Intracellular forms of Trypanosoma lewisi have been found in the mid-gut cells of the flea, Nosopsyllus fasciatus. These stages have been found only infrequently, and in only two of the four strains of T. lewisi studied.

Attempts to find similar stages in Trypanosoma nabiasi, evotomys and microti were not successful.

The possible significance of these forms is discussed.

The financial assistance of the Agricultural Research Council is gratefully acknowledged. The author is also indebted to Dr P. Tate for his enthusiasm, help and guidance throughout the course of the study.

Intraerythrocytic granules in the blood of Sudanese fowls first seen by Balfour (1908) were named Aegyptianella pullorum by Carpano (1928).

Electron microscopy of a Rhodesian strain of A. pullorum indicated the similarity between these organisms and the Rickettsia-PLT group of organisms, also their dissimilarity to Sporozoa which we have studied. The organism was coccoid or coccobacillary, measuring up to 0·6 μm in diameter, and in longitudinal sections 1·5–3 μm. Pleomorphic and dividing forms were observed. Each organism was surrounded by two unit membranes with short internal invaginations from the inner membrane only. A layer of ribosome-like particles varying in density and depth gave the impression of an outer cortex. Irregular extensions of this layer towards the centre of the organism followed the lines of fine DNA-like filaments forming a web lattice throughout the substance of the organism. There was no well-defined nucleus as seen in Protozoa. Single as well as groups of organisms were situated in vacuoles lined by a membrane similar to the erythrocytes' own outer membrane. No intranuclear organisms were seen.

The similarity of these organisms to symbionts studied by Roshdy and Bird in both adult and immature stages of A. persicus is illustrated. From this evidence, and its apparent likeness to the tick-borne fever agent, there is good reason for including A. pullorum among the Rickettsiaceae. The position suggested for it is between the Rickettsia and PLT group of organisms, closely allied to Wolbachia, the agent of tick-borne fever (Cytoecetes sp.) and the agent of bovine heart water disease (Cowdria ruminantium).

We thank Mr R. Jacobson, F.I.M.L.T. for his willing assistance in conducting the sensitivity test, also Miss L. Lanigan and Mr D. Day for their technical and photographic assistance.

It is well known that the defence reactions of insects to metazoan parasites are principally cellular rather than humoral; and it has recently been shown that the habitual parasitoids of an insect have means of resisting or avoiding the defence reactions of their host (Salt, 1968). Those two findings suffest that it may be necessary to re-examine the situation with regard to protozoan parasites, including some genera for which insects act as vectors between vertebrate hosts. Two examples will serve to draw attention to the problems involved.

Eimeria tenella infections were established in the chorioallantoic membranes CAM) of developing chick embryos. At different times after infection the CAM ells were cultured in vitro in modified 199 medium. Different types of schizont were grown, some similar to those occurring in infections of the usual in vivo site and others markedly different. In schizogony of one type the merozoites appeared to develop by growing out from the periphery of cellular masses; a process similar to that described by Hammond et al. (1966) for E. bovis.

Gametocytes and infective oocysts were also grown by the tissue culture of CAM cells but only in cells infected 4–6 days previously in embryo; CAM cells infected for only 3 days before culture supported the growth of large numbers of schizonts of different types.

The majority of the stages grown developed within densely populated areas of epithelial-like cells. A temperature of 41 °C was found to be necessary for the growth of the parasite. The nutritional requirements for the growth of E. tenella are fairly well provided by the medium used. The relatively simple procedure described should be of value for the observation of E. tenella at various stages of its growth and provide a means of in vitro testing of antiparasitic substances.

I wish to thank Dr P. P. Levine for his interest and encouragement during the course of the work and Mr D. L'Amoreaux for technical assistance.

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 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.

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.

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 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.

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.

The morphology and biology of the various stages of the life-cycle of Trypanosoma evotomys has been studied in laboratory bred vertebrate and invertebrate hosts. The morphology of T. evotomys in the blood of the bank-vole, Clethrionomys glareolus, is typical of that of a trypanosome of the subgenus Herpetosoma, but the shape and size of the trypanosome in C. glareolus is more variable during the reproductive phase of the infection than in the adult phase. The duration of the infection of T. evotomys in C. glareolus varies from 22 to 31 days in experimental infections and the incubation period of the infection initiated by metatrypanosomes is 5–6 days. The peak of the parasitaemia varied from 15000 to 55000 parasites per mm3 of blood. The reproductive stages of T. evotomys are found in the spleen and lymphoid tissue (appendix) of the bank-vole, and reproduction is by equal, binary or multiple fission in the amastigote stage. T. evotomys was not transmissible to M. agrestis, mice or rats. Cyclical development of T. evotomys took place in the flea, Nosopsyllus fasciatus, and metatrypanosomes developed in the intestine of the flea. No intracellular forms of T. evotomys were found in the mid-gut of the flea.

The encouragement, help and advice of Dr P. Tate is gratefully acknowledged. Financial support was received from the Agricultural Research Council to whom thanks are also due.

Dense larval suspensions have a higher Qo2 than dilute ones, probably due to an oxygen debt acquired during preliminary washings. Ammonia (0·1M) did not affect the respiratory rate of the infective larvae of S. ratti. Mechanical agitation of larvae reduced their rate of respiration.

The partial pressure: respiration curve of the infective larvae was sigmoidal, that of larval homogenates was a rectangular hyperbola. At low partial pressures of oxygen the respiratory rate depends to a certain extent on the rate of change of the partial pressure.

In 100% oxygen the respiratory rate of the larvae is at first stimulated and then falls to its initial level. On returning to air after 100% oxygen, the respiratory rate is at first depressed and then recovers.

The infective larvae of S. ratti develop an oxygen debt when kept under anaerobic conditions. During the recovery period, which lasts for about 1 h, the Qo2 may increase by up to 100%.

The respiration of the infective larvae increases as the temperature is increased. When larvae are maintained at 37 °C there is a gradual decrease in their rate of respiration and this change is apparently irreversible.

The rate of respiration of the larvae is slightly inhibited at low pH values and slightly stimulated at high pH values.

Moderate levels of carbon dioxide (5%) have no effect on the respiratory rate of the infective larvae of S. ratti, high levels of carbon dioxide (10–20%), have a definite narcotic effect on the larvae.

The respiration of the infective larvae is unaffected by osmotic pressures up to an equivalent of 1% sodium chloride. Above this level the respiratory rate is at first stimulated then decreases.

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

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.

Foleyella confusa sp.nov. is described from Rana limnocharis on Luzon, near Manila. It differs from F. duboisi (Gedoelst) in arrangement and number of anal papillae, in spicule size in relation to length of body, and in overall size. Icosiella hoogstraali sp.nov. from Rana macrodon on Palawan differs from others in the genus in having a left spicule 1·955 mm long and a single preanal papilla. Gnathostoma spinigerum and Cosmocerca japonica are also reported.

We thank Dr Roy C. Anderson, University of Guelph, for reading the manuscript and offering his comments on it.

The authors wish to acknowledge the field support by Dr D. S. Rabor, Department of Biology, Silliman University, Dumaguete City, Negros Oriental, Republic of the Philippines, and the technicians of the Parasitology Department of Naval Medical Research Unit no. 2 for general assistance in obtaining and examining hosts.

Initial work for this study was supported by funding under Public Law 480, Section 104 (c), by funds provided by the Bureau of Medicine and Surgery, Navy Department Work Unit MR 005·20–0098, and by Contract No. NR 103–690/N0014–66-C 0094, between the Office of Naval Research, Department of the Navy, and the Southwest Foundation for Research and Education.

Final efforts were sponsored by Department of the Army, Contract No. DADA 17–68–C–8094.

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.

Two strains of E. tenella (Weybridge and Houghton), which have been maintained by laboratory passage for at least 20 years without exposure to drugs, were compared.

Oocyst dimensions and morphology were similar and cross-protection experiments showed that the strains could not be distinguished immunologically.

One strain (Weybridge) caused greater caecal haemorrhage than the other when small doses of oocysts were given and was less completely controlled by some quinoline and pyridine drugs.

Acknowledgements are due to Dr M. E. Rose of the Houghton Poultry Research Station who kindly made the Houghton strain of E. tenella available to us with details of its maintenance.

The statistical calculations were made by Miss C. N. Hebert.

A study was made of some of the factors which affect the growth of Plasmodium knowlesi in blood diluted in the Harvard medium. Some slight enhancement of growth and re-invasion of new host cells was obtained when P. knowlesi was incubated in low oxygen tensions. Some growth but no re-invasion was obtained under anaerobic conditions. High oxygen tensions were also inhibitory.

The lipids present in the plasma are important for the growth of P. knowlesi in vitro. Stearic acid and cholesterol are required by the parasite but do not account for all the growth promoting properties of plasma. A requirement for vitamin B12 was also demonstrated.

I should like to thank Dr F. Hawking and Dr J. Williamson for their advice and for reading through the manuscript, Mr T. J. Scott-Finnigan for technical assistance, Dr K. Mervyn (Glaxo Research Ltd.) for samples of vitamin B12 analogues and the Hachmeister Company (Pittsburgh) for a sample of TEM 4T. This work received financial assistance from the World Health Organization.

An improved perfusion apparatus for the cultivation of malaria parasites is described. It consists of a sandwich of three chambers, of which the outer two contained medium and the inner contained the infected blood sample.

Using this apparatus and rocker-dilution cultures it has been shown that medium ‘199’ and medium ‘NCTC 135’ are as good as the Harvard medium for the cultivation of P. knowlesi in vitro.

The asexual cycle of P. knowlesi took slightly longer in vitro than in vivo. The stage of development at which the parasite was inoculated into culture affected the amount of reinvasion of new host cells. Thus, a greater amount of re-invasion was obtained with an inoculum of schizonts than with one of ring stage parasites. The rate of multiplication of the parasite decreased at each successive subculture.

I should like to thank Dr F. Hawking and Dr J. Williamson for their advice, Mr T. J. Scott-Finnigan for technical assistance, Mr F. R. Wanless and Mr C. D. Sutton for taking the photographs, and Mr F. A. New for drawing the figures. The perfusion apparatus was made by Mr T. Harding of the Engineering Division, National Institute for Medical Research. This work received financial assistance from the World Health Organization.

Ophidascaris filaria sensu lato previously redefined in the wide sense is now subdivided into four species as follows: (1) O. filaria sensu stricto in Python molurus in India; (2) O. infundibulicola (Linstow, 1903 with emended definition) in P. reticulatus in Singapore, Malaya and Thailand; (3) O. baylisi Robinson, 1934, emend. in P. reticulatus in Malaya, Singapore and Thailand; (4) O. moreliae (new species) in Morelia spilotes variegatus and Aspidites melanocephalus in Australia. The four species are differentiated particularly by their colour, the pattern of the folds of the intestine, the relative length of the spicules and vagina, the eggs, and the extent of development of the larvae in mice.

The writer gratefully acknowledges the cooperation of Professor A. G. Chabaud and his colleagues at the Museum National d'Histoire naturelle, Paris, in giving helpful suggestions and criticism and also for making Dujardin's specimens and drawings available. Thanks are also due to the following: Dr G. Hartwich, Zoological Museum of the Humboldt University of Berlin, for sending syntypes of Ascaris rubicunda; to Dr V. Zaman, Singapore, Dr Fred Dunn, Kuala Lumpur, Dr A. R. Omar, Ipoh, Dr Pradon Chatikavanij and Dr Suvajra Vajrasthira of Bangkok, Dr A. S. Dissanaike, Colombo, and Dr G. A. Schad, Calcutta, for help in obtaining specimens. Facilities to carry out this work were provided by the south-east Asian Ministers of Education Council at the Faculty of Tropical Medicine, through the Dean, Professor Chamlong Harinasuta, Bangkok. Acknowledgement is also made to the able technical assistance of Miss Marian Hollis. Financial assistance for this study was provided under grants from the Australian Research Grants Committee and the United States Department of Health, Education and Welfare no. A 107023–02.

Evidence was obtained that copulation of Nippostrongylus brasiliensis did not occur until at least 104 h after rats had been subcutaneously infected with 1000 3rd-stage larvae. The majority of worms copulated for the first time approximately 120 h after infection.

Females usually, but not invariably, contained infertile eggs in the uterus before the first insemination occurred. Fertile eggs were not seen in the uterus until sperm had ascended from the vagina to the seminal receptacle, a process which appeared to take about 3 h.

On average each female in a normal mixed infection was capable of producing about 1000 fertile eggs every 24 h. Females recovered from rats on the 10th day after infection and transplanted into uninfected rats, were capable of producing an average of 3500 fertile eggs each without further insemination. When newly matured females, which had received a single full insemination, were isolated from males they produced on average, between 1000 and 1500 fertile eggs each.

From these observations it was concluded that in normal mixed infections females must copulate at least once every other day and probably once a day, in order to maintain fertile egg output. It was also deduced that females are capable of storing from 4000 to 4500 sperm in the seminal receptacle.

There was clear evidence of reduced reproductive efficiency in both male and female worms recovered from rats during the expulsion phase of the infection.

The author wishes to thank Professor J. J. C. Buckley and Dr J. A. McFadzean for their suggestions, criticism and encouragement and Miss G. Merchant and Mrs P. Counsel for able technical assistance.