell. The chromatin aggregates into the peripheral part of each nucleus, which then elongates to form the nucleus of the of microgamete. That part of the gametocyte nucleus which is devoid of chromatin, probably gives rise to the perforatorium and three flagella of the gamete. Residual material is left in the gametocyte cytoplasm in the form of a gametogenic cyst.‘Wall-forming’ bodies are present in the earliest macrogametocyte and ‘dark bodies’ are developed later. After fertilization these bodies migrate to the periphery of the zygote. In E. tenella the oocyst wall is trilaminate. The limiting membrane of the zygote separates from the cell to become the outermost membrane of the cyst wall. The middle layer of the cyst wall is developed from the ‘dark bodies’, and the ‘wall forming’ bodies give rise to the inner layer of the oocyst wall.

Seasonal incidence and development of the cestode Caryophyllaeus laticeps (Pallas) in the River Avon.

Abstract: An account is given of the seasonal occurrence, maturation and population changes of Caryophyllaeus laticeps in its intermediate host Psammoryctes barbatus and in its definitive host Leuciscus leuciscus in the River Avon. Infective larvae were found in the tubificid in all months except August, but occurrence and maturation of C. laticeps in fish showed a clear seasonal periodicity. Infections were acquired between December and March, the parasite matured in April and May and disappeared by July. Maturation of C. laticeps appeared to be governed by changes in fish hormone levels. Changes in the parasite population in fish bore no relationship to the availability of infective larvae nor probably to the feeding behaviour of fish but were closely correlated with water temperature changes. The most important factor controlling the dynamics of the C. laticeps–dace system appears to be a temperature dependent rejection response to the parasites by dace, and it is suggested that a similar mechanism may operate to control the dynamics of other freshwater fish–tapeworm systems.I should like to thank the Avon and Dorset River Authority, and in particular Mr A. S. Grater, for their help in capturing the fish and for their ready co-operation.

Acquired immunity to Babesia microti and Babesia rodhaini in mice.

Abstract: Mice infected with Babesia rodhaini can be cured with a single injection of Diampron and are immune to challenge with the same species. Mice infected with B. microti recover naturally from their infections and are also immune to challenge. The immunity extends to the heterologous species in both infections. In some cases parasites persist at a low level after recovery but in others the immunity is of a true sterile type. Immunity once established is unaffected by splenectomy in mice which have recovered from B. rodhaini and challenged with either B. rodhaini or B. microti. We wish to express our thanks to May and Baker Ltd. who kindly provided us with the Diampron used in these experiments.

Immunological variation between two strains of Eimeria acervulina.

Abstract: Two strains of Eimeria acervulina were compared. The more recently isolated strain had a higher reproductive potential and produced slightly longer oocysts. Otherwise both strains conformed to the characteristics of the species.The two strains were not immunologically identical and birds which were solidly immune to one strain would support a degree of infection with the other.Acknowledgements are due to Mr C. C. Norton for technical assistance and to Dr Helen Hein who provided access to experimental data prior to publication and discussed some aspects of the results. The statistical calculations were carried out by Miss N. Hebert.

Compound sensilla in monogenean skin parasites.

Abstract: The fine structure of two kinds of compound presumed sense organs from the heads of three skin parasitic monogeneans Gyrodactylus sp. Entobdella soleae (larva only) and Acanthocotyle elegans is described. One kind of compound receptor consists of a number of associated sensilla, each ending in a single cilium (the spike sensilla of Gyrodactylus and the cone sensilla of E. soleae oncomiracidium).The other kind of compound organ is made up of one or a few neurones only, each of which bears many cilia (pit organs of E. soleae oncomiracidium and feeding organ sensilla of Acanthocotyle elegans). The spike sensilla of Gyrodactylus have also been studied using a Cambridge Instrument Co. Stereoscan electron microscope and by phase-contrast microscopy. The ciliary endings of all these sense organs are highly modified and have lost the 9 + 2 structure, being packed with many fibres. The fibre arrangement in the cilia of the cone sensillae of E. soleae oncomiracidium and the feeding organ sensilla of A. elegans has been compared with that in the ciliary endings of other invertebrate mechano- and chemoreceptors. The possibility that the spike sensilla of Gyrodactylus may be chemoreceptors has been discussed but it is considered premature to attempt to assign functions to the other sense organs studied. Electron dense membrane-bound inclusions occurring specifically in the nerves supplying the spike sensilla of Gyrodactylus may be neurosecretory.

The morphology, formation and development of cysts of Entamoeba.

Abstract: Parasitic amoebae of the genus Entamoeba are transmitted from one host to another as cysts. The active amoeboid forms are so adapted to life within their particular hosts that, outside the host, they soon die, but encysted stages survive conditions fatal to the unprotected amoebae. The cyst is an integral and critical phase in the life-cycle of Entamoeba; its attributes directly influence the dissemination of the organism, and are an important aspect in the epidemiology of amoebiasis. The following account of the morphology, formation and development of the cysts of Entamoeba is an assessment of present knowledge, and is not a comprehensive review of all available observations.

The fine-structure of the epimastigote forms of Trypanosoma lewisi in the rectum of the flea, Nosopsyllus fasciatus.

Abstract: The fine-structure of the epimastigote forms of Trypanosoma lewisi in the rectum of the flea, Nosopsyllus fasciatus is described.The parasites are attached to the rectal wall by penetration of the flagella between the highly folded wall of the rectum, by the wedging of the expanded flagella in these folds and irregularities of the wall and by a zonula adherens functional complex between the flagellar membrane and the cuticle of the rectum.The parasites remain associated with each other by their flagella. The daughter axonemes produced during division are retained within the parent flagellar membrane; this delayed division of the flagella results in up to four axonemes being found within a single flagellar membrane. The parasites accumulate together in masses in the lumen of the rectum and their surfaces interdigitate resulting in the mass of parasites being retained together.The financial assistance of the Agricultural Research Council is gratefully acknowledged. My sincere thanks are due also to my supervisor Dr Tate for his help, encouragement and enthusiasm through the course of this work.

Lipid fractions in adult Schistosoma mansoni.

Abstract: Total lipid from adult male and adult paired Schistosoma mansoni has been fractionated by thin-layer chromatography. Phospholipid, free sterol, and triglyceride were major components of both mixtures. A remarkably small amount of sterol ester was found in adult flukes considering the prevalence of this compound in mouse blood. Free fatty acids were found to be a minor component. Cholesterol was the major free sterol present.The technical assistance of Mrs Katye Ross Summerlin is gratefully acknowledged.

Life-cycles of helminth parasites using Gammarus lacustris as an intermediate host in a Canadian lake.

Abstract: The systematics and life-cycles of the helminths of a local population of Gammarus lacustris in a eutrophic lake near Edmonton, Alberta, is reported as part of a larger study of the composition and seasonal dynamics of the helminth fauna of gammarids.A total of 12 species of helminths, including eight cestodes, one nematode and three acanthocephalans, were recovered. Of these, eleven were new host records, ten were assigned to an intermediate host species for the first time, and one, Hymenolepis albertensis sp.nov., was described for the first time.Adults of all twelve helminths were raised in experimentally infested birds and the life-cycles of five species (Lateriporus clerci, L. skrjabini, Hymenolepis albertensis sp.nov., Fimbriaria fasciolaris and Polymorphus marilis) were completed in the laboratory. The larvae are described, and the developmental period in the gammarids, prepatent period and life span of the adults are given for many of the helminths.The rate of development of the cysticercoids of Lateriporus skrjabini was shown to be directly related to the size of the gammarid and inversely related to the intensity of infestation.The proboscis-hook formula was not a good diagnostic character for the separation of the three acanthocephalans, Polymorphus contortus, P. marilis and P. paradoxus; however, the size of the largest hook and the structure of the cystacanth body-wall were good diagnostic characters.I am indebted to Dr J. C. Holmes for advice and encouragement at all stages of the study. I also wish to thank Drs S. Prudhoe and D. R. R. Burt for their editorial assistance, Mr L. Graham for many helpful suggestions and information on the natural definitive hosts of the species encountered, Mr R. Podesta for his laboratory assistance, and Miss E. D. Senio for caring for the ducklings during their first few days of life. The study was supported by the Francis F. Reeve Foundation Graduate Bursary, the Queen Elizabeth Education Scholarship Fund, by the Department of Zoology through a Teaching Assistantship, by a grant from the R. B. Miller Biological Station Fund and by an N.R.C. operating grant (A–1464) to Dr J. C. Holmes.

Host specificity and rejection of Schistocephalus solidus .

Abstract: 1. Procercoids of Schistocephalus solidus reared in copepods were found to infect marine and freshwater varieties of Gasterosteus aculeatus, and Pungitius pungitius, but not Barbus ‘Schuberti’.2. In primary infections growth of S. solidus plerocercoids in P. pungitius was much slower than in G. aculeatus and ceased after 6–8 days; plerocercoids were dead by day 11 at 19·5 °C, and by day 14 at 10 °C.3. A challenge infection, following two previous infections given 14 and 28 days before, becomes established in G. aculeatus, but in P. pungitius plerocercoids are rejected within 3–5 days.4. The possible rejection mechanism involved in P. pungitius to primary and tertiary infections is discussed and electronmicrographic evidence presented of a degenerative change in the tegument visible by 8 days in a primary infection. Prior to 8 days the survival and growth of S. solidus plerocercoids surgically transferred to G. aculeatus, indicated no irreversible damage had occurred.

The regulation of egg output by Ostertagia ostertagi in calves infected once only

Abstract: Analysis of the egg counts of calves infected with one dose of larvae showed that they followed exactly the same course as those of calves infected daily, rising to a peak 25–30 days after infection and then decreasing logarithmically.An experiment in which calves were infected on a single occasion with different numbers of worms showed that faecal egg counts were not a reflection of worm numbers but were limited by a separate mechanism. Evidence is discussed which suggests that the number of eggs present in a female worm is not an entirely satisfactory measure of its rate of ovulation.

The effect of cortisone on the worm burdens of calves infected daily with Ostertagia ostertagi.

Abstract: Of two groups of calves, one was regularly treated with the cortisone-derivative, β-methazone. This treatment greatly reduced the ability of the calves to produce antibody and gave rise to a number of side effects. Calves of both groups were given 1500 infective larvae of Ostertagia ostertagi daily. A milder regime of cortisone administrations using prednisolone was employed in a second experiment in which two comparisons were made. Worm burdens of calves infected with 1500 larvae daily and receiving cortisone were compared with those of control calves infected at the same rate and cortisone-treated calves receiving 1500 larvae per day were compared with cortisone-treated calves infected at twice this rate. By periodic post-mortem examination it was shown that even in cortisone-treated animals the worm burden was regulated at a level only slightly higher than in control calves, apparently by a density-dependant loss of worms. A more rapid loss of worms which occurred for a period in the control calves was prevented in cortisone-treated animals unless the infection rate was high. The limit set by the normal host to the total egg output of its worm population was raised by cortisone treatment, very greatly in the first experiment, rather less in the second in which the cortisone treatment was less severe. The effect of host resistance on the length of worms and on the incidence of females without vulval flaps was greatly reduced by cortisone. The inhibition of development was not affected.

Host-parasite interactions and the feeding of blood-sucking arthropods.

Abstract: The range of interactions between blood-sucking arthropods and their hosts is enormous and their importance to man would be difficult to overestimate. From the tundra to the tropics man, and animals in which he has a vital nutritional or economic interest, represent to the blood-sucking arthropod a source of animal protein. By their activities in obtaining a blood meal the host may be subjected to minimal inconvenience, acute irritation, or suffer severely from blood loss or from the effects of toxins introduced during feeding. Superimposed upon these situations is an enormous variety of disease agents which may be transmitted to the host by the parasite during or after feeding.

Ultrastructural studies on the sporozoite of Eimeria tenella

Abstract: The structure of the sporozoite of Eimeria tenella has been studied with the aid of the electron microscope, using both sectioned fixed material and negatively stained or shadowed whole cells. An apical ring gives rise to a series of 24 fibrils which extend posteriorly under a double bilamellar membrane. Within the apical ring lies a conoid with spirally arranged tubules and an apical pore; under some conditions at least this conoid can be pushed forward beyond the apical ring. Originating within the conoid are a group of club-shaped organelles which extend backwards into the cell to lie among an extensive regular array of micronemes. The nucleus lies between two apparently structureless paranuclear bodies, and scattered throughout the remaining cytoplasm are mitochondria with tubular cristae, oval structures containing amylopectin, occasional lipid droplets, and vesicles containing nondescript material possibly undergoing digestion and which may have a possible connexion with a lateral micropyle. I am indebted to Mrs Muriel Bentley for much technical collaboration and to Mrs McLaren for making her manuscript available before publication.

Skin penetration by cercariae of the bird schistosome Austrobilharzia terrigalensis: the stimulatory effect of cholesterol.

Abstract: An artificial skin of hardened gelatine was used to examine the factors affecting penetration of skin by cercariae of the bird schistosome, Austrobilharzia terrigalensis.The percentage of cercariae able to penetrate through a gelatine membrane was increased by a factor of 3–4 by coating the membrane with a thin layer of lipid collected from the surface of chicken skin.The free sterol fraction, isolated from chicken skin surface lipid by thin-layer chromatography, stimulated penetration to the same extent as whole skin lipid.Cholesterol was detected in the sterol fraction by mass spectrometry and pure cholesterol had the full stimulating effect on cercarial penetration.Skin lipid, from which free sterols had been removed, lost the stimulatory effect on cercariae, but full activity was recovered by adding cholesterol to the sterol-free lipid. Fractions of skin lipid containing free fatty acids or triglycerides, wax esters and sterol esters similarly failed to stimulate penetration.These results establish that penetration of A. terrigalensis cercariae is greatly stimulated by the free sterols present in the surface lipid of chicken skin but cholesterol may not be the only active sterol. Cholestanol and the plant sterols campesterol and β-sitosterol were also detected in chicken skin surface lipid. These sterols were not tested for activity on cercariae because samples free from cholesterol could not be obtained.Some cercariae were able to penetrate plain gelatine membranes not coated with cholesterol but small amounts of free sterol were detected in the gelatine itself. This sterol could not be completely removed by prolonged solvent extraction and consequently it is not known whether any cercariae are able to penetrate in the complete absence of sterols.Temperature had a marked effect on penetration of cercariae; lowering the temperature from 40 to 25 °C reduced the number of successful penetrants by a factor of 4.I am grateful to Miss V. Bowen for excellent technical assistance and to Dr J. McLeod of the Research School of Chemistry, Australian National University, who performed and interpreted the mass spectrometry. This investigation was partly supported by a research grant from the World Health Organization.

Parasites as biological models.

Abstract: The value of metazoan parasites as models for the investigation of fundamental biological phenomena is discussed. The fields in which they are of particular value include: immunology, neuromuscular physiology, nucleo-cytoplasmic relationships, genetics, cyto-differentiation, comparative biochemistry.After reviewing some examples of experimental organisms in trematodes and nematodes, the cestode, Echinococcus granulosus, in particular, is examined as a model.Problems of particular interest in this organism are: the function of the rostellar gland, whose cells release a secretion which has its origin in the nuclei of these cells; the mode of action of the suckers, which in vivo are thought to remain in a state of sustained contraction; the mode of digestion at the host–parasite interface—membrane digestion appears to be involved; the various modes of membrane transport through the tegument; factors controlling cystic or strobilar differentiation, a problem viewed in the light of the Jacob–Monad model of gene action.A hypothetical control circuit for differentiation of the stages in parasite life-cycles in general is also put forward.Some preliminary experiments on the in vitro culture of Taenia serialis, are described. The factors controlling differentiation into cystic or strobilar directions appear to be generally similar to those which operate for Echinococcus granulosus.It was stressed that many other species of parasites could also serve as models for investigating similar or related problems.Some of the studies described here were supported by grants from the Australian Wool Board, the U.S. Public Health Service (Grant no. A 107452–02) and the Rural Credits Development Fund.I am grateful to Mr D. Heath for the provision of the micrographs of Taenia pisiformis and T. serialis in P1. 2, figs. 2, 3.

The migration of Hymenolepis diminuta in the rat's intestine during normal development and following surgical transplantation.

Abstract: The normal position of attachment of the scolex of H. diminuta from day 6 to day 27 in single worm infections in male Wistar rats was determined. A migration forward from a position of attachment 30–40% down the length of the small intestine to the 10–20% region during day 7 to day 14 was confirmed. A concomitant and slightly faster movement backwards of the posterior of the worm led to the mid-point of the strobila changing from 35 to 50% down the intestine. By day 18 the worms were mature and occupied the whole small intestine except the anterior 10% and the posterior 5–10%. The worms weighed approximately 350 mg dry weight (= 1550 mg fresh weight) at maturity, no significant weight increase occurred thereafter. 6½ and 7½ day-old worms were transplanted from donor rats into the duodenum or posterior ileum. Nearly 100% recovery of the single worm transplants in recipient rats was obtained. Both anterior and posterior implanted worms returned to the region of the intestine from which they had been removed. Migration was measured over periods of 13–72 h and was found to be complete within 24 h. Worms transplanted into the duodenum continued to grow at nearly the normal rate but worm transplants into the posterior ileum lost nearly a day's growth. The significance of this point is discussed. Opium was found to prevent migration of the tapeworm, but ligation of the bile duct did not prevent the tapeworm migrating forward from a point of insertion near the ileo-caecal junction. The evidence confirms the concept that a tapeworm is a dynamic organism capable of recognizing a specific region of the intestine, of detecting when it is not in this region, of being able to interpret one or more stimuli to indicate direction and of having a motor system sufficiently coordinated to overcome peristalsis. We gratefully acknowledge financial support by the Medical Research Council (London) and technical assistance from Miss Helen Stallard, B.Sc., Miss Pat Grant and Miss Gillian Moore. One of us (Trond Braten) wishes also to thank the University of Glasgow for the award of a Visiting Fellowship.

Studies on the host-parasite interface of strigeoid trematodes. IV. The ultrastructure of the lappets of Apatemon gracilis minor Yamaguti, 1933.

Abstract: The ultrastructure of the lappets of Apatemon gracilis minor Yamaguti, 1933, has been described The tegument covering the lappets is characterized by the presence of long seta-like structures and unicellular gland cells which exhibit a holocrine type of secretion The setae contain extensions of the tegument cytoplasm and are supported by a bundle of fibrils The unicellular gland cells are pear-shaped with the tapering neck supported by a ring of 50–60 microtubules The secretion consists of relatively large, membrane bounded bodies containing a fine granular material The lappets are regarded as representing a specialized host–parasite interface in which adaptation exists for attachment to the host tissue and the discharge of secretion from the parasite at this region The concept of the specialized host–parasite interface is discussed and extended to include Apatemon gracilis minor The author wishes to acknowledge the research grant provided by the SRC for the purchase of a vacuum coating unit and an AEI EM 6 electron microscope The progress of this study was greatly facilitated by the excellent assistance of Mr T Davies and Miss C Green The stereoscan micrograph (P1 1, fig 2) is published by permission of the Cambridge Instrument Company

Nematode parasites of Oceanica: V. Four new species from fishes of Palawan, P.I., with a proposal for Oceanicucullanus Gen.nov.*

Abstract: Camallanus marinus sp.nov. is described from eleven specimens recovered from a variety of marine fishes. It is the only species in Camallanus s.s. with a single spicule. Spinitectus palawanensis sp.nov. is described from two males and one female found in a scombrid fish. It is characterized by having 12–17 rings of 60–65 spines, a spicule ratio of 5:1, and ten pairs of caudal papillae. Cucullanus lutjani, sp.nov. is described from two specimens from lutjanid fishes. It is distinct in lacking a preanal sucker, in possessing a gubernaculum and eight pairs of cloacal papillae.Oceanicucullanus padfica gen. et sp.nov. is described from two specimens found in cyprinid fish. It is most similar to Indocucullanus, differing from it in the absence of a gubernaculum, and in possessing three pairs of teeth in the radii of the oesophagus. Philometra (Philometra) lateolabracis Yamaguti, 1935; Terranova sp.; and Procamallanus spp. are reported for the first time in the Philippines.We wish to thank Dr D. S. Rabor, Silliman University, Damageute, Negros Oriental, Republic of the Philippines, for general support in field activities, and Dr Loren Wood, Curator of Fishes, and Dr Robert F. Inger, Curator of Reptiles, Chicago Museum of Natural History, for identification of hosts. Acknowledgement is also made to Dr Betty June Myers for technical assistance and advice in handling materials.

The structure and development of Histomonas meleagridis (Mastigamoebidae: Protozoa) in the female reproductive tract of its intermediate host, Heterakis gallinarum (Nematoda).

Abstract: The ultrastrcuture and development of the protozoan Histomonas meleagridis in the reproductive system of the female nematode, Heterakis gallinarum, have been described. It has been shown that there is a distinct cycle of events in the reproductive system. The stage of Histomonas in the germinal zone of the ovary is extracellular; the protozoan feeds and multiplies here. The histomonads move down the ovary with the oogonia and become intracellular when they penetrate the developing oocytes in the growth zone of the ovary. This is apparently an active penetration of the oocyte by the parasite. Feeding and division of the histomonad occurs in the oocyte and also in the newly formed egg; apparently a gradual reduction in size occurs along the reproductive system. These stages in the parasite are similar to the tissue-inhabiting stages in the bird but are much smaller. There are no mitochondria present.There is strong circumstantial evidence that the nematode is able to repair damage to the wall of the reproductive system and to the oolemma caused by the protozoan.I wish to thank Dr E. E. Lund of Beltsville Parasitological Laboratory, Maryland, U.S.A., who very kindly sent me some of his Histomonas-free strain of Heterakis and Dr L. P. Joyner and Mr C. C. Norton of the Central Veterinary Laboratory, Weybridge, for the strain of Heterakis which was found to transmit Histomonas wenrichii. I also wish to thank Mrs B. Fisher, Mr B. Millard and Mr M. Shirley for technical assistance and Mr P. Rogers for assistance with the photography.

The development of Haemonchus contortus in vitro. I. The effect of pH and pCO2 on the rate of development to the fourth-stage larva.

Abstract: The development of the exsheathed third-stage larva of Haemonchus contortus to the fourth stage in vitro has been investigated in half-strength Ringer's solution and in solutions with ionic concentrations similar to those found in the abomasum during normal secretion of acid and during heavy infestations of Haemonchus. The concentration of carbon dioxide required for the optimum development of the larva has been shown to be dependent on the pH of the medium. At a low pH the optimum pC02 is low and the larva is less sensitive to a range of carbon dioxide concentrations than at a pH near neutrality. Gastric mucin and histamine have been shown to have an inhibiting and a potentiating effect, respectively, on development, in media at pH 6.

Studies on the host-parasite interface of strigeoid trematodes. V. Regional differentiation of the adhesive organ of Apatemon gracilis minor Yamaguti, 1933.

Abstract: The adhesive organ of Apatemon gracilis minor Yamaguti, 1933, consists of two lobes lying in a cup-shaped fore-body. The cytoplasmic tegument covering the apposing faces of the lobes is different from that covering their outer surfaces. The covering of the outer surface corresponds to the general tegument present on the rest of the body, whereas that on the inner surfaces is finely pitted and in certain regions is elevated to form a coarse reticulum. This specialized surface is covered externally by a plasma membrane and is in continuity with nucleated cell bodies lying below the basement layer. The cell bodies contain large quantities of granular endoplasmic reticulum as well as several Golgi complexes and numerous mitochondria. Masses of secretion bodies are present and these also occur in the extensions to the external tegument as well as within the external tegument of the lobes. When the parasite is attached the inner faces of the lobes come into contact with the vascular lamina propria of the host. The possible biological role of this specialized host–parasite interface is discussed and it is suggested that this surface specialization may form a morphological basis for the ‘placental’ function suggested for the adhesive organ by earlier workers.The author wishes to acknowledge the research grant provided by the S.R.C. for the purchase of a vacuum coating unit and an AEI EM 6 electron microscope. The progress of this study was greatly facilitated by the excellent assistance of Mr T. Davies and Miss C. Green. The Stereoscan micrographs are published by permission of the Cambridge Instrument Company.