There was no difference between the worm burdens or the lengths of the worms recovered from sexually immature male and female mice infected with Amplicaecum robertsi.

In mice older than 70 days there were fewer larvae recovered from the female than the male host. The larvae were also longer in the male than in the female mouse.

The suckling mouse harboured fewer larvae than the weanling; the susceptibility of the mouse continued to increase with increasing age until the one-hundredth day in the males and the seventy-fifth day in the females. In animals older than this an age resistance was apparent. It is suggested that the much lower susceptibility of the suckling mouse is due to the small gut and villi size reducing the establishment of the larvae.

Worm growth also varied with host age, the longest larvae being recovered from the suckling mice and successively smaller worms from each older age group up to the seventy-fifth day, beyond which there was no significant change in worm growth with age of the host. There was no difference between the worm burdens and the lengths of the worms recovered from 90-day-old mice. The injection of testosterone had little or no effect on the host–parasite relationship of gonad-ectomized animals. Larvae from gonadectomized mice were significantly shorter than those from the intact hosts.

Adults of Nematospiroides dubius recovered from female mice were smaller than those from male mice. In general, a similar result was obtained for infections in rats. Longer worms were recovered from gonadectomized rats than from intact animals; the difference between the growth of the worms in male and female hosts was still apparent.

Treatment of castrate male rats with testosterone was more favourable to the growth of the male than the female worm. Oestradiol treatment of spayed female rats caused a stunting of both sexes of the nematode. However, this effect was least noticeable among the female worms.

An increase in the age of the rat resulted in a decrease in the lengths of the worms recovered. The worms grew to greater lengths in 25-day-old mice than in rats of the same age and sex.

It is suggested that the sex hormones may have two effects on worm growth. First, an indirect effect involving the nutrition of the worm, and secondly, a direct effect of the hormones themselves on the tissues of the worm.

This work was done during the tenure of a Department of Scientific and Industrial Research Studentship. My thanks are due to Dr E. T. B. Francis for his help and criticism, and to Professor I. Chester Jones, in whose department the work was done, for the facilities he provided.

Male mice harboured more third-stage larvae of Amplicaecum robertsi than females; these larvae also grew longer than did those in female mice. Gonadectomy removed the differences, evident between the sexes, in both the numbers recovered and the growth of the third-stage larva, between male and female mice. The worms recovered from the gonadectomized hosts were smaller than those from the intact male and female host.

Pregnant female mice harboured more worms than either the male or the non-pregnant female mouse. These worms were also smaller than those from the male and non-pregnant females. There was no difference in the growth of the larvae from gonadectomized and pregnant hosts.

The treatment of spayed female mice with progesterone resulted in an increase in the size of worm population as compared with the numbers of worms recovered from intact female mice. The worms from the progesterone-treated spayed females were also longer than those from the spayed female hosts.

Treatment of normal female mice with progesterone resulted in an increased recovery of worms compared with the numbers of worms recovered from normal female animals. The worms were stunted but they were of a size comparable to those recovered from pregnant female mice.

It is suggested that the increase in the size of worm populations and the stunting of the worms recovered from pregnant female mice is associated with the effects of progesterone in relation to the hormone balance associated with the ovary at the time of pregnancy.

When infected with equal numbers of eggs of Amplicaecum robertsi normal male mice harboured more larvae than normal females. Treatment of the mice with thiouracil and thyroxine eliminated the differences between the sizes of the worm populations shown between the sexes in normal mice.

Thiouracil treatment increased the susceptibility of mice to A. robertsi infection, but caused a stunting of the larvae. Thyroxine treatment had no effect on the host–parasite relationship.

It is suggested that the differential ability of the larvae to hatch, penetrate the tissues of the host and migrate to the liver in thiouracil- and thyroxine-treated mice was responsible for the differences in the sizes of the larval populations observed.

This investigation was carried out during the tenure of a Senior Research Fellowship from the Wool Research Board of Australia. I should like to thank Professor J. F. A. Sprent for his continued interest and Mr C. Wilkinson for his careful assistance.

Excystment of the metacercaria of Fasciola hepatica is an active process and occurs in two stages—activation and emergence. Activation is initiated by high concentrations of carbon dioxide, reducing conditions and a temperature about 39 °C. The reducing conditions increase the rate of action of the other two stimuli. The carbon dioxide stimulus need only be applied for 5 min, but the exposure time to the reducing conditions has to be of the order of 30 min. Changes in the order of application of the stimuli carbon dioxide and redox potential have no effect.

The second phase, emergence, is triggered by bile. Metacercariae were held in an activated condition for 24 h, and when bile was added emergence took place normally.

During excystment the metacercariae exhibit a complex behaviour pattern. After activation there is an initial period of rotatory activity, but after about 20 min a quiescent phase ensues when the metacercariae contract away from the cyst wall at an imperceptible rate. This behaviour occurs in response to stimulus from a high concentration of carbon dioxide and a temperature about 39 °C. When the metacercariae are exposed to bile the second phase of activity is initiated, consisting of antero-posterior thrusting movements directed against the ventral side of the cyst wall. Within about 15 min the juvenile flukes escape through a small circular hole in the ventral surface of the cyst wall which corresponds to the ventral plug region.

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.

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.

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.

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.

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.

Fasciola tragelaphi sp.nov. from the bile ducts of a Tragelaphus spekei shot in the Chobe swamps of Rhodesia differs from other species of the genus in its very large size (10.2–12.0 cm long in fixed and mounted specimens), in the shape of its body and in the distribution and arrangement of the reproductive organs. The testes lie almost entirely in the posterior half of the body, well away from the female genitalia, which lie mostly in the anterior third of the worm. The ovary is always bilateral, and the coiled uterus runs in a straight line between the caeca to the genital atrium. A prepharyngeal pouch found in this species is compared with that found in other members of the genus Fasciola and in related genera.

We gratefully acknowledge the help given during the preparation of this paper by the staff of the Bureau, particularly Mr G. Dimmock for technical assistance. We would also like to thank Mr S. Prudhoe for reading the manuscript and offering useful advice, Mr B. Bracegirdle for producing the photograph of the whole worm reproduced in the Plate, fig. 1 and to Bechuanaland Hunters Ltd., particularly I. M. Henderson, for obtaining further specimens for examination.

The distribution of Trichostrongylus colubriformis in the intestine of the non-immune guinea-pig is described. At day 10 of the primary infection the majority of larvae were found in the anterior half of the small intestine with progressively smaller numbers in all parts of the gut posterior to this. However, soon after day 10 a migration of the worm population took place so that by day 15, before there had been a reduction in the total present, the majority of worms were found in the posterior small intestine. Thereafter the posterior movement continued, but while the numbers in the large intestine increased the movement then coincided with the beginning of the expulsion of the worms.

By the irradiation of guinea-pigs prior to infection the posterior migration was prevented, indicating that the latter was the result of a host response.

The independent migration of two generations of parasites was noted and two possible explanations are put forward.

This work was carried out during the tenure of a Research Training Scholarship awarded by the Animal Health Trust.

The author would like to thank Professor W. I. B. Beveridge in whose Department this work was carried out, and also Mr King of the Department of Radio-therapeutics, University of Cambridge, who irradiated the guinea-pigs used in Expt. 3.

A series of line drawings of the unsporulated and sporulated oocysts of the species of Eimeria found in cattle and sheep in Britain has been compiled.

The incidence of the different species in faecal samples from animals, of which only a minority were known to be suffering from enteritis or diarrhoea, has been recorded. Samples from sheep frequently contained five or six different species among which those that are believed to be pathogenic (E. arloingi, E. crandallis, E. ninaekohlyakimovae) were the most common.

The samples from cattle most frequently contained two or three species. E. bovis and E. zuerni, which are associated with acute disease, were the most common.

Lecithaster musteli sp.nov. from the five-bearded rockling, Onos mustelus (L.), at Mumbles Head, Swansea, is described and compared with other species of the genus.

The genus Lecithaster Lühe, 1901 is reviewed. An emended definition of the genus is given with keys, diagnostic features, and brief notes of the nine recognized species. L. gibbosus (Rud.) Lühe, 1901 is briefly redescribed.

L. intermedius Szidat, 1954 is considered to be a synonym of L. gibbosus (Rud.) Lühe, 1901.

I am deeply indebted to Dr B. L. James for his advice and indispensable assistance throughout the work and the preparation of this paper. I am also grateful to Professor E. W. Knight-Jones for his interest and for providing the working facilities, and to Mr S. Prudhoe, British Museum (Natural History), for his advice, for allowing me to examine slides of L. gibbosus and for the use of the library.

For the tick Hyalomma anatolicum anatolicum, at constant temperature and humidity, the pattern of oviposition is constant, and consists of a peak period, usually after 2–3 days, followed by a gradual decline.

Oviposition proceeds more rapidly in constant darkness than in continuous light.

With increased temperature the oviposition period is decreased, but with increased humidity the period is extended.

The largest eggs are laid from 0 to 48 h after the peak of egg production, and no correlation has been observed between female size and egg size.

The number of eggs produced by a tick is directly proportional to its engorged weight, and since the engorged female weight is a direct reflexion of its unfed weight, the number of eggs laid is directly dependent on the size of the unfed female.

Guinea-pigs show partial immune response to adults of H. anatolicum anatolicum when the ticks are fed on them repeatedly. This minimizes the amount of nutrients imbibed and accordingly egg-laying potential is not realized.

The material on which this study was made was presented to us by Dr R. P. Chaudhuri, and we gratefully acknowledge his kindness.

The sporozoites of Eimeria acervulina and E. maxima when injected into the caeca of chickens fail to produce infections. Sporozoites of E. mivati injected into the caeca invade the walls and the life-cycle is completed. These observations are discussed in their relation to the fate of sporozoites injected into the allantois of chick embryos. The possible use of these techniques in confirming the identity of a species is discussed.

We wish to thank Mr B. J. Millard for technical assistance with the preparation of the sporozoites.

Four species of Eimeria from fowls were able to develop in chicken embryos following the inoculation of sporozoites. Three species (E. brunetti, E. mivati and E. tenella) were able to complete the whole of their endogenous cycle in embryos. E. necatrix developed to late schizogony, E. acervulina and E. maxima did not develop.

The life-cycles appeared to be delayed, and in the case of E. tenella embryo mortality was associated with heavy parasitism of the chorioallantoic membranes. Oocysts recovered from urate deposits in the allantoic cavity sporulated normally and induced infections in chickens.

Attempts to infect turkey and quail embryos with E. tenella or chicken and turkey embryos with E. stiedae were unsuccessful.

The implications of the findings are discussed.

I wish to thank Dr C. Horton-Smith for his encouragement and guidance with this work; Dr M. E. Rose for help in the preparation of the manuscript and Mr B. J. Millard for careful technical assistance.

In the mouse pinworm, Aspiculuris tetraptera, the basic features of the process of osmotic regulation is similar to the process in other nematodes such as Parascaris equorum, Rhabditis terrestris and Hammerschmidtiella diesingi. The ability to osmoregulate is, however, utilized more effectively in a hypertonic than in a hypotonic solution.

The body fluid is isotonic with 35 % sea water. The relative accuracy of this value is confirmed by direct measurements of the depression of freezing point of the body fluid. This is approximately 0.68–0.69 °C, which is approximately 0.12 % NaCl, which is higher than the value for Ascaris (= 0.655 °C).

My thanks are due to Dr D. L. Lee for helpful discussions on some aspects of this study.

Young and old rediae of Parorchis acanthus Nicoll, from the digestive gland and gonad of Nucella lapillus (L.) have been described. The young redia has an anterior collar-like fold and mid-ventral birth papilla the movements of which are controlled by a special arrangement of muscles. The birth papilla is ‘solid’ but in older rediae is replaced by a pore. The pharynx is of fairly uniform size irrespective of the size of the redia and the intestine, in the young redia, is relatively muchlarger than in the older redia which facts suggest that relatively more food may be absorbed by the mouth in the young stage. The redia cavity develops as a slit in the central cell mass. As it enlarges the intestine remains close to the dorsal wall and developing embryos are carried, among the cells, towards the surface dropping into the cavity, later, to complete their development. Up to fifty-five cercariae may be present in a redia. Simple nervous and excretory systems are present.

The tegument consists of one layer; the surface bears numerous unusual branched microvilli. The syncytial protoplasm contains numerous electron dense mitochondria, vacuoles and granules. Half-desmosomes, not recorded for any other parasitic platyhelminth, occur fairly regularly on the lower limiting layer. Below the uninterrupted basement membrane are circular and longitudinal muscles and, lining the cavity, two or three layers of flattened cells containing numerous mitochondria, agranular and granular endoplasmic reticulum and Golgi complexes. Large lipid globules accumulate in the cells.

It is suggested that, in the older redia, active transport of nutrient materials through the tegument may take place.