The sudden death on 27 February 1963 of David Keilin deprived the scientific world of a biologist the span of whose scientific activities is unlikely to be equalled, much less excelled, in the future. These activities extended from descriptive morphology of protists, fungi and insects to the biochemistry of respiratory enzymes and metallo-protein compounds. It is, consequently, not possible for one person to deal adequately with all aspects of his work and in this appreciation of the man and his scientific achievements the main emphasis will be on his biological work. That bearing on parasitology will be considered in some detail, whereas the later, more biochemical, work will be considered briefly as fuller accounts of it have already been given by E. F. Hartree (Biochem. J.89, 1–5), T. R. R. Mann (Biogr. Mem. Fellows R. Soc.10, 185–207) and E. C. Slater (Enzymologia, 26, 313–20). In the brief survey of his biochemical work I have made much use of summaries and reports written by Keilin himself and I have often given the essence of lines of work summed up in his own words which express his conclusions with the elegance, lucidity and brevity characteristic of his style. As I was closely associated with David Keilin and in almost daily contact with him for just on forty years, it is natural that the following account will to some extent reflect my own interests and those aspects of his personality which became manifested during long and intimate contact both in private life and collaboration in scientific and administrative work.

Dictyocotyle coeliaca is recorded for the first time from Raja fullonica and R. batis. It is suggested that normally D. coeliaca may be restricted to waters deeper than about 50 fathoms and that this should be considered in attempts to incubate its eggs in the laboratory. No Calicotyle kroyeri were present in female R. radiata which contained eggs in their uteri and a brief account is given of the apparent differential distribution of the species in male and female rays. Up to 64 specimens of C. kroyeri per fish are recorded from the cloacae of R. radiata (previous records refer to an average of about four specimens per host). In a brief discussion of the controversial papers on these two monogeneans reasons are given for retaining the two in separate genera.

I am most grateful to the following for providing excellent facilities for the work. The Director and staff, in particular Dr B. B. Rae and Dr Z. Kabata, Marine Laboratory, Aberdeen; the Director and Staff, Plymouth Marine Laboratory; Professor James Brough, University College, Cardiff; the Director, Commonwealth Bureau of Helminthology; and Mr H. Atkinson of Messrs J. Marr and Son, Fleetwood. Additional specimens were provided by Mr J. Slinn, Marine Biological Station, Port Erin, and by Mr J. W. Smith of this Bureau.

When sporozoites of E. necatrix or E. brunetti are injected into the caeca of coccidia-free chickens they penetrate the caecal epithelium and initiate life-cycles which are similar to those developing in the usual sites in the small intestine. Viable oocysts of E. necatrix are produced which can be sporulated and, when given to coccidia-free chickens, produce characteristic infections in the small intestine. Possible implications of this investigation are discussed.

The results are described of a study of the distribution of the antigens in F. hepatica at various stages of development, and in the liver and spleen of experimentally infected rats using globulins labelled with fluorescein isothiocyanate, from rats and sheep infected with F. hepatica.

Possible uses of this technique in aiding the diagnosis of fascioliasis are indicated.

I wish to thank Dr A. W. J. Broome for a supply of the metacercariae of F. hepatica, and Dr J. S. S. Lowe who kindly isolated and labelled the globulins with fluorescein isothiocyanate. Miss Enid Eccleston gave valuable technical assistance.

Laboratory experiments indicated that third-stage larvae of T. axei and T colubriformis were able to penetrate soil to a depth of 12 in. or more, although most larvae remained in the upper 2 in. No geotaxis was observed.

Heavy downflows of water caused severe mortalities but did not affect the rate of movement of surviving larvae through the soil. Very limited random lateral movement occurred in turf, being more pronounced in the presence of faeces. Prolonged desiccation was extremely lethal to infective larvae. The activity of larvae was directly and linearly proportional to normal temperatures. The average activity of groups of larvae declined with time.

The author wishes to acknowledge the assistance and facilities provided by Professor B. G. Peters and the other members of staff of Imperial College Field Station, and to thank Dr C. W. R. Spedding for helpful criticism and supplying culture lambs; and Dr T. E. Gibson and Dr P. Silverman for supplying pure cultures of trichostrongyle larvae.

A fourfold enhancement of resistance to diamino-diphenylsulphone has been produced over a period of 15 months in strain A of P. gallinaceum by subjecting the strain, maintained in a state of patent infection, to increasing doses of the drug.

The DDS-resistant strain was cross-resistant to the minimum effective dose of sulphadiazine and slightly resistant to twice this dose, but it showed only a slight enhancement of resistance to pyrimethamine and proguanil.

The antimalarial action of DDS was antagonized by p–A.B. in the ratio of 100 to 1. DDS, in the minimum effective dose, was antagonized completely by folic acid if given in equal doses, but not by smaller doses of the antagonist; the antagonism was not competitive.

The relationship of cross-resistance between p–A.B.-inhibited sulphonamides and proguanil and pyrimethamine is discussed.

The cyst wall which encloses the metacercaria of Fasciola hepatica consists of four major layers, one of which is further divisible into three sublayers. These layers are numbered I to IV, the first being the external layer. Layers I and II form the outer cyst which may be separated from the inner cyst formed from layers III and IV.

Layer I, the thick incomplete external layer, covers the metacercaria dorsally and laterally. It is composed of tanned protein.

Layer II is a thin fibrous layer closely adherent to the inner surface of layer I and is composed of mucoprotein and acid mucopolysaccharide.

Layer III is made up of three separate sublayers, the relative widths of which vary in different regions of the cyst. In general IIIa consists of mucoprotein, IIIb of acid mucopolysaccharide and IIIc of neutral mucopolysaccharide.

In the dorsal and lateral regions, layer IV appears to be formed of lamellae held in a protein–lipid matrix. The lamellae are composed of protein stabilized by disulphide linkages.

In the ventral region of layer IV there is a thickened mucopolysaccharide area, the ventral plug, through which the metacercaria excysts.

The relative importance of these resistant layers in providing protection against desiccation, toxic substances and attack by other organisms is discussed.

This work was carried out during the tenure of a Commonwealth Post Graduate Scholarship. The author is grateful to Professor J. D. Smyth and Dr J. A. Clegg for advice and encouragement during the course of this work and in preparation of the manuscript.

This paper records the presence of a new species of trypanosome in the lizard Uranoscodon superciliosa L. (family Iguanidae), caught near Codajaz, Amazonas, Brazil, in August 1964 during the Guy's Hospital Expedition to the Lower Amazon.

Examination of a methanol-fixed and Giemsa-stained smear of blood taken from the heart of the lizard showed a heavy infection of trypanosomes. Camera lucida drawings of twenty were made, measurements being determined with dividers and a map-measuring wheel. The animal was preserved in 10% formalin, portions of liver, spleen, brain, heart, and lung being later extracted. Sections 5 μ in thickness were cut and stained in Ehrlich's haematoxylin and eosin, and in Giemsa's stain.

The chief distinguishing feature of the trypanosomes was their comparatively enormous size, up to 124 μ in total length, including the free flagellum, and 19 μ in width. The cytoplasm stained deep blue in Giemsa's stain, there being little indication of any vacuoles or inclusions. The posterior end was generally less intensely stained, and in certain cases pale, narrow, channel-like extensions from this region forward into the main body of the organism could be detected (Fig. 1). The nucleus, elongated and rather variable in shape, was distinguished by being less deeply coloured than the surrounding cytoplasm; it lay slightly posterior to the mid-point of the body, its longest diameter being parallel to the longitudinal axis of the animal. The kinetoplast, situated slightly posterior to the nucleus, was usually of circular shape, staining deep red; from it arose the flagellum, which passed forward along the margin of an undulating membrane and terminated in a free portion at the anterior end. Measurements of the organisms are given in Table 1.

Stream net samples in the Esk estuary reveal the infection, in winter, of the Gammarus zaddachi zaddachi Spooner, 1947 (Crustacea, Amphipoda) population alone with the plerocercoids of Diplocotyle sp. (Eucestoda, Spathebethriidea). Large plerocercoids are found in both sexes of the gammarids but the parasitized females are significantly larger than the non-infected ones and their fertility may be reduced.

I wish to express my thanks to Professor I. Chester-Jones of the University of Sheffield Department of Zoology in whose laboratories this work was carried out, and to Dr F. Segrove (of the same Department) for advice and constructive criticism.

A standard culture technique is described for obtaining the various developmental stages of Trichostrongylus colubriformis, and a suitable technique for assessing the effect of methyl bromide fumigations is outlined.

Doses of 10, 25 and 150 mg. hr./l. were required to give an LD50 for eggs, pre-infective and infective larvae respectively. The age of infective larvae, the relative humidity and the temperature all influenced the toxicity of the fumigant. Soil, and more especially faeces, sorb the fumigant and probably protect larvae from its full effect. Infective larvae of T. axei, T. retortaeformis and H. contortus were also found to be susceptible to methyl bromide.

The activity of larvae surviving fumigation was considerably reduced, but these larvae were still capable of infecting guinea pigs.

Methyl bromide killed common pasture grasses at doses of 150–250 or more mg. hr./l., depending on the temperature. Chloropicrin was found to be highly toxic to grass and unsuitable where the pasture is required undamaged.

The results are discussed in the light of current theories of the mode of action of methyl bromide on insects and what is known at present of the physiology of nematodes.

It is concluded that under certain conditions, methyl bromide might be used to eliminate infestations of trichostrongyle larvae on pasture, without killing the pasture.

I am indebted to Professor B. G. Peters and other members of the staff of the Imperial College Field Station and Dr C. W. R. Spedding of the Grassland Research Institute, Hurley, for their help in this work and their advice and criticism.

In a small-scale trial in which boxes of grass infested with Trichostrongylus colubriformis were treated with methyl bromide, a dose of 100 mg. hr./l. prevented guinea-pigs picking up the infection when they grazed the grass at a later date. A practical method is described for fumigating small areas of pasture. In a field trial, sheep were used to test plots of pasture fumigated with doses which did not kill the grass but the results were inconclusive. These experiments are discussed and also the possible value and applications of this technique.

I am indebted to Professor B. G. Peters and to the other members of the Imperial College Field Station, particularly Dr A. B. Page and Dr O. F. Lubatti, for their help and criticism during the work. Thanks are also due to Dr William Davies, the Director of the Grassland Research Institute, Hurley, for permission to carry out the field experiment there, and also to Dr C. R. W. Spedding and Mr T. Brown for their co-operative assistance and advice throughout this work.

Unidentified helminth larvae in the eyes of the North Sea whiting, Gadus merlangus L., were recorded in an unpublished survey of parasites of gadoid fish by Dr Z. Kabata of the Marine Laboratory, Aberdeen, in 1957. Since then these larvae have been observed frequently and during the first 6 months of 1964 an effort was made to identify them. They proved to be the plerocercoids ofa trypanorhynch cestode.

The plerocercoids are usually free in the aqueous or vitreous humour, but occasionally one is found attached to the lens. Very often both eyes of the same fish are infested, the largest number of larvae recorded from one eye being ten, from a fish taken in Shetland waters. The other eye of the same fish contained eight larvae.

In several plerocercoids examined the proboscides were evaginated, thus enabling detailed drawings to be made of the armature at a magnification of × 900. A number of specimens were fixed in Gilson's fixative; transverse and longitudinal sections were cut and stained with Mallory's triple stain; whole mounts were stained with borax carmine.

Early larval stages are completely undifferentiated; they develop, later, to the stage shown in Fig. 1. A plerocercoid with a fully developed scolex measures 3·5–8·0 mm long by 0·7–1·6 mm broad.

The larva is composed mainly of faintly staining tissue resembling the parenchyma of adult cestodes. The scolex is withdrawn into a cavity, the receptaculum scolecis, in the anterior half of the plerocercoid. At the anterior extremity is a deep invagination from which, in early larval stages, the receptaculum scolecis begins to develop.

Penetration and maturation of Schistosoma mansoni were compared in four strains of mice under controlled conditions, with the C3H as the reference strain. Differences were observed in (1) average numbers of cercariae penetrating, (2) percentage of individual mice penetrated by at least 90% of the available cercariae, (3) worm burdens, (4) maturation of penetrants, and (5) consistency of mice with reference to these characteristics. Criteria 2, 4 and 5 have not been evaluated in this way before.

Best average penetration (47%), greatest consistency, and highest percentage of samples with at least 90% of the cercariae penetrating were in mice of the C3H strain (85%). Of the other strains the Swiss albino strain most resembled strain C3H. Beige and hairless mice were comparatively resistant to penetration since only 60 and 30% respectively of these hosts had at least 90% of the cercariae penetrating. Hairless mice gave the best average worm recovery (23%) and parasite maturation (53%) and beige the poorest (15 and 34%). Mice of the Swiss and beige strains were highly variable in terms of parasite maturation.

The establishment of reproducible differences in infections of hosts at the sub-specific level provides basic information for investigation of factors which influence penetration and maturation and which are thus involved in host specificity and innate resistance in closely related animals.

The authors are grateful to Dr Margaret Deringer, National Cancer Institute, National Institutes of Health, for the hairless mice and to Mildred Walters, Naval Medical Research Institute, for assistance in the production of cercariae.

A number of experiments was carried out on Polycercus lumbrici to induce evagination of the cysticercoid in vitro. The results indicate that in nature the containing cyst is ruptured by pressure and abrasion in the gizzard of the bird host and that evagination is induced mainly by the action of gastric juice followed by pancreatic juice, heat assisting the process by exciting vigorous movements in the larvae. By a process of elimination it has been shown that the effective agents are the pepsin in the gastric juice and pancreatin in the pancreatic juice. The latter probably causes contraction of the muscles of the wall of the caudal vesicle which forces out the scolex.

An account is given of the incidence and intensity of infection of the gulls Larus argentatus Pont., L. fuscus L., and L. marinus L. with Cestoda on the coast of Wales. Eleven species of cestodes were found in the three species of gulls, nine in L. argentatus, eight in L. fuscus and six in L. marinus. Nine new host records and two new British records are listed.

The presence of the cestode parasites did not appear to be harmful to the birds.

Ecological and systematic notes are given on each of the cestode species. The basic cestode fauna of L. argentatus, L. fuscus and L. marinus on the coast of Wales comprised three species, namely Tetrabothrius cylindraceus (Rud.), T. erostris (voennberg) and Anomotaenia micracantha (Krabbe).

The factors influencing the intensity and incidence of infection are considered and it is concluded that infection is determined mainly by the food and feeding habits of the host. It is shown that the number of cestodes per bird may be represented by a negative binomial distribution. There is some evidence to suggest that high intensities of infection were associated with high incidences. Three main stages in the development of the helminth-fauna of gulls are described and are related to the age and activity of the bird.

It is a pleasure to record our personal thanks to those to whom we are indebted, namely the Nature Conservancy and the West Wales Naturalists' Trust for permission to work on Skomer Island, to Mr D. R. Saunders, Warden of Skomer Island, for help in the collection of some of the Great Black-backed Gulls, to Mr P. Hope Jones who collected the gulls from Malltraeth, and to Professor J. G. Baer, Departement de Zoologie, Université de Neuchâtel, Professor W. E. Kershaw, Department of Parasitology and Entomology, University of Liverpool, and Mr Stephen Prudhoe, British Museum (Natural History), for the loan of specimens, and to Mr M. D. B. Burt, Department of Natural History, University of St Andrews, for identifying the specimens of Paricterotaenia paradoxa.

The participation of one of us (M. P. H.) in this work was made possible through the generous assistance of a D.S.I.R. research studentship, which is gratefully acknowledged.

The development of Archigetes limnodrili in species of Limnodrilus is described. There is no free-living larva and eggs are ingested by the tubificids. Growth and development is completed within the body cavity of the annelid, and egg liberation is accomplished by release of the parasite and decay of its body.

Breeding of A. limnodrili takes place throughout the year. In the localities investigated there was no evidence that a fish host was required in the life-cycle. Progenesis was the only type of development encountered in Britain.

A. limnodrili exhibits an unusual degree of host specificity, being found only in species of Limnodrilus. It is suggested that this is due to differences in the composition of the coelom or intestine of Limnodrilus compared to other genera.

The degree of infection in all localities is very low, and shows no regular seasonal variation. There is no similarity in the seasonal changes in different localities.

The relationship between the host and parasite is a stable one, and there is little mutual damage. Factors contributing to this stability are discussed.

The development of A. limnodrili is compared with that of other species of Archigetes, and the life-history discussed with particular reference to the phenomenon of progenesis.

I wish to thank Professor R. J. Pumphrey in whose Department this work was carried out, and Dr J. C. Chubb for his constant advice and criticism. I also wish to thank Dr K. H. Mann and the University of Reading for provision of specimens and permitting me the use of their facilities. The work was carried out during the tenure of a Nature Conservancy Research Studentship.

The genus Acanthobothrium van Beneden, 1850, seems to be in a state of slight confusion due, possibly, to some inadequate descriptions which have led to subsequent misidentifications of some of the species. The type species A. coronatum was first recorded and very briefly described by Rudolphi (1819) as Bothriocephalus coronatus from ‘Squali stellaris, Squali squatinus and Torpedinis marmoratae“. Since then specimens have been assigned to this species which probably belong elsewhere; the list of synonyms and the host records are extensive. Baer (1948) divided the species of the genus into two groups, namely, those parasitic in sharks and those in rays. This seems to be a reasonable distinction as specificity is very marked in cestodes and especially in those of elasmobranchs. It was thought that a detailed study of the type species, presented here, would be a useful preliminary to a consideration of the genus as a whole.

Specimens of A. coronatum were obtained from the spiral valve of Scyliorhinus stellaris (L.) caught in Cardigan Bay in July 1956 and at Plymouth in August 1958 and August and September 1959. The number of worms, complete with scoleces varied from 6 to 10 per fish and in one instance 14. Many fragments of two to three proglottids and many individual proglottids were also present. The scoleces were attached in the first turn of the spiral valve and the strobila adhered feebly along the whole length, sometimes as far back as the rectal gland, leaving on removal a distinct groove in the mucosa.

The route of migration of Cercaria X within the host snail, Lymnaea stagnalis, has been demonstrated by means of serial sections and a method of numerical assessment of the distribution of cercariae in the various tissues and organs of the body.

The main pathway along which migrating cercariae travel is the blood system, which differs from the route in the fish second intermediate host. The cercariae enter the main visceral haemocoel by rupture of the wall of the digestive gland and from there enter the main venous vessels deploying through the rectal sinus, sub-renal sinus, lung and heart. The cercariae travel from the heart via the mantle arteries to localize in the mantle sinuses prior to emergence.

The inner surface of the mantle, and in particular its leading edge, is the main site of emergence, and escape of cercariae is, in the final stages at least, an active process. It is suggested that passage of parasites in the blood stream is partly active.

Migration and subsequent emergence is probably facilitated by the activity of the oral cap of spines, the general activity of the cercaria and the activity of the oral and ventral suckers.

A tissue response to the presence of cercariae in the blood vessels and tissues of the mantle is evident, the cells responsible for this being, possibly, actively phagocytizing amoebocytes of the mollusc's blood. The swellings in the epidermis of the mantle in heavy infections may be the result of an accumulation of these cells.

Grateful acknowledgement is expressed to Professor J. Brough for the facilities and encouragement provided during the course of this study. This work was carried out during the tenure by A. J. Probert of a D.S.I.R. Research studentship.