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.

Three new species of dactylogyrid monogeneans with three anchors are described: two of them, Eutrianchoratus magnum and E. minutus, from Ophiocephalus obscurus, and Heteronchocleidus ctenopomae from the anabantid Ctenopoma kingsleyae, all from south Ghana.

Analysis of the variation in the morphological patterns of the anchors and the bars in the known species of the Heteronchocleidinae suggests gradual specialization from an ancyrocephaliid type with two pairs of anchors to types with three anchors arranged in triangular symmetry.

Heteronchocleidinae occur on both ophiocephaliid and anabantiid fish in Southeast Asia as well as in West Africa.

The biochemical constitution of the copepod parasite Mytilicola intestinalis was determined and seasonal changes were detected which, in general, could be correlated with the life-history of the parasite.

Biochemical constituents have been discussed in relation to those in other endoparasites and free-living crustaceans. It was concluded that carbohydrates and lipids both formed important stores of energy in M. intestinalis. Protein levels were reduced compared with those in free-living forms.

This work forms part of a Ph.D. thesis in the University of London. It was completed during the tenure of a Research Assistantship at The Polytechnic, Regent Street, London W. 1.

The aim of the present contribution has been to draw attention to the technique of covariance analysis in the study of factors influencing the oviposition of ixodid ticks.

The example given as an illustration shows the high degree of sensitivity of the technique and its effectiveness in removing the bias due to the sample differences in the mean female weights of different treatment groups. This bias has not been removed in any published work on egg-production studies of ticks, so preventing precise comparisons of different conditions to be made.

My thanks are due to Dr D. E. Moorhouse for helpful advice and to the Australian Meat Research Committee for generous financial support. The cooperation and assistance of Mr W. J. Roulston and his staff of the Division of Entomology, C.S.I.R.O. is gratefully acknowledged.

The behaviour of the sand martin flea Ceratophyllus styx jordani Smit was studied in relation to its ecology.

The cocooned resting imago in the host's old nest is the main over-wintering stage. Mechanical disturbance of the cocoons by the exploratory habits of the newly returned sand martins elicits emergence of the imagines in spring. The seasonal rise in temperature is not, by itself, important in causing emergence, as it does not become effective until many weeks after the martins have returned, by which time undisturbed fleas are likely to have died inside their cocoons.

When the imagines break out of their cocoons they are negatively photo-tactic, but become positively phototactic within 24 h. This response takes them outwards along the martin's disused burrow until the increasing intensity of light reduces their activity so that they aggregate on the lower lip of the entrance. Positive phototaxis prevents them dispersing downwards from the entrance.

Periodically the fleas bury themselves in sand; this response may function for water conservation. The proportion buried is greatest at night.

The sand martin's habit of hovering close to a succession of entrances renders it accessible to the fleas, whose main host-finding response is an outward jump from the burrow entrance. This jump is released by a sudden decrease in light intensity and is directed towards dark objects. Vibration and air currents do not release jumping.

Dispersal from aggregations in disused burrows may occur by transport on the host, or by spontaneous horizontal emigration along the cliff face, or by falling after an unsuccessful host-finding jump. Fleas which have fallen become negatively geotactic and positively anemotactic. Fleas wandering on the cliff face visually detect burrow entrances up to 30 cm away, and turn towards them. Preference for moister sand and, possibly, a negative phototactic response, may induce the fleas to remain in newly found burrows.

Small aggregations of fleas also occur at the entrances of burrows currently in use by martins. Fleas circulate between the entrance and nest chamber of these burrows, until the martin begins to incubate its eggs. The entrance aggregation then disappears and fleas accumulate in the nest chamber. There is some interchange of fleas between infested burrows in the martin's pre-incubation period.

The general pattern of behaviour resembles that of C. gallinae. Behavioural differences between the two species are related to the ecology of their hosts. C. styx is adapted to dispersal and host finding in its host's breeding site, whereas C. gallinae is adapted to reach foraging birds. This difference partly accounts for the narrow host specificity of C. styx and the wide host range of C. gallinae.

My grateful thanks are due to Chris. Mead and Giles Pepler for information on sand martins' behaviour and migratory arrival. I would also like to thank Brian Little, who introduced me to several colonies in the Tyne Valley. The valuable advice of the Hon. Miriam Rothschild and Dr E. T. Burtt is especially acknowledged.

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.

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.

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.

Ultrastructural observations on the intestine of Metastrongylus sp. have shown that it is composed of a single layer of epithelial cells each with a basally placed nucleus and resting on a basement layer. The basal plasma membrane of each cell is highly infolded and at the cell apex the lateral plasma membranes of adjacent cells are thickened to form a terminal bar.

The brush border consists of numerous microvilli which possess dilated tips. The tips contain a granular material and the evidence obtained suggests that they are released into the lumen indicating a form of micro-apocrine secretion. Golgi complexes, glycogen, lipid droplets and granular endoplasmic reticulum are present in the intestinal cells. The Golgi complexes are associated with the production of an electron dense body. Other inclusions are cytolysome-like in their structure. The discharge of lamellar bodies into the intestinal lumen is described and the role of the intestine in secretion, absorption, and excretion is discussed.

The authors wish to acknowledge the co-operation and facilities provided by Professor J. Brough in the Department of Zoology and Mr J. H. M. Hughes (Veterinary Officer) for allowing unrestricted access to infected material at the Roath abattoir, Cardiff. Grateful acknowledgement is also expressed for the financial support given to one of us (T. J.) by the Pig Industry Development Authority. Any views expressed in this paper are not necessarily those of the Authority.

The authors also wish to express their appreciation of the excellent technical assistance provided by Mr T. W. Davies.

Two experiments are described in each of which three groups of calves received infective larvae of Ostertagia ostertagi daily, each group at a different rate. Changes in worm burdens were followed by the periodic slaughter of calves.

Numbers of adult worms quickly rose to and remained at levels which bore a positive relation to the infection rate. There was evidence that a turnover of worms was occurring and it appeared that burdens of adult worms were regulated by a loss of worms which depended on the number present. As the host's experience of infection increased, the adult worms present were smaller and an increasing proportion of females lacked a fully developed vulval flap. Evidence is presented which suggests that these were effects of resistance and that they depended on separate mechanisms.

Faecal egg counts of groups carrying different numbers of worms rose to the same peak and then declined according to the same logarithmic curve. This finding, together with data concerning the number of eggs in the uteri of the worms, suggested that the egg output of the entire population was restricted by the host to a limit which depended on previous egg-laying.

Infection did not have an adverse effect on the growth of the calves unless the infection rate exceeded a critical value. The resistance of calves to infection was impaired by clinical ostertagiasis.

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.

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 infective larvae of S. ratti were non-feeding and none of a wide variety of possible substrates had any effect on the respiratory rate of intact worms. Sodium succinate increased the oxygen uptake of larval homogenates.

The cytochrome chain inhibitors, sodium cyanide, sodium azide and urethane all reversibly inhibited the respiration of the infective larvae. The inhibition caused by carbon monoxide was photo-reversible and 2,4-dinitrophenol inhibited respiration rather than stimulated it. Methylene blue increased the oxygen uptake of homogenates, but had no effect on the intact larva. Iodoacetamide and sodium arsenite irreversibly inhibited respiration, whilst sodium fluoride, sodium pyrophosphate and sodium malonate were without effect on the intact larvae.

This work was carried out during the tenure of a Medical Research Council Scholarship.

In two experiments in which two different strains of T. annulata were used it was possible to induce theileriasis by injection of the crushed infected unfed H. dromedarii. The nature of the disease corresponded with that transmitted naturally by the feeding of the tick vector.

The faecal egg counts of seven groups of calves exposed to infection with O. ostertagi on pastures were found to follow a similar course to those of calves experimentally infected on one occasion or of calves infected daily. They declined from an early peak according to an apparently logarithmic curve. It was concluded that faecal egg counts were largely independent of worm numbers and tended to follow a stereotyped pattern. Acute clinical ostertagiasis was as liable to lead to high egg counts as was poor thriving due to the other causes. The relevance of these findings to the epidemiology of ostertagiasis and to the use of faecal egg counts in the diagnosis of parasitic gastro-enteritis are discussed.

The author thanks Miss C. N. Herbert for the calculations and curve fitting in this paper.

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.

The durations of the various phases of the life-history of the ixodid tick H. anatolicum anatolicum are described with particular reference to the modifying influence of temperature and relative humidity. An increase in the ambient temperature was found to decrease the length of the feeding, preoviposition, oviposition, larval and nymphal moulting periods and the egg-incubation period. The effect of change in relative humidity was less marked, the only significant effect being on the oviposition period which was protracted when the relative humidity was increased.

Daily percentage water loss and longevity were found to be more or less inversely proportional. Thus at high temperatures and low relative humidities a large percentage of water was lost daily with consequent reduced life span.

I wish to thank Dr R. P. Chaudhuri for providing me with the specimens of H. anatolicum anatolicum from which the culture was started, and Professor D. R. Arthur for his advice and assistance. I am also grateful to the Science Research Council for the provision of a research grant.

Immunization was performed against Schistosoma mansoni or S. japonicum infection in rhesus monkeys either with living schistosomula and young adult worms or with only schistosomula by intra-peritoneal injection. Each monkey was immunized four or more times and subsequently challenged. A significant degree of resistance against the challenge was produced. The injected worms lived in the peritoneal cavity for a short period, then died and disintegrated therein, thus obviating their inhabiting the circulatory system. The functional antigens were evidently derived from the metabolic products of the immunizing living schistosomes in the peritoneal cavity.

The results of the present experiment indicate that the main immune mechanism induced by the intraperitoneal injection of living schistosomula and young adult worms is that which inhibits the development of the late stage of schistosomula and the early stage of adults which have originated from the challenge infection. This immune mechanism may be induced by the functional antigens either from both the schistosomula and adult worms or from the schistosomula alone.

Toxocara canis larvae infecting dogs undergo tracheal migration following their ingestion in an experimental intermediate host. Development of the parasite then proceeds to the adult stage in the small intestine. Adult dogs are therefore capable of distributing fertile T. canis ova which, if ingested, could cause larva mirans in humans and other animals.

This work was financed by research grants from the University of Queensland and the National Health and Medical Research Council of Australia. The writer wishes to acknowledge the encouragement and criticism of Professor J. F. A. Sprent.

Babesia rodhaini remains in the blood of most rats from 2 to 12 months after the acute primary parasitaemia has subsided. By analogy with some trypanosome and malaria infections it is suggested that piroplasms may have the capacity for antigenic variation, thus allowing the parasite to survive in the host for so long albeit as a chronic infection. A population of B. rodhaini was isolated which was considerably less sensitive in passive transfer experiments to two pools of serum from recovered rats than the normal passage population.

I thank Dr S. F. Barnett for his advice during the work and Dr F. Hawking and my colleagues at Mill Hill for criticizing the manuscript. This work was carried out during the tenure of an Agricultural Research Council Studentship.