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.

Psilochasmus oxyurus, type species of the genus Psilochasmus Lühe, 1909, has been redescribed from whole mounts and transverse, horizontal and sagittal sections. Twenty-five worms from one common mallard, Anas platyrhynchos L. and 12 from one greater scaup, Nyroca marila L. are considered as conspecific with P. oxyurus, although they show some variations in body size and shape of the testes. Of the other eight known species of Psilochasmus, five have already been synonymized and the remaining three, namely, P. indicus Gupta, 1957, P. alii Jaiswal, 1957, and P. megacetabulus Jaiswal, 1957, are declared synonyms of P. oxyurus.

By means of serial sections of experimentally infected Eisenia foetida with Metastrongylus spp. The distribution of larvae within the host has been determined. The exact position in the host intestine at which larvae penetrate has been shown to be the crop. The preferred site for localization and development of the parasite has further been shown to be the lamellar sinuses of the calciferous glands, although larvae were recovered from the dorsal blood vessel, hearts and perienteric blood sinuses. The way in which larvae pass from the gut to their definitive positions in the host has been discussed and it is supposted that this is determined by the configuration of the blood system rather than by a predilection for an environment low in free carbon dioxide. The pathology of infection and the distribution of alkaline and acid phosphatases, lipases, non-specific esterases RNA, DNA and haemaglobin have been described. The distribution of phosphatases in the gut is suggested to be concerned in the uptake of nutrients. The presence of an active zone of enzyme activity in the subcuticular regions of the parasite has been shown and the significance of this discussed. The parasite has been shown to feed directly on host blood.

A simple apparatus for the collection of helminth eggs from faeces by means of a gas-circulated saturated salt solution is described. Nearly all the Nematodirus spp. eggs in both bovine and ovine faeces were recovered and shown to be viable.

The infective larvae of S. ratti had a high Qo2 (9·8 μl/mg dry weight/h at 20 °C). Their R.Q. was 0·74 indicating that lipid was the main metabolite.

As the infective larvae aged there was an exponential decrease in their infectivity and a corresponding decrease with age in the Qo2 rate of activity and rate of substrate utilization. The R.Q. of fresh and aged larvae was the same showing that there was no change in the substrate being utilized.

The infective larvae of S. ratti contained 25% lipid 3% glycogen and 0·25% free sugars (dry weight). The free sugars consisted of glucose and trehalose in approximately equal amounts. Both lipid and glycogen decreased as the larvae aged.

Analyses showed all the normal lipid components to be present, the major lipid fraction detected was free fatty acid. As the larvae aged there was a big decrease in the amounts of triglyceride and free fatty acid. There was, however, no difference in the amounts of cholesterol or in the phospholipid composition of fresh and aged larvae.

The fatty-acid composition of the larvae was analysed; 68% of the total fatty acids were unsaturated and the major fatty acid was C18:1. As the larvae aged, the fatty acids were not all catabolized at the same rate; the acid C18:1 was the most heavily catabolized decreasing by 75% in 8 days, whilst C20:1 decreased by only half this amount in the same time.

The distribution of lipid and glycogen was investigated histochemically. Lipid droplets were confined to the posterior two-thirds of the infective larvae and occurred in the lateral lines, non-contractile parts of the muscles and in the cells of the intestine. As the larvae aged the lipid in the body wall and lateral lines disappeared first. Dark granules, possibly excretory products, accumulated in old larvae. Glycogen was found in the lateral lines throughout the length of the larva.

I should like to thank Dr P. Tate for his constant advice and encouragement. This work was carried out during the tenure of a Medical Research Council scholarship.

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.

An experiment has been conducted to examine the effects of discontinuous infection with H. contortus in sheep on the development of resistance to infection. When sheep were given four doses of 3000 larvae at fortnightly intervals and these sensitizing infections were removed on day 56, there was no evidence of resistance at that time nor of any residual effect of the sensitizing infections on the response to challenge at day 140. When, however, the sensitizing infections were permitted to persist until day 132, there was strong resistance to the establishment of a challenge infection whether or not the sensitizing worm burden was removed prior to challenge, and, in the case of superimposed challenge, resistance was coupled with rejection of most of the sensitizing population. When a single infection with 3000 larvae was followed 22 days later with a further single dose of 3000 larvae, there was no evidence of resistance whether or not the first infection was removed with anthelmintic. If the interval between two doses of 3000 larvae was extended to 76 days there was resistance to the establishment of the second dose if this was superimposed on the first, but not if the first infection was removed 8 days prior to the second dose.

It is suggested that the manifestations of resistance appear in a sequence of (i) resistance only to a superimposed infection, followed by (ii) resistance to a challenge infection in the presence or absence of a previously existing worm burden, coupled in the former case with rejection of much of the existing population. This sequence could be related crudely to a product of larval dose and time. Although larval dose and time were to some extent confounded in the design of the experiment strong resistance was associated with prolonged uninterrupted infection.

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.

The IgM levels in rabbits infected with Trypanosoma gambiense are raised. Agglutinating antibody has been found in the IgM immunoglobulin fraction of sera obtained from these infected rabbits. Agglutinating antibody was first found in the IgM fraction and later in the infection appeared in both the IgM and IgG fractions. Precipitating antibody to blood trypanosome antigens was found solely in the IgG fraction.

On the basis of the data presented here a working hypothesis is developed to explain the raised IgM immunoglobulin levels found in the sera of rabbits and humans infected with T. gambiense.

Regional differences in the tegumentary tissue of Moniezia expansa, as revealed at the light-microscope level by histological and histochemical techniques, are described and evidence for secretory activity by the interproglottidal glands is presented.

In very immature proglottides the interproglottidal glands are at the ‘precryptic’ stage. Gland cells may be differentiated from other tegumentary cells by their high RNA content and in certain gland cells the presence of an alcian blue staining material.

In mature proglottides the glands consist of rosette-like clusters of cells around crypt-like intuckings of the tegument. Two types of cells are found in the gland, small alcian blue-staining cells which are most numerous in the neck region of the crypt, and larger cells, the predominant gland cells, which do not stain with alcian blue but possess non-specific esterase activity. No other tegumentary cells in Moniezia exhibit this activity. Esterase and phosphatase activity is found in the tegument and crypt of the glands and in the interproglottidal folds.

The non-enzyme histochemistry confirms and extends the observations of previous workers.

Cytochrome oxidase and succinic dehydrogenase were detected in the tegumentary cells and tegument. Very strong reactions were given in the neck and scolex, with a progressive diminution of activity posteriorly along the strobila. Very low activities were recorded in the tegument of the glands.

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.

Of 414 Bermuda fishes belonging to 58 species 69 belonging to 20 species were infested with adult or larval cestodes. Five species of adult and five species of larval cestodes were recovered. There are 18 new host records and seven new records for Bermuda. Four of the adult cestodes were found in the cub shark Carcharinus platyodon (Poey). Larval cestodes were found in 66 individuals of 18 species of fishes mainly groupers and rock fishes from the outer reefs and deep water where sharks abound. It is suggested that the restriction of cestodes of Bermuda fishes is due to the isolation of the fishes into the deep water population of the outer reefs and beyond and the shallow water population of the inner reefs.

Acompsocephalum tortum (Linton) was the only cestode from a teleost Synodus intermedius (Agassiz). It has been included in a new family in the Pseudophyllidea but has affinities, also, with the Spathebothridea. It is described and its significance relative to the evolution of the strobilate cestodes is discussed.

Ultrastructural studies on adult Nippostrongylus brasiliensis removed at varying stages after infection from rats given a single dose of 2000 infective larvae have shown that changes in their cytology occur and that these can be correlated with the onset and development of immunity to this nematode in the host. The most noticeable changes are resorption of spermatozoa by the male reproductive tract, a drop in egg production, changes in the cytology of the intestinal cells, and the appearance of many large droplets, which look like droplets of lipid, in the cytoplasm of the hypodermis, muscles, excretory glands, reproductive system and the intestine. There are no obvious changes in the structure of the cuticle of the nematode during the course of infection. Possible reasons for the onset of these changes in the nematode are discussed.

Much of this work was carried out during the tenure of grants from the U.S. Public Health Service (grant number AI 04275) and from the Agricultural Research Council while I was working at the Molteno Institute in Cambridge. I wish to thank the late Professor J. D. Boyd and Dr C. C. D. Shute of the Department of Anatomy, Cambridge, for permission to use the electron microscopes, Mrs B. Fisher for technical assistance and Mr Paul Rogers for assistance with the photography.

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.

When egg-masses of Lucilia illustris, taken from carcasses, were mounted on the body of living vertebrates (Clarius batrachus, Bufo melanostictus, Rana tigrina, Hemidactylus fluviviridis, Columba livia and Suncus murinus), the larvae hatched in the majority of cases under normal conditions of room temperature, and entered into the body of the experimental hosts, ultimately killing them. It appeared that the larvae fed on the brain first before turning their attention to other soft tissues of the hosts. The specimens fed on living tissue had a shorter period of development than those bred on dead animals and the period of development was shortest when the host was a living warm-blooded animal.

Thanks are due to Mr H. Oldroyd of the British Museum of Natural History, London, for kindly identifying the fly. The authors also wish to thank Dr T. N. Ghosh of the Calcutta School of Tropical Medicine, who helped with literature; and Dr S. K. Das Gupta, of the Darjeeling Government College, who helped with suggestions at the time of preparing the manuscript.

The immunogenicity of larval Trichuris muris in Schofield strain albino mice has been studied using infections abbreviated by chemotherapy. When mice were exposed to an initial infection lasting from 7 to 14 days their immunity to reinfection increased progressively, but the immunity stimulated by abbreviated infections was less than that stimulated by uninterrupted immunizing infections which last for approximately 16–18 days. The immune response of mice to T. muris appears to be stimulated by, and effective against, larvae of all ages and does not show stage specificity.

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.