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.

Two new species of Ardeicola Clay, 1925 (Mallophaga, Ischnocera), A. keniensis and A. africanus, ectoparasites of African ibises, Lampribis olivacea akleyorum and L. rara respectively, are described. These species are closely related to each other, and the characters which distinguish them have been enumerated.

We are most grateful to Dr Theresa Clay, Department of Entomology, British Museum (Natural History), for the loan of material and for reading the typescript.

The first full account of the life-cycle of an Eimeria sp. from the pheasant is given. The strain, isolated from pheasant chicks by single oocyst inoculation is identified as E. phasiani. It is shown that there are three distinct asexual stages prior to the onset of the sexual stage.

I should like to express my sincere gratitude to Dr E. U. Canning for her supervision and helpful advice, and to Professor O. W. Richards, F.R.S., for permission to work at the Imperial College Field Station. I am also grateful to Mr T. H. Blank, of the Eley Game Advisory Service, Fordingbridge, Hants, who gave day-old pheasant chicks for this study.

The work was financed by a scholarship from the Agricultural Research Council.

The development and survival of the eggs and larvae of T. colubriformis was studied by spreading the daily faecal output of an infected sheep for 1 or 2 weeks on grass plots 6 ft square. The observations were carried out for 3 years and broadly similar results were obtained in each year. Eggs placed on the ground from November to February failed to develop. A large proportion of the eggs placed on the ground in March disintegrate but those which develop do so slowly. Infective larvae are not produced for 8–12 weeks and they are relatively short lived. As conditions become more favourable development becomes more rapid, taking 8 weeks in April 6 weeks in May and only 1 week in June, July and August. A larger proportion of the eggs develop into infective larvae and their time of survival increases. On most plots larvae did not die out until the following April. The prolonged period of snow during the winter of 1962–63 tended to preserve larvae rather than kill them. The relevance of these results to the control of trichostrongylosis is discussed and it is concluded that it would be impossible under British conditions to devise a system of rotational grazing which would satisfactorily control trichostrongylosis and at the same time ensure economic utilization of the herbage.

We are grateful to Mr C. R. Phillips and Mr T. Doy for technical assistance during the course of this investigation.

A marked difference in the rate and extent of growth of the third-stage larvae of Amplicaecum robertsi was observed in the liver of indigenous Australian rats, as compared with non-indigenous rats. This difference was not correlated with the number of larvae recovered. The suggestion is made that the greater growth in indigenous rodents might be due to the availability or accessibility of a growth factor in the former group, or a manifestation of ‘adaptation tolerance’, an immunological unresponsiveness arising from adaptation in a geographically isolated host-parasite association. Observations on incidence of natural infection supported the hypothesis in relation to Rattus assimilis but not to Melomys cervinipes.

Thanks are due to Dr C. Dobson for help with the presentation of results.

Eimeria phasiani was shown to be pathogenic to young pheasant poults. Doses of 100000 or more oocysts proved fatal to birds up to 3 weeks old. Lesser doses produced losses in the live-weight gain of the host.

Two separate strains of E. phasiani were completely susceptible to 0·006% (w/w) Sulphaquinoxaline, but less so to 0·006% (w/w) Amprolium and 0·0125% (w/w) Zoalene.

I should like to express my sincere gratitude to Dr E. U. Canning for her supervision and helpful advice, and to Professor O. W. Richards, F.R.S., for permission to work at the Imperial College Field Station. I am also grateful to Mr T. H. Blank, of the Eley Game Advisory Service, Fordingbridge, Hants, who gave day-old pheasant chicks for this study.

The work was financed by a scholarship from the Agricultural Research Council.

Cercaria helvetica XXXIII Dubois and the cercaria of Sphaerostoma bramae (Müll.) are redescribed and their metacercariae, which occur in various invertebrates, are described. Additional observations are made on C. parvus Khan and C. tarda Khan and their metacercariae, occurring mainly in arthropods, are described. C. wentloogensis sp.nov. has four pairs of penetration glands, a slender, thickened stylet 0·03 mm long and the flame-cell formula

2[(2 + 2 + 2) + (2 + 2 + 2)].

C. rumniensis sp.nov. has four pairs of penetration glands lying dorsally to the acetabulum, a slender thickened stylet 0·03 mm long and at least 20 flame cells arranged in pairs. C. octoglandulata sp.nov. is a virgulate species with four pairs of penetration glands, a partially thickened stylet 0·029–0·032 mm long and the flame-cell formula 2[(3 + 3 + 3) + (3 + 3 + 3)]. Each of these new species develops in sporocysts in the digestive gland of the prosobranch mollusc Bithynia tentaculata. A cercaria occurring in the digestive gland of B. tentaculata and assigned to the Microphallidae on the structure of its metacercaria is considered to encyst in the digestive gland without emerging. It bears no stylet or penetration glands and the cysts occur in the snail in exceedingly large numbers.

All of these cercariae were found in freshwater molluscs collected at the Wentloog level near Cardiff, South Wales.

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.

A tenfold enhancement of resistance to primaquine was obtained by maintaining a strain of Plasmodium gallinaceum in a state of acute infection by serial passage of infected blood through young chicks treated with gradually increasing doses of the drug.

No loss in resistance to primaquine was observed when the resistant strain was transmitted through mosquitoes, though there was some loss in resistance to the maximum tolerated dose, but not to lower doses, when the strain was maintained in a state of acute infection through untreated chicks for 41 weeks.

The primaquine-resistant strain was cross-resistant to lower effective doses of pamaquin, and slightly less sensitive to quinine than the parent strain but the loss in sensitivity to chloroquine was only marginal. Sensitivity to proguanil, dihydrotriazine and pyrimethamine was normal.

Attempts were made to produce a chloroquine-resistant strain of P. gallinaceum using different doses of the drug but no change in sensitivity was observed though the experiments were continued for more than a year. An attempt to produce a chloroquine-resistant strain by treatment of a proguanil-resistant strain with chloroquine also failed.

The problem of resistance to quinoline compounds in different species of Plasmodium is discussed.

The growth rate of Schistocephalus solidus was measured in culture at 4°, 7°, 12°, 17°, 23°, 27°, 30°, 33°, 35° and 40 °C. Between 7° and 23 °C the rate of growth increased nearly exponentially, with a Q10 of 3·6. Between 23° and 27 °C growth rate was maximal; above 27 °C, after 8 days in culture, weight increase was shown only by worms which were too small to mature. Even in these worms the weight increase was very small and was probably due to partial maturation which occurs from 30 °C upwards. The larger the plerocercoid and the higher the temperature up to 40 °C the bigger the percentage weight loss, due to earlier maturation.

Incubation of plerocercoids for 4 days at 35 °C, although inducing maturation and suppressing somatic growth, did not destroy the ability of the worm to grow when returned to 23 °C.

Two hypotheses to explain the pattern of morphogenesis in Schistocephalus plerocercoids and adults are propounded; the evidence favours the existence of two enzyme systems, one controlling somatic growth with peak efficiency near 23 °C and the other controlling maturation with peak efficiency near 40 °C.

The authors gratefully acknowledge financial support for this work from Shell International Petroleum Company Limited. It is also a pleasure to thank Miss Patricia Grant for technical help.

Acanthocotylid (monogenean) parasites inhabit the skin of rays, and the lack of cilia on the infective larvae of these parasites may be an adaptation to the host's bottom-living habits.

Freshly hatched larvae must remain on the sea-bottom until a ray settles on top of them, when the larvae then have the opportunity to attach themselves to the host's ventral surface. Acanthocotyle lobianchi, which infects Raia montagui and R. clavata at Plymouth, becomes sexually mature on the host's ventral surface and rarely wanders from it, but A. elegans, which is found on R. clavata only, migrates to the dorsal surface of the host before reaching sexual maturity.

The larvae of acanthocotylids use their haptoral hooklets to attach themselves to the host's epidermis, but as the parasites increase in size the load on these hooks becomes acute. This load is relieved not as in other monogeneans by the development of hamuli but by the transformation of the posterior third of the larval body into an accessory ‘pseudohaptor’.

I am obliged to the Director and Staff of the Plymouth Laboratory for laboratory facilities and to Mr J. E. Green for his interest and help on many occasions.

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.

Resistance to E. necatrix induced by complete life-cycle infections was transferable from duodenum to caeca and vice versa.

Complete life-cycle infections with E. tenella protected against subsequent attempts to infect with 2nd-generation merozoites. A similar experiment using E. necatrix showed a partial effect.

E. necatrix gametogony-only infections did not protect against subsequent gametogony-only infections and did not protect against schizogony in the duodenum. They did suppress oocyst production resulting from entire life-cycle infections.

Satisfactory oocyst production from the intra-caecal inoculation of second generation E. necatrix merozoites was not obtained.

Repeated oral infections with E. tenella or E. necatrix reduced subsequent cross-infections with E. necatrix or E. tenella respectively by approximately half. Both schizogony and gametogony stages were affected.

A similar result was obtained with E. tenella oocyst inoculation when previous infections were initiated by the intra-caecal injection of E. necatrix sporozoites.

Oocyst production from the oral administration of E. tenella oocysts to birds previously infected with gametogony stages only of E. necatrix was much reduced but the schizogony stages of the challenge infection were not appreciably affected. Previous repeated caecal inoculation of E. necatrix merozoites did not reduce oocyst production from subsequent inoculations of E. tenella merozoites or of E. necatrix merozoites.

Thanks are due to Mr P. L. Long for his interest and advice. The excellent technical assistance of Mr Brian Millard and other members of the Parasitology Department is also gratefully acknowledged.

The capsule, which surrounds the acanthocephalan parasite Moniliformis dubius in its intermediate host, the cockroach, Periplaneta americana, has been studied in the electron microscope. Some observations were also made with the light microscope.

The capsule surrounding the developing parasite (acanthella) taken from the haemocoel of its host consists largely of closely packed vesicles, which are empty of electron-dense contents.

The vesicles arise from the surface membrane of the insect's haemocytes, which surround the parasite. Long tubular protrusions arise from the haemocyte's surface and, on separating, swell to form rounded vesicles.

The capsule, which surrounds the later resting stage (cystacanth), has apparently been stretched by the growth of the parasite after haemocytic activity has declined. An amorphous, electron–dense component is also evident at this stage.

We are grateful to Mrs A. Baker and Miss R. Jantunen for their technical assistance and to Dr A. C. S. Crossley for his comments on the work.

Experimentally infected female mice and naturally infected female jackrabbits are more susceptible to infection with Taenia multiceps than males. It appears that cortisone has the greatest effect in altering rates in mice when injected between days 1 and 14 following exposure of mice. Cortisone elevates infection rates in male mice but not in females. Cortisone does, however, alter the distribution pattern in a similar manner in both sexes. The biology of coenuriasis in rabbits and mice is discussed and compared.

I wish to thank Dr J. R. Egerton of the Merck Institute, Rahway, New Jersey, for supplying cortisone used in this study. I also thank Dr Robert Sullivan and Dr Ralph Amen for their criticisms and suggestions in the preparation of this manuscript.

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.

The oncomiracidium of the monogenean skin parasite Entobdella soleae finds its flatfish host, Solea solea, by chemoreception. The free-swimming larva responds to a specific substance secreted by the skin of the common sole, attaches itself to this skin and immediately sheds its ciliated epidermal cells. Larvae respond in the same way to agar jelly which has been in contact with the skin of S. solea.

The oncomiracidia attach to S. solea skin in preference to that of other soleid fishes (Buglossidium luteum and Solea variegata), pleuronectid fishes (Limanda limanda, Pleuronectes platessa) and elasmobranch flatfishes (Raia spp.).

Larvae respond strongly to isolated epidermis from Solea solea but show no response to the fish's cornea, indicating that the attractive substance is produced by the mucus cells in the fish's epidermis.

The larvae attach with equal readiness to skin from the upper and lower surfaces of S. solea. Thus the preponderance of young parasites on the upper surfaces of soles is due not to a preference for the upper skin but to the fact that the lower skin is in contact with the substratum and cannot be reached by the larvae.

These results led to speculations on the way in which host specificity evolved in the Monogenea.

I am indebted to Mr J. E. Green of the Plymouth Laboratory for setting up a tank containing infected soles and for maintaining the tank and feeding the fishes for many months.

I am also grateful to the Directors and Staff of the Plymouth Laboratory and the Fisheries Laboratory, Lowestoft, for hospitality and assistance. I am particularly grateful to Mr J. Riley of the Lowestoft Laboratory for providing various flatfishes.