Sheep infected with Oesophagostomum columbianum showed anorexia, loss of weight, diarrhoea and macroscopically obvious oedema and inflammation of the alimentary tract. Arthus-like lesions caused by the larvae developed particularly in the large intestine.

The gross immunological responses of the sheep were associated with antibodies in greater titre within the mucus of the gut than in the serum. Acquired immunity was manifest through an antibody-antigen reaction which was localized mostly in the large intestine and which involved proliferation of the lymphoid tissue of the gut and an increased synthesis of protein. Lower titres of antibody in serum than in mucus may indicate leakage of antibody away from the gut. The direct effects of antibody were established by the respiratory inhibition of the 3rd-stage larvae. The presence of antibody was associated with particularly high concentrations of β globulin after first and β and γ globulins after second infection. Specific antibody was demonstrated in these protein fractions by blockage of electrophoretic movement by larval antigen in the buffer.

The antibody-antigen reaction may act as a trigger which releases the nonspecific effects of excess mucus production and oedema which causes mechanical dislodgement of the infection. The natural and specific respiratory inhibitory effects of mucus on the worm would facilitate this dislodgement process.

The greatest cellular reactions occurred where there was an active infection. Usually these reactions were associated with oedema, hyperaemia and an increase in the size of the mesenteric lymph nodes.

A new coccidium Caryospora demansiae sp.nov. is described from the Australian snake Demansia psammophis (Elapidae). This is the first report of this genus from Australia. A host check-list of the genus Caryospora is given and the characters of the known species are tabulated.

I would like to thank Mr O. Kelly and Mr J. Winter of the Zoology Department, University of Queensland, for help with supplying the snakes and to Dr H. M. D. Hoyte of the Parasitology Department, University of Queensland, for his careful reading of the manuscript.

Three new species of Isospora from Australian lizards are described, viz. Isospora amphiboluri from Amphibolurus barbatus (Agamidae), I. egerniae from Egernia whitii (Scincidae), and I. ablephari from Ablepharus boutonii (Scincidae) and Heteronota bineoi (Gekkonidae). A further Isospora sp. is mentioned from Phyllurus cornutus (Gekkonidae). A table of comparative data of lacertilian Isospora is given, and also a host check-list.

I would like to thank Miss J. Arnold of the Department of Zoology, University of Western Australia, for her identification of many of the lizards, and Miss S. Curtis for her drawings. Finally I wish to thank Dr H. M. D. Hoyte, Department of Parasitology, University of Queensland, for his interest, and helpful criticism of the manuscript.

Five new species of Eimeria are described, viz. E. ablephari from Ablepharus boutonii (Scincidae), E. egerniae from Egernia whitii (Scincidae), E. gehyrae from Gehyra variegata (Gekkonidae), E. lampropholidus from Lampropholis guichenoti and E. leiolopismatis from Leiolopisma challengeri. The characters of these species and the other coccidia from lizards are tabulated and a host check-list is given. Gall bladder parasitism is discussed as is the significance of complete sporogony occurring within the host's body. Some ideas concerning the possible events occurring in the evolution of the Haemosporina are presented.

I wish to thank Miss J. Arnold of the Zoology Department, University of Western Australia, for her identification of many of the lizards and Miss S. Curtis for her drawings. Finally I wish to thank Dr H. M. D. Hoyte, Department of Parasitology, University of Queensland, for his interest and helpful criticism of the manuscript.

The ultrastructure of the intra-erythrocytic stage of a haemogregarine in the gecko, Gehyra variegata, has been studied and numerous points of similarity to Plasmodium, Toxoplasma, Sarcocystis, Lankesterella, Eimeria and the M-organism were found. The protozoon was invested by a triple-layered pellicle, and possessed at least two micropyles. A conoid and apical rings were observed at the anterior end. Other cell constituents included nucleus, microtubules, endoplasmic reticulum, mitochondria, toxonemes and a variety of vacuoles. Loosely surrounding the parasite was an unusual convoluted membrane, which in appropriate planes of section, displayed corrugations on both surfaces. It was not clear whether this membranous structure should be regarded as an integral part of the protozoon or as an external coat which could be discarded upon emergence from the host red cell. Within its external sheath, the parasite lay in a periparasitic vacuole which was limited by a conventional unit membrane from the host red cell cytoplasm. The parasite did not exhibit any evidence of pinocytosis or phagocytosis.

Our sincere thanks are due to Dr E. H. Mercer, Professor J. D. Smyth and Dr M. J. Mackerras for advice. Miss N. Carroll gave technical assistance.

Observations have been made on the frequency distribution of strongyloid infective larvae in random samples of pasture herbage collected on four occasions from a quarter-acre field grazed by five sheep carrying mixed strongyloid infections. The distribution of numbers of infective larvae of the genera Haemonchus, Trichostrongylus and Cooperia in each of the samples has been examined. The variance of each series was found to be substantially larger than its mean and, hence, the distributions were all highly overdispersed statistically. Of a number of frequency distribution models, the ‘truncated lognormal’ was found to provide the best empirical fit to the sample distributions.

The implications of these findings for determining the significance of the difference between two random sample estimates of the abundance of infective larvae in pasture, have been discussed. It has been shown that, because of the limit imposed by technical considerations on the maximum size of herbage samples, and because the distribution of infective larvae in pasture departs markedly from a random one, only quite large differences between the means of two random sample estimates are likely to be significant. However, because the numbers of infective larvae of the different strongyloid genera are highly correlated within sample units, considerably smaller differences between the means for different genera, within the same sample, can be detected as significant.

From a consideration of the complexity of the biological components generating populations of infective larvae on pasture, it has been concluded that simple frequency distribution models can contribute little towards an improved understanding of the population dynamics of strongyloid infective larvae in pastures grazed by sheep.

Meiogymnophallvs minutus (Cobbold, 1859) comb, nov., from Cardium edule L. and Haematopus ostralegus occidentalis Neumann is shown to be synonymous with Lecithodendrium somateriae (non Levinsen, 1881) Jameson, 1902 in partim, Cercaria margaritae Lebour, 1907, Oymnophalloides oedemiae (Jameson & Nicoll, 1913) James, 1964, Cercaria cambrensis Cole, 1938, and Gymnophallus nereicola Rebecq & Prévot, 1962. The intensity of infestation of cockles, with the metacercariae, increases with the size and particularly with the age of this host and varies with the seasonal migrations of the final host. The connexion between the metacercaria and the adult is demonstrated experimentally. Domestic ducklings used as experimental hosts were susceptible to infection when fed with minced cockles but not when fed on baby chick crumbs.

We are very grateful to Professor E. W. Knight-Jones for the provision of working facilities and for help with the manuscript and to the Science Research Council for a grant to one of us (E.A.B.) which made the work possible.

Infections are described of Gasterosteus aculeatus in a pond at Leeds with the plerocercoid larvae of Schistocephalus solidus.

Very heavy infections were found in the summer of 1962 and early 1963; many fish harboured over 50 worms and the maximum was 130.

For each individual host the relative percentage parasite-weight to host-weight has been determined (parasitization index). In more than 50% of the fish the P.I. exceeded 25; the maximum value was 68·5. The relationship between P.I., numbers of worms and weight of worms is described.

Some effects of parasitization on the host are described; they include gross distension of the body, reduction of liver weight, reduction of packed cell volume of erythrocytes and delay in oocyte maturation. In some heavily parasitized individuals spawning is apparently inhibited.

Concurrent infections with the microsporidian Glugea anomala are described.

Our thanks are due to Mr A. Rennie, B.Sc., of the Yorkshire Ouse and Hull River Authority for drawing our attention to the source of infected sticklebacks, to Mr A. O. Holliday for the preparation of photographs and to Mr G. R. Standley for assistance in the preparation of the slide shown in PI. 1, fig. 3. We also wish to thank Dr J. N. Ball, Department of Zoology, University of Sheffield, for his helpful comments on the gonads of infected fish, and Professor J. M. Dodd, Department of Zoology, University of Leeds, for providing facilities and much helpful discussion. One of us (C. A.) gratefully acknowledges the receipt of a Science Research Council Research Studentship and Fellowship.

The humidity reaction of the larvae of Xenopsylla cheopis Rothsch. was studied in an air-chamber with a 0–100% relative humidity gradient.

Highly significant differences (P < 0·001) between the reactions in the various R.H. ranges was found. The most preferred relative humidity along this gradient was 100%. Relatively few larvae were collected at each R.H. in the range between 0 and 90% R.H.

The reaction of the larvae under the experimental conditions was seen to be of orthokinetic type.

Methods are described for the freezing of trichomonads, suspended in a liverinfusion broth, to – 20 °C and – 70 °C, and storage at these temperatures. After 9 months’ storage at – 70 °C, with dimethyl sulphoxide, six strains of T. vaginalis and three strains of T. foetus were alive; three trichomonas strains frozen earlier were also alive after 13 months. Four of the T. vaginalis strains survived equally well for 6 months at – 70 °C, in the presence of either dimethyl sulphoxide or glycerol.

Although trichomonads may be maintained by subculture in a modified Diamond medium, storage at – 70 °C is more economical and less tedious; – 20 °C is not a satisfactory storage temperature for T. vaginalis or T. foetus, even in the presence of dimethyl sulphoxide or glycerol.

The severity of subcutaneous lesions in mice produced by six freshly isolated strains of T. vaginalis, and two out of three stock culture strains of T. foetus, was enhanced by either the addition of agar to the suspending medium or administration of cortisone to the mice. One strain of T. foetus produced lesions only in the presence of agar, cortisone, on the dose schedule used, being ineffective.

After storage at – 70 °C or cultivation in agar medium for 6 months, the virulence of trichomonads of either species was little affected, and lesions in mice were enhanced by agar in a similar degree to that found originally.

For enhancing the severity of subcutaneous lesions in mice the agar method, involving only one injection, is simpler than the repeated administration of cortisone to infected mice; agar was shown to be more effective than cortisone with small infecting doses of one strain of T. vaginalis quantitatively tested.

The subcutaneous injection of mice with trichomonads suspended in a liver-infusion broth containing 0·06% agar regularly produces lesions of the degree of severity required for the evaluation of compounds for therapeutic activity.

We express our gratitude to Mr S. A. Price for constructive criticism and helpful suggestions during the preparation of the manuscript. Dr S. R. M. Bushby kindly provided details of the dimethyl sulphoxide method and apparatus in use in his laboratory for the low-temperature preservation of tumour cells and trichomonads. For supplying the trichomonas strains we are grateful to Dr L. P. Joyner, Miss M. Moore-Tucker and Mr C. S. Ledwich. We thank Mr W. A. Freeman for advice on the composition of the inoculum medium.

Rats thymectomized at birth were infected with Nippostrongylus brasiliensis at 6 weeks of age. In some, but not all, rats egg production was greatly prolonged after the primary infection. Thymectomized rats were less resistant than controls to second infections.

In rats infected at birth the development of resistance was significantly delayed. Neonatally infected rats, when they were 6 weeks old, were less resistant to reinfection than adult controls. Rats infected three times in the neonatal period were more resistant to challenge at 6 weeks of age than rats infected once at birth, but less resistant than adult controls. The degree of immunity induced by a single neonatal infection was variable; some rats showed little or no resistance to reinfection 6 weeks later.

Antibody formation, measured by the occurrence of reagins, was altered in the same way by both neonatal thymectomy and neonatal infection. In the thymectomized (immunologically deficient) and newborn (immunologically immature) rats, reagin formation following initial infection was delayed; there was a reduced anamnestic response after challenge; mean titres were lower and more variable than in control animals; and in some rats circulating reagins were never detected.

In thymectomized rats the capacity to demonstrate anaphylaxis was as variable as in control rats, but in the few thymectomized rats in which egg production was prolonged, anaphylaxis was slight at a time when most intact rats had developed the capacity to show anaphylaxis. Rats infected at birth and adult rats infected at the same time were equally susceptible to systemic anaphylaxis 6 weeks later, although by this time there was a marked difference in the degree of resistance to the parasite developed by the two groups.

In adult rats splenectomy before, during or after initial infection had no effect either on the rate of development, or the degree of immunity induced by primary or secondary infections, or on antibody formation.

We would like to thank Miss W. Griffiths and A. J. Edwards for their willing technical assistance. This work was done whilst one of us (B.M.O.) was the holder of a Wellcome Fellowship of the Animal Health Trust.

E. praeox has been isolated in Britain for the first time and is similar in regard to its oocyst size, prepatent time, life-cycle and the development of host immunity to the reports of Tyzzer et al. (1932). Oocysts had mean dimensions of 20·4 × 17·45 μm and the first oocysts were discharged 83½ h after infection.

Oocyst production and life-cycle studies suggest that at least three generations of schizogony precede gametogony and that at least one further generation of schizogony is needed to explain the oocyst reproduction resulting from a small dose of oocysts. Young chickens 1½–3 weeks of age were not such suitable hosts as older chickens (6 weeks) and the reproduction of the parasite was seriously reduced when large numbers (e.g. 105, 106) were used to infect chickens. A high degree of resistance to reinfection occurred after a single exposure to infection.

Sporozoite infections of the caeca or cloaca resulted in the infection of the usual small intestine site; there was no development at the site of inoculation and attempts to infect the embryo allantois did not succeed.

I wish to thank Dr M. Elaine Rose for the surgery required for the caecal infections, for her interest throughout and for help with the manuscript; Dr R. F. Gordon for help with the manuscript and permission to publish and Mr B. J. Millard for skilled technical assistance.

Fowls were completely resistant to a third inoculation with E. maxima oocysts given after two graded and spaced infections. Resistance was complete to a fourth inoculation of E. brunetti oocysts after a series of three infections.

Birds made resistant to E. maxima and then inoculated with E. brunetti showed very little evidence of this infection when the intestines were examined on the sixth day and they passed very few oocysts in the faeces. The resistance of E. brunetti-immunized chickens to E. maxima infection was not quite so strong. The results indicate that, under the conditions of the experiment, infection with either of these species will protect in some degree against subsequent infection with the other. These results suggest that the species specificity of acquired resistance is not so complete as previously reported.

I am indebted to my colleague, Mr P. L. Long, for his interest throughout the experiment and, in particular, for assistance with the examination of the intestines. The excellent technical assistance of other members of the Parasitology Department is also gratefully acknowledged. Mr Paul Rogers took the photographs.

The anatomy and function of the pharyngeal ridges, which occur in the larvae of saprophagous Cyclorrhapha but are generally absent in biontophagous forms, were investigated.

The ridges extend throughout the length of the ventral aspect of the pharynx and lie in an evagination of its floor. They enclose channels which narrow and disappear posteriorly but are open anteriorly. Each ridge consists of a longitudinal plate projecting dorsally from the floor of the pharynx, the bifurcated upper edge of the plate bears, one on each side, a row of lateral lamellae, each about 0·3 μm thick. They are set vertically at regular intervals of just less than 1 μm. The lateral lamellae of adjacent ridges are closely apposed and completely roof over the channels between the ridges. Anteriorly the upper arms of the ridges are fused together.

The lamellae form a sieve which retains particles of 0·6 μm and above in size as the relaxation of the pharyngeal dilator muscles allows the roof to descend so forcing liquid downward between the lamellae. In Calliphora vicina (Syn. C. erythrocephala) the sieve concentrates a suspension of 2700 particles of 4–7 μm per mm3 between five and six times. The natural food consists largely of bacteria.

Fungus spores and pollen grains of selected sizes were used to demonstrate the movements of particulate material in the pharynx and to define the upper size limit of the particles ingested.

Swallowing is effected by simultaneous relaxation of the anterior dilator and transverse muscles of the pharynx, with contraction of the posterior dilator muscles; the transverse muscles are otherwise maintained in a state of contraction closing the posterior exit from the pharynx.

Starved third-instar larvae of C. vicina can ingest 50% of their body weight in 6 min.

The larvae of the Cyclorrhapha which possess pharyngeal ridges are essentially particle feeders. The ridges form a sieve by means of which food is concentrated and the amount of non-nutritious liquid ingested is reduced. The lack of pharyngeal ridges in larvae feeding on living material is correlated with the constant and high nutritive value of both the solid and the liquid fractions of the food.

I should like to thank Dr P. Tate for his help and encouragement during the course of this work which was carried out during the tenure of a Postgraduate Studentship awarded by the Agricultural Research Council. Thanks are also due to the staff of the Plant Pathology subdepartment of the Botany School, Cambridge, for their help in obtaining fungus spores of suitable sizes.

The haemocytes of Gammarus, which are part of the environment of P. minutes, have been found to react by encapsulation, against a variety of foreign bodies, wounded tissues and interspecific transplants. Cuticular wounds appear to present the most urgent stimulus to the haemocytes. The haemocytic reaction is not suppressed by X-irradiation. Differences in haemolymph salt concentration between G. pulex and G. duebeni do not appear to be responsible for the failure of transplants of tissue from G. pulex to G. duebeni. It is concluded that the haemocytic reaction of Gammarus is very similar in response and nature to that of insects.

Healthy P. minutus does not stimulate the haemocytic reactions of its hosts, but wounded or damaged parasites are encapsulated. The capsule of connective tissue, which surrounds the parasite during its development in the haemocoele of the intermediate host, appears to be essential for the parasite's survival.

I am most grateful to Dr P. Tate for helpful discussions, and Dr George Salt, F.R.S., for his advice and criticism of the manuscript. I also wish to thank Drs H. Laser and B. E. B. Moseley for irradiating the amphipods for me.