In the present experiment the response of sheep to a single infection with Haemonchus contortus at graded doses, and their responses to two subsequent infections with 3000 infective larvae, were studied. The response of the sheep was investigated by estimation of faecal worm egg counts on alternate days throughout the course of the experiment and by following the development of the parasite by differential worm counts performed on animals slaughtered at crucial times.

Initial infections were established with graded doses of 500–2700 infective larvae. The proportion of infective larvae which became established at each dose, development of the worm burden and fecundity per adult worm were similar for each dose.

Following first challenge infection with 3000 infective larvae there were no effects which could be related to the previous level of infection. Whereas there was no difference in egg production per female worm, a small but consistent increase in the number of retarded larvae was observed in the previously infected animals (6% of the burden was in the 4th larval stage at day 56) compared with the previously uninfected control animals (2% in the 4th stage at day 56).

Following a second challenge with 3000 infective larvae an increase in the number of retarded larvae was again observed, and, in addition, egg production per female worm was reduced compared with egg production in previously uninfected sheep given a similar challenge infection.

The results of the initial infections suggest that control of the growth and development of H. contortus infection by factors independent of an active host response does not occur in worm burdens established from doses as high as 2700 infective larvae.

From lizards caught in the Codajaz region, Amazonas, Brazil, the following malaria parasites are described:

Plasmodium rhadinurum Thompson & Huff, 1944 from Iguana iguana L.

A species of Plasmodium from Ameiva ameiva L.

A species of Plasmodium from Cnemidophorus lemniscatus (Daudin).

I wish to express my thanks to the Instituto Nacional de Pesquisas da Amazonia (I.N.P.A.) at Manaus for their help in the collection of material; to Professor P. C. C. Garnham, C.M.G., F.R.S., for assistance in identification, and to Mr P. E. Nesbitt for advice on staining procedures. This work was carried out during the tenure of a Medical Research Council Scholarship for Training in Research Methods.

Arborocystis nov.nom. is proposed to replace Dendrocystis Rees, 1962, a junior homonym of Dendrocystis Bather, 1889.

The authors wish to thank Dr David L. Pawson, Division of Marine Invertebrates, Smithsonian Institution, Washington, D.C., for furnishing data from Barrande (1887).

The male and female of Rhipicephalus reichenowi are re-described and the nymph and larva are described for the first time. The affinities of the species within the genus are discussed.

Its developmental periods under laboratory conditions are given and all known records of its hosts and distribution in the field are noted.

The author wishes to thank the Director of the East African Veterinary Research Organization for permission to publish this paper. She is also most grateful to everyone who submitted field collections of this species to her and to Dr Hoogstraal for permission to quote unpublished information; to Dr G. Theiler for her helpful suggestions during the preparation of the manuscript, and to Mr G. M. Kohls, Dr C. M. Clifford and Dr H. Hoogstraal, who also checked the manuscript before publication.

A comparison has been made of the attenuating effect on N. brasiliensis larvae of X-rays (HVL, 8 mm Al) and γ-rays from 60Co over the range 10–80 kr. In the dose range where attenuation was manifested as a decrease in total worm burden the γ-rays had an RBE (relative biological effect) of 0-7 relative to the X-rays as unity.

Because of the enhancement of infectivity occurring at low radiation doses, attenuation in this region is better assessed in terms of alteration to the sex-ratio of the parasite rather than effect on total population.

This work was supported by grants from the Agricultural Research Council and the Horse Race Betting Levy Board, and was carried out during the tenure by one of the authors (T. Kassai) of a British Council exchange scholarship.

Thanks are due to Dr T. Martin Young and Mr J. S. Orr of the Radiotherapy Department, Western Infirmary, Glasgow, for access to radiation sources and helpful advice.

The nervous system of Acanthobothrium coronatum consists of paired bilobed cerebral ganglia in the scolex joined by a transverse and a dorsal and ventral commissure, and a number of longitudinal nerve cords joined by ring commissures from which arise delicate nerves supplying various organs.

Bipolar neurons are present in the transverse commissure and scattered throughout the longitudinal nerves in the strobila.

Multipolar neurons occur outside and in close association with the nerve cords.

A band of ganglionic cells lies on the inner side of the lateral nerve cords in the scolex.

Binding cells are present around the longitudinal nerve cords and ring commissures in the strobila.

Stretch receptors are present in the anterior half of the scolex in association with the powerful muscles moving the bothridia and hooks.

It is established that the genera Amira Lane, 1914, and Amiroides Strand, 1929, are synonyms of the genus Khalilia Neveu-Lemaire, 1924.

The two valid species, K. sameera and K. pileata, are redescribed.

K. sameera is designated as the type-species of the genus.

K. buta and K. rhinocerotis are considered to be synonyms of K. sameera, whilst K. straelini is considered to be a species dubia.

I wish to express my thanks to Dr W. G. Inglis, British Museum (Natural History), for his advice and criticism, and to Mr S. Prudhoe, British Museum (Natural History), for reading this paper in manuscript.

I would also like to thank Dr P. L. G. Benoit, Musée Royal de I'Afrique Centrale, Tervuren, Belgium, for the loan of material used in this study, and to Dr R. P. Lee, University College Dublin, Ireland, for the donation of specimens of K. sameera.

Dark-adapted third-stage Trichonema larvae were orthokinetically stimulated by light. The response soon reached a maximum and then gradually decreased, the larvae then being inactive as long as they were kept in light. The rate of response was independent of intensity, and was a constant for any one age. Inactive larvae were activated by a mechanical stimulus, and were therefore only insensitive to light.

The larvae needed 3 h adaptation in darkness before responding fully to a further light stimulus. Dark-adaptation required continuous darkness; larvae exposed to a flashing light responded as if in continuous light. The presence of a light-sensitive mechanism is postulated.

I thank Professor B. G. Peters for discussion of this work, and for reading the manuscript.

The biology and life-cycle of Holostephanus lühei is briefly described. The similarities in morphology between the adhesive organ gland cells of H. lühei and Cyathocotyle bushiensis are stressed. Alkaline and acid phosphatases were demonstrated in the adhesive organ gland cells and cuticle. Acid phosphatase also occurs in the caecal cells. Non-specific esterase, sensitive to E600 10−5M and Mipafox 10−3M, is present in the gland cells. The caeca contain a non-specific esterase sensitive to AgNO3 10−2M and PCMB 10−4M. Leucine aminopeptidase is present in the gland cells. The in vitro studies confirmed that the secretory products pass to the exterior of the parasite and have a histolytic action.

The four strigeids studied, Cyathocotyle bushiensis, Holostephanus lühei, Diplostomum spathaceum and Apatemon gracilis minor, are compared and the biological role of the secreted enzymes are discussed.

My sincere thanks are due to Dr D. A. Erasmus for suggesting this line of investigation and for his encouragement during the study. I am also grateful for his performing, on my behalf, the infection experiments and criticizing the manuscript. I wish to thank Professor J. Brough for his interest in the progress of the work, and the University College, Cardiff, for a grant from William E. Morgan's Bequest.

Reduction of neotetrazolium in the presence of succinate is used as a quantitative assay for succinic dehydrogenase activity in homogenates of Hymenolepis microstoma, and nitro blue tetrazolium and thiazolyl blue tetrazolium-cobalt are employed with cryostat sections for its histochemical localization in adults and cysticercoids. The highest concentrations of the enzyme occur in the anterior region of the worm (scolex, neck, and immature proglottids), lesser amounts in the region of mature and early gravid segments, and least amounts in the gravid region of the strobila. Primary sites of succinic dehydrogenase activity are the mitochondrial layer of the cuticle and the cells of the subcuticle. In the anterior region the entire parenchyma shows uniformly high concentrations of enzyme activity. This reaction decreases posteriorly until, in the gravid region, only the subcuticle and the mitochondrial layer of the cuticle display activity. Intense activity was also noted in the layer just under the shells of the eggs. The ovary and immature testes show high enzyme concentrations; however, in the testes, the concentration diminishes as the organs mature. In the scolex, the rostellum and the muscles of the suckers show higher enzyme activity than the surrounding tissues. The scolex, inner membrane, and tail of the cysticercoid show uniformly high concentrations of formazan.

Some callus tissues induced by 2,4–D and derived from plants resistant to plant-parasitic nematodes lost their resistance to the nematodes. Red clover callus supported a large population of A. ritzemabosi, whereas red clover seedlings did not. Six races of D. dipsaci were cultured on lucerne and red clover callus tissues and reproduced rapidly, although races usually multiplied more on callus from susceptible than from resistant plants. A. ritzemabosi, normally only a foliage parasite, reproduced equally well in stem and root callus. H. rostochiensis did not reproduce in callus culture.

Nematodes multiplied most in callus that grew fastest; both reproduction of A. ritzemabosi and the growth of callus were greatest with 0.125 mg/1 of 2,4–D. Reproduction was inhibited by 5.0 mg/100 ml of 2,4–D. 2,4–D influenced nematode reproduction indirectly by making callus, which provides a better environment for nematode feeding and reproduction.

Nematode extracts added to an agar medium caused callus formation on red clover seedlings and nematodes feeding on these tissues reproduced faster than on normal seedlings. Hence, the substances secreted by a nematode into a plant may act on the tissues in a manner similar to 2,4–D. The host–parasite relationship is probably partially controlled by the host's plant-growth substances and the effect on them of the nematode's secretions.

We thank Mr C. T. Drakes for assistance, and Mr K. Smith of Imperial Chemical Industries for advice with the initial callus culture.

Investigations on the host specificity of plerocercoids of Schistocephalus solidus were carried out using the technique of surgically transferring plerocercoids from the body cavity of Gasterosteus aculeatus to various other fish. Plerocercoids survived in all cases when transferred from G. aculeatus to other G. aculeatus; when tranferred to Pungitius pungitius the worms survived for long periods but failed to grow. Plerocercoids transferred to Coitus gobio, Nemacheilus barbatula, Phoxinus phoxinus, Salmo trutta, Coregonus clupeoides, Perca fluviatilis, Rutilus rutilus and Esox lucius always died within 2–10 days after being transferred. Electron-microscopic examinations of the tegument of plerocercoids transferred to new hosts showed: in G. aculeatus normal appearance throughout the experiment; in P. pungitius degeneration of the microtrichs after 6 days; and in S. trutta complete destruction of the tegument in 7 days.

Plerocercoids of the genus Diphyllobothrium survived the transfer from Gasterosteus aculeatus to Salmo trutta and continued to grow in their new host.

Infection of fish with S. solidus by feeding infected copepods and by aspetic injection of procercoids into the body cavity of the fish were also tried. Gasterosteus aculeatus became infected using both these methods but it was not possible to infect Pungitius pungitius.

The main adhesive apparatus of Diplectanum aequans is described and a functional interpretation is offered.

Two sets of organs are involved in adult animals: 4 hamuli (median hooks) and the 3 associated accessory bars, and 2 squamodiscs.

The two pairs of hamuli are ‘gaffed’ into the gill tissue of the host by the following mechanism: muscular contraction produces a posterior movement of the anterior ends of the hamuli which are so pivoted that the points of the hamuli are caused to extend out from the haptor.

The squamodiscs of adults are operated by extrinsic muscles which produce posterior movement in the anterior region of each squamodisc. This tends to push the parasite out from its position between adjacent secondary gill lamellae.

The tendency to dislodgement is opposed by the two pairs of hamuli which ‘gaff’ into the base of the secondary lamellae. In addition, the more posterior spines on the squamodiscs become embedded in the gill tissue and so provide further resistance to the movement of the anterior region of the squamodisc. Thus the attachment of adults requires both sets of adhesive organs functioning in a complementary manner.

Juveniles do not possess squamodiscs. They are considered to adhere to their hosts primarily with their hamuli.

I would like to acknowledge the help given to me by the Director and Staff of the Marine Biological Association Laboratory and also the Proprietors of Cook and Sons Ltd., of Salcombe, Devon.

Single infections of Trichinella spiralis of 48 h duration did not produce a significant degree of immunity in mice.

Single or triple light infections of 7 h duration which, it is suggested, were composed entirely of fourth-stage larvae, did not produce significant immunity.

Single infections of 3 or 4 days duration did produce immunity. It is suggested that the fourth-stage intestinal larvae and the fourth moult are not the most important sources of immunizing antigen in a T. spiralis infection.

I would like to express my gratitude to H.M. Treasury and to the Agricultural Research Council for financial support during the time that this work was being conducted. I would also like to thank Professor W. I. B. Beveridge, within whose department this work was conducted, for the facilities provided. The helpful advice of my colleagues R. M. Connan and R. S. Phillips was of great value during the time that this manuscript was being prepared for publication.

The chemical and physical nature of monogenean attachment sclerites has been investigated by chromatography, histochemistry, solubility studies and X-ray diffraction, and these sclerites have been shown to be scleroproteinaceous and not, as the bulk of the literature suggests, chitinous in composition.

Hooks sclerites (i.e. marginal hooks and hamuli), which are the most primitive type of monogenean attachment sclerites and arise early on ontogeny, have been shown to be keratinous; clamp sclerites, accessory bars and the spines arming the pseudohaptors and squamodisks of certain genera which appear to have been acquired later in the phylogeny of the group to supplement the efficiency of the haptoral hooks, contain no cystine sulphur.

While the chemical nature of the non-cystine-containing skeletal structures has not been fully elucidated, clamp substance was found to differ from that of the other scleroproteins with which it was compared, namely sclerotin, vertebrate elastin, reticulin, collagen and keratin. Clamp substance does not appear to be quinonetanned; acid proteins maybe important in the stabilization of this scleroprotein.

Evidence for the occurrence of a variety of S–S bearing scleroproteins throughout the invertebrates has been collated and the problems involved in adequately defining a keratin have been discussed. The implications of the apparent affinity of monogenean hook protein to vertebrate keratins, which in general are intracellular and epidermal derivatives have been considered and some speculations have been made about the possible differences in the development of hook and clamp structures.

Contrary to the prevalent opinion that monogenean sclerites are ‘cuticularizations’, evidence has been presented that these structures are in fact deeply seated and unlikely to be derivations of a superficial cuticle.

The chemical differences elucidated between the median and lateral sclerites of the chimaericolid adhesive organ have encouraged speculation on the evolution of the different types of attachment organ throughout the polyopisthocotylineans.

Outside the Monogenea, studies on the sulphur content of digenean spines and stylets and on the larval hooklets of the ‘cestodarians’ Gyrocotyle, Amphilina and Archigetes, and of eucestodes, have helped to substantiate the view that the cestodes and monogeneans are more closely related to each other than either of these groups is related to the digeneans.

I wish to thank the Director and staff of the Plymouth Laboratory and in particular Mr J. E. Green, for invaluable assistance. I am also indebted to Dr R. W. H. Small for his guidance on X-ray diffraction technique, to Dr J. Morris of the M.R.C. Skin Unit for his advice on the chromatography and to Dr J. Llewellyn for his interest and helpful criticism throughout.

The response of sheep given six sensitizing infections each of 3000 infective larvae of H. contortus at fortnightly intervals to a challenge infection of 3000 infective larvae has been studied. While a large proportion of the challenge infection (ca. 3000 parasites) persisted to termination of the experiment on day 141 in sheep (group 3) in which the challenge was superimposed upon the sensitizing infections, only a mean of 437 worms, including both 4th-stage larvae and adults, was recorded at this time in another group (group 5) in which the sensitizing infections were terminated by anthelmintic treatment on day 90, i.e. 8 days before challenge. The proportion of the total worm burdens of the challenge infections arrested at the 4th larval stage was increased for both groups (25% for group 3 and 18% for group 5) by comparison with unsensitized control groups (4% for group 6 and 2% for group 7).

As the effect of immunological control was expressed more vigorously in the group (group 5) in which the sensitizing infections were terminated before challenge it seems likely that recovery of immunological competence occurred in these animals following removal of the sensitizing infections. This conclusion suggests that, owing to the persistence of the sensitizing infections and therefore exposure of the immune mechanism to continuous antigenic insult, the animals which received the superimposed challenge infection had entered a phase of immunological exhaustion during the challenge period.

Besnoitia sauriana sp.nov. is described from the tissues and organs of Basiliscus vittatus, with the type locality Baking Pot, British Honduras. It is placed in a new family, Besnoitiidae in the order Toxoplasmea.

The proliferative stage is thought to take place in the alveoli of the lungs. The cystic stage begins as the greatly altered host cell, containing a single round or oval body. The host-cell membrane becomes greatly thickened, the nuclei multiply and enlarge and the cytoplasm assumes a granular texture. The parasites (‘spores’) multiply by binary fission, enclosed in a fine membrane, within this cell, which eventually they entirely fill.

The ‘spores’ easily contaminate the blood and may be mistaken for Schellackia or Toxoplasma in blood smears of infected lizards.

The free-living phase of the life-cycle of Nematodirus helvetianus was studied out of doors on grass plots and the effects of temperature and humidity on the free-living stages were studied in the laboratory. The results of these observations are discussed in relation to the bionomics of N. battus and N. filicollis, and tentative deductions made regarding the epidemiology of N. helvetianus infection.

I wish to thank Mr J. Bailey for technical assistance.

The monogenean parasite Tetraonchus monenteron attaches itself to the gills of the pike (Esoz lucius) by means of two pairs of large hooks (= hamuli). One pair of hooks is directed ventrally and the outer (flanking) pair is directed dorsally so that the hooks impale two adjacent secondary gill lamellage. Each lateral pair of hooks (one ventral and one dorsal hook) is operated by a single extrinsic muscle, the tendinous region of which is threaded through two fibrous loops (= Pulleys), one attached to each hook. Each tendon on entering the haptor also passes beneath transverse fibres joining the anterior ends of the two ventral hooks and these fibres probably act as a third pulley. This complex pulley system has a high mechanical advantage so that the energy used by the parasite to hold its hooks in position against the strong gill ventilating current of the host is minimal.

The mechanics of the adheasive apparatus of T. monenteron are compared with those of the monogenean gill parasite Amphibdelloides maccallumi, in, which the pulley system is less complex.

I would like to express my thanks to Mr G. W. Vane who kindly caught some pike for me, and to Dr P. C. Croghan and Mr S. A. Hodson for helpful discussion on the mechanical aspects of the adhesive apparatus.

The percentage change in weight, at 38·2 °C, in different concentrations of sodium chloride at different intervals, of a trematode, Gastrothylax crumenifer, from the reticulum of buffaloes has been recorded.

A state of approximate isotonicity is reached between 0·4 and 0·5% sodium chloride.

The osmotic pressure has also been expressed in terms of freezing-point depression.