The pathogenicity and pathology of Ascaridia galli infections of poultry were studied using repeated doses of varying numbers of eggs. No significant effect due to infection was seen on p.c.v. and Hb. values of infected chickens. Haemo concentration occurred parallel with sporadic diarrhoea, which follows high levels of repeated dosing. This phenomenon complicated the interpretation of blood protein values. A/G ratios were, however, worked out and analysed. No significant differences were noticed between the groups and it was concluded that under the levels of inoculation employed during the study there were no significant effects of A. galli infections on blood protein values as well as p.c.v. and Hb.values of infected chickens.

Symptomatic developments, macroscopic lesions and pathological changes of intestinal tissues due to infection were followed up in detail. They were most severe in chickens on the least nutritive diet and sustaining the highest levels of repeated doses. Pathogenicity of larval and adult parasites was described. The presence of large numbers of adult parasites was associated with intestinal obstruction in host. This was seen mostly in chickens on low nutrition and sustaining either low or high levels of repeated inoculation, and very rarely also in chickens under high plane of nutrition but sustaining repeated inoculation of small numbers of eggs. Cellular infiltration as a result of infection was found to be extremely rare.It was present as fibroblasts and only in association with dead larvae.

The author is indebted to the Department of Technical Cooperation, under whose Fellowship the study was carried out; the Department of Zoology, Edinburgh University, for facilities provided; the Ministry of Agriculture, Fisheries and Food; Lasswode Poultry Research Laboratories, for the free supply of chickens used here; Dr J. A. Campbell of Edinburgh University for his keen interest, encouragement and advice throughout this work; and finally to G. Obiwelozor for reproducing the photographs from the original manuscript.

The study of the ultrastructure of the blood stages of P.tropiduri Aragao & Neiva, 1909 shows close similarities between this species and P.floridense, another lizard malaria parasite. The absence of a rounded structure in the merozoites of P.tropiduri, and their tendency not to project beyond the original area occupiedby the schizont are the ultrastructural characteristics that differentiate the two species.

The close relationships of homology between P.tropiduri and the Plasmodium of birds are emphasized, and new evidence on their phylogenetic relationships is offered.

P.tropiduri has a mode of ingestion and digestion similar to that of P. elongatum. When the size of the cytostome, the size of the boluses produced by the phagotrophy, and the size of the digestive vacuoles are compared, they are seen to be nearly identical in the two species.

The comparative study of the structure of the cytostomes of the merozoites and the trophozoites, and of their relationships with the membranes that limit the parasites, gives new evidence that the external membrane of the trophozoites schizonts, and gametocytes is derived from the host cell.

An exflagellatory apparatus is described from the microgametocytes of P.tropiduri, and its relationships with similar structures described from the gametocytes of avian Plasmodium and from Coccidia are discussed.

A model of host–parasite relationships is proposed and its properties examined.

1. Serial sections were examined of thoraces of a selected strain of Aedes aegypti infected with either Brugia pahangi or subperiodic B. malayi and of wild-caught Mansonia uniformis infected with the latter parasite.

2. A few badly damaged flight muscle fibres are seen in the early stages of infection but many more fibres are destroyed at the time when the mature filarial larvae leave the muscle and enter the haemocoel. In the intervening period, when the larvae are growing within the muscle fibres, only minor damage develops, affecting nuclei and/or mitochondria. The nature of such minor damage and the mechanism of complete muscle breakdown differs in the two species of mosquitoes.

3. It was concluded that the muscle damage was sufficiently severe to explain the significant mortality often observed among experimentally infected mosquitoes at the time when the mature filarial larvae enter the haemocoel.

I am very grateful to Dr W. W. Macdonald, of the Liverpool School of Tropical Medicine, who supplied the Aedes aegypti mosquitoes and who has given invaluable help and advice throughout this study. Mr Cheong Weng Hooi and his colleagues in Kuala Lumpur kindly supplied, and partly processed, the Mansonia uniformis mosquitoes. Miss M. A. Johnson assisted with the care of the A. aegypti colony and Mrs J. Oliver helped with some of the histological work.

The relationship between the age of infective O.cunictili larvae and the proportion that remained retarded in a subsequent infection was investigated. The proportion of retarded larvae recovered was found to depend on the length of time and the temperature at which infective larvae were stored prior to infection. At 4 °C the level of retardation was found to increase with increase in the length of storage until a maximum was reached. Further storage tended to decrease the proportion of larvae that remained retarded. These effects were found to be less marked when the larvae were stored at a higher temperature. At 17 °C the maximum level of retardation reached was lower and the process of the increase and the decrease in retardation was of shorter duration. An increase in the number of larvae from 200 to 6000/rabbit did not appreciably change the proportion that remained retarded.

We are most thankful for the advice and criticisms of Dr J. D. Dunsmore. Theskilled technical assistance of Mr Ted Eaton is gratefully acknowledged. This workwas supported by the Ontario Department of Agriculture and Food.

The covering layer of Amphibdella flavolineata has been found to be a syncytial cytoplasmic epidermis bearing scattered microvilli and connecting with parenchymally situated ‘cell’ bodies or cytons by means of conspicuous, microtubulelined cell processes. There was no evidence from a comparison of the covering epidermis of the juvenile blood living form and that of the gill living adult that this layer in the juvenile was especially modified for life in the host blood system.The epidermis of adult and juvenile worms was found to be remarkably similar, the main difference being that that of the adult was slightly thicker and bears longer microvilli. Apart from some lamellate bodies of doubtful significance, the inclusions found in the epidermis of adult and juvenile forms were also very similar.The outer epidermis of the juvenile monogenean was found to be quite different from that of Schistosoma mansoni in that it is not permeated by incursive channels and contains many more mitochondria than the epidermis of the digenean. The epidermal cell bodies of Amphibdella tend to have electron-lucent areas especially in the ‘ apical cell’ regions leading to the cell processes that connect with the outer layer. Electron-dense granules secreted by the epidermal cells accumulate in these ‘clear’ regions before being transferred to the outer layer. There is some slight evidence that these electron-lucent regions may have contained glycogen. The epidermal ‘cell’ bodies of the juvenile tend to be multinucleate, whilst those of the adult are usually uninucleate.

Thanks are due to the Director and staff of the Marine Biological Laboratory, Plymouth for providing accommodation and material. The assistance of Mr George Best of the Plymouth Laboratory and Mr Harry Edge of King's College London Botany Department, who maintain the microscopes used in this work, is especially appreciated.

In the laboratory, transmission of Schistosoma mansoni to and from Biomphalaria glabrata can occur in artificial sea water at or below a concentration of 12-5%. Miracidia can also emerge and infect snails in 25 % sea water, but this concentration is inimical to the survival of the snails or their eggs. The parasite thus appears to be better adapted to brackish water than is the intermediate host. Epidemiological studies are needed to assess the limits imposed by brackish waters on the natural transmission of schistosomiasis.

Observations on the egg-content of 78 counted populations of Metastrongylus apri showed that there was a positive correlation between population size and mean egg-content per female. In bilateral infections within one host, a significant tendency was shown for the larger of the two populations to outstrip the smaller in mean egg-content per female. There was no correlation between sex-ratio and mean egg-content. It is suggested that the effect of the host's defences against individual parasites is mitigated when large numbers of parasites are present.

A positive correlation was found between the sex-ratios of populations in left and right lungs, and there was a significant predilection of M. apri for the right lung.

I am indebted to Dr S. Peto, Microbiological Research Establishment, Porton, Wilts, for statistical advice, and to T. Wall and Sons Ltd., Atlas Road, London, N.W. 10, for access to abattoir material.

A population of three-spined sticklebacks, Gasterosteus aculealus, from a pond in Somerset was sampled about every three weeks for 19 months, a total of 1863 fish being collected. Three species of parasites were found in the fish: Schistocephalus solidus, Diplostomum gasterostei and Echinorhynchus clavula.

Seasonal changes in the incidence and intensity of the three parasitic infections are described. In general the infections increased in summer and autumn and decreased in winter and spring, although in Schistocephalus the differences in the level of infection between the two years were greater than between seasons.

The largest increases in the Schistocephalus and Echinorhynchus infections occurred in August and September and in the Diplostomum infection in May and June. These increases could be related to the feeding activity of the sticklebacks and to the abundance and level of infection of the Cyclops, Asellus and Limnaea pereger populations.

Reduction in the sizes of the infections was by predation and death of heavily infected fish in the case of Schistocephalus and Diplostomum and mainly by death of individual worms in Echinorhynchus.

These mechanisms are shown to be adapted to the life cycles of the parasites.

I am most grateful to Dr H. D. Crofton for his help and encouragement during this investigation and to Dr R. A. Avery for useful discussions and advice. This work was supported by a Science Research Council research studentship and a NATO studentship.

An ascaridoid nematode Cotylascaris thylacini is described from the thylacine or Tasmanian marsupial wolf. Only female specimens were available. The new genus is distinguished by two cup-like structures on the anterior margin of each lip. It is suggested that these structures may comprise an adaptation to the unusual form of the intestinal wall in the thylacine. The significance of the ascaridoid nematodes of marsupials is discussed in relation to the possible route of entry of marsupials into Australia.

Financial assistance for this study was provided under grant No. 16 from the Australian Research Grants Committee.

The nematode, Heterotylenehus autumnalis, generally failed to develop successfully in Musca domestica L., Orthellia caesarion (Meigen) and Ravinia l'herminieri(Robineau-Desvoidy). Failure is attributed mainly to host resistance in the form of haemocytic encapsulation of the small gamogenetic female nematode. In a few instances, the nematode was able to complete development successfully in housefly and Orthellia, but was not capable of leaving the host. These few instances of development in the unnatural, experimental hosts are attributed in part to general ’weakening’ of the host by superparasitism. In many cases superparasitism resulted in the death of the fly larvae or pupae.

Tracheolar proliferation was usually associated with the encapsulated nematodes and there was evidence that many of them were expelled from the host during pupation. We also observed nematodes with cephalic caps of dark material, presumably melanin, and some that were tightly girdled with this material. A host reaction, apparently never before reported for a parasitized insect, was the darkening (presumably melanization) of the larval anal organ.

The authors would like to express their appreciation to Dr A. J. Nappi, Biology Department, State University of New York, Oswego, New York, and Dr G. 0. Poinar, Jr., Department of Entomology and Parasitology, University of California, Berkeley, California, for reading an early draft of the manuscript and offering critical comments. The senior author would like to express his deep appreciation to Dr W. R. Nickle, Nematology Investigations, Crops Research Division, Agr. Res. Serv., USDA, Beltsville, Maryland, and to Dr H. Welch, Zoology Department, University of Manitoba, Winnipeg, Canada, for their help and encouragement during the course of this study. This study was supported in part by a grant from the University of Connecticut Research Foundation.

All three species of parasites found in sticklebacks from Priddy had overdispersed distributions, similar in shape to host–parasite distributions described by other authors.

The distributions of both Echinorhynchus clavula and Diplostomum gasterostei were fitted closely by the negative binomial. The values of k were similar for both species, being always less than unity; the Diplostomum distributions were more overdispersed and therefore had higher values of p.

The Schistocephalus distributions were mostly described by the log normal, but the high percentage infection and relatively low overdispersion meant that the fit was often not close. There was also some truncation at the upper end.

The relationship between the parameters of the negative binomial (p and k) and the shape and parameters of the original distribution have been investigated, both theoretically and with respect to the distributions of the stickleback parasites.In general p varied as the overdispersion and k as the percentage infection.

The advantages of an overdispersed distribution to the host and parasite populations are discussed and related to the life cycles of the parasites.

I am most grateful to Dr H. D. Crofton for his help and encouragement during this study, and to the staff of the Computer Unit, University of Bristol for their assistance with programming problems. This work was supported by a Science Research Council research studentship and a NATO studentship.

Specimens of nematodes from the stomach, oesophagus and trachea of the cat, previously reported and described by Led et al. (1968), were found to resemble L. major more closely than other species in the genus. In the shape of the interlabia and the number of pits on the surface of the eggs, they conformed to this species. Accordingly they were identified as L. major although they differed from L. major in the width of the lips in relation to the postlabial collar and their more slender appearance, especially in the tail region. This is the third report of L. major and extends the host range and geographical distribution of this species.

Reinfection of the insects Tenebrio molitor and Schistocerca gregaria by the Eugregarines Gregaria polymorpha and G. garnhami respectively is not caused by the retention of the gametocyst and the release of sporocysts inside the gut. The gametocyst has to pass out from the insect gut, dehisce, and the sporocysts have to be ingested in order for reinfection to occur. Thus it is possible to stop reinfection by preventing the host from ingesting sporocysts. The development of spore ducts on the gametocyst of G. garnhami is inhibited if the gametocysts are prevented from leaving the hindgut of the locusts.

I would like to thank Professor L. H. Finlayson for his advice for many helpful discussions.

A population of Gasterosteus aculeatus from Priddy pool in Somerset was infected with three species of parasites: Schistocephalus solidus, Diplostomum gasterostei and Echinorhynchus clavula. The variations in the incidence and intensity of the infections with the sex, age and size of the sticklebacks are described.

The infections with the three parasites were very similar in male and female sticklebacks. The weights of Schistocephalus were greater in the females, however;the mean weight of the plerocercoids was larger suggesting that they grew more rapidly in female fish.

The Schistocephalus infection increased in small fish, under 1 year of age, but in older larger ones the infection decreased. These changes can be related both to the feeding habits of the fish and to the weight relationships of host and parasite.It was found that the weight of Schistocephalus did not greatly exceed that of the fish, and also that the combined weight of fish plus Schistocephalus was rarely greater than 3 g. Therefore the proportional weight of Schistocephalus, or parasite index, was lower in sticklebacks heavier than about 1·5 g.

The Diplostomum and Echinorhynchus infections both increased with age and size of fish, but the largest sticklebacks were not the most heavily infected. These increases can be related to the feeding habits of the fish in the case of Echinorhynchus and to accumulation with time in the case of Diplostomum.

In all three species the largest and oldest fish were not the most heavily infected.It is suggested that the old fish are becoming senile and are therefore less able to support large parasite burdens.

I am very grateful to Dr H. D. Crofton for his constant advice and encouragement during this study, and to Dr R. A. Avery for useful discussions. This work was supported by a Science Research Council research studentship and by a NATO studentship.

The distribution and fine structure of gland cells in the miracidium of Fasciola hepatica is described. There is a large flask-shaped, multinucleate, apical gland lying ventrally in the anterior. On each side of this are pairs of uninucleate accessory gland cells. Both apical gland and accessory gland cells communicate with the tip of the apical papilla. A third group of vesiculated gland cells opens by ducts at the base of the apical papilla. The miracidium is covered with a surface mucous coat and this appears to be directly extruded from the underlying ciliated epithelial cells. The findings are discussed with relevance to attachment to, and penetration of, the snail intermediate host.

Clavella uncinata (Muller) (Crustacea: copepoda) was found infesting 24% of 1352 whiting Odontogadus merlangus (L.) examined from the North Irish sea.A study of the dimensions of the adult female and a comparison of the morphology of copepodid larvae from the Clavella of whiting and pollack Pollachius pollachius (L.) suggests that the Clavella from these gadoids are all one species. Observations on the process of feeding and histochemical analysis of the gut contents indicate that the parasites feed by browsing on the superficial tissues of the whiting and that the mouthparts are used more for the gathering of food towards the mouth than for cutting. Observations show that the hatching of eggs and the behaviour of the nauplii are remarkably similar to these functions in free-living copepods. Examination of Clavella from adult and young whiting, and from living specimens kept alive on the fish, suggests that there are definite cycles of development, that the eggs take about 21 days to mature and that adult Clavella has a life span of no more than 6 months.

I would like to thank the Masters and crews of the R.V. Cuma and R.V. Silver Spray and the staff of the Marine Biological Station, Port Erin for their help in the collection of material. I am especially grateful to Mr D. J. Slinn who guided me throughout this work and to Mr R. Moore for his technical assistance.

The phylogenetic relationships of the Acanthocephala are reviewed in the light of recent ultrastructural evidence. Comparisons of spermatozoan ultrastructure suggest that the Acanthocephala cannot be closely related to the Platyhelminthes and that the similarities which exist between cestodes and acanthocephalans are the result of convergence. Similarities in the spermatozoan ultrastructure of acanthocephalans, rotifers, nematomorphs and nematodes probably have a phylogenetic significance and suggest that the acanthocephalans have a close common ancestor with some at least of the Aschelminthes. The case for this affinity is strengthened by ultrastructural evidence concerning the body walls and muscles of acanthocephalans, nematodes and rotifers.

I am indebted to Professor D. R. Arthur and Professor F. R. Whatley at King's College, London, for the provision of facilities. The work reported in this paper was undertaken during the tenure of a Research Fellowship from the Nuffield Foundation.

The population dynamics of Ascaridia galli infections of poultry was studied under repeated doses of 10, 100 and 1000 eggs. Daily doses were continued for 6 weeks only. At low dose rates patency is noted by the fifth week. At higher dose rates there is an accumulation of larvae whose development has been arrested at a precise developmental stage, i.e. the third larval stage. Sporadic development of some of these larvae to adults occurs later. Arguments are produced to show that this inhibition of larval development is a resistance and not a density-dependent phenomenon. The epidemiological significance of this inhibition is discussed. This resistance phenomenon is shown to occur in single infection experiments where 1000 eggs were used. This gives rise to the development of a worm population of two facies. The whole finding is suggested to be related in some way to the phenomenon of ‘tissue phase’, which is accepted as a normal life-cycle behaviour of this parasite. A brief review of previous work on this is given and it is postulated that the ‘tissue phase’ phenomenon is not an obligatory part of the normal life-cycle of the parasite but represents a reaction to factors of resistance inherent in chickens and controlled to a large extent by a complex dynamic interplay of dose size and the chickens.

I wish to thank the Department of Technical Co-operation and the Federal Government of Nigeria for financing this project, Dr Boulton of the Poultry Research Centre, Edinburgh, for advice on nutrition, and the Department of Zoology, especially Dr J. A. Campbell, University of Edinburgh, for facilities and advice on the work. I am greatly indebted to the Ministry of Agriculture, Fisheries and Food, Poultry Research Laboratories, Lasswade, for the free supply of chickens used in this work.

Farm-reared Leuciscus idus were experimentally infected with the intestinal tapeworm Caryophyllaeus laticeps, and maintained at different temperatures. At 12 °C and lower temperatures there was an initial loss of parasites after infection, but the survivors persisted for up to 1 month. At 18 °C only temporary survival of the parasite was possible, and worms were killed and rejected after 3 days. Experimental transplants to fresh hosts after 2 days indicated that the factors responsible for the death of the parasites became effective within this period.Neither inhibition of peristalsis nor host starvation affected the parasite's survival. The speed of the response to the parasite indicates that circulating antibodies produced in response to C. laticeps and directed specifically against it cannot be involved in the rejection. Other possible mechanisms of rejection are discussed, together with the significance of the results to the interpretation of data based on field observations.

I should like to thank Mr J. Harris for allowing me to refer to some of his unpublished data and for helpful discussions.