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.

A group of thirty-nine Merino-Border Leicester cross-bred lambs were given six sensitizing doses each of 3000 infective Haemonchus contortus larvae. Previous studies showed that this treatment produced a state of immunological exhaustion (Dineen & Wagland, 1966a). In the present study the regeneration of the immunological response was followed by challenging the sensitized animals and previously uninfected control animals with 3000 infective larvae at 2, 4, 8 and 16 weeks after termination of the sensitizing infections with an anthelmintic. Faecal egg counts were carried out on infected animals twice weekly throughout the course of the experiment, and differential worm counts were performed on groups of animals slaughtered 48 days after challenge.

The results show that maximum regeneration of the immune response occurred when periods of 4 and 8 weeks were permitted to elapse between removal of the sensitizing infections and challenge. At 16 weeks the response of the sensitized animals to challenge infection was not significantly different from that of the control animals.

In contrast to results recorded in the previous communication, a high mortality (49%) occurred among the sheep during sensitization. In the earlier experiment the sensitizing infections were given at fortnightly intervals whereas they were given at weekly intervals in the present experiment.

Interpretation of the study is based upon the threshold behaviour of immunological responsiveness in parasite immunity, the occurrence of an immunological latent period and the relationship of this period to the developmental rate of the parasite, the susceptibility of the 4th larval stage to immunological attack, and the immunological privilege of the adult worm. These phenomena have been described in previous communications in the present series.

We are greatly indebted to Mr Emil Teleki and Misses Helen Giller and Lindy Stothart for their excellent technical assistance.

The effect of hexachlorophene, a fasciolicide, on some enzyme systems in the liver fluke was studied. The enzymes were adenosine triphosphatase, succinate oxidase and cholinesterase. Adenosine triphosphatase was activated, whereas succinate oxidase and cholinesterase were inhibited when homogenized flukes were incubated in some concentrations of hexachlorophene. Of the three enzyme systems studied in flukes killed by hexachlorophene, only the succinate system showed reduced activity, whereas the other two showed almost normal activities. The study indicates that biochemical changes are associated with the effect of hexachlorophene on the liver fluke.

Thanks are due to Dr C. Grant for critical examination of the language, to Miss Maarja Kippar for valuable help with the experiments and to Mrs Astrid Holmström for the typing of the manuscript.

This work was supported by grants A 1108/B 904 and A 1635/B 1299 from Jordbrukets Forskningsråd.

The wall of the trunk, that of the praesoma, and the lemnisci of Acanthocephalus ranae have been studied by electron microscopy. Striations visible in sections of the body wall under the light microscope do not correspond with the ‘striped layer’ revealed by the electron microscope.

A new region, the ‘canal layer’, has been described. This contains canals running into the body wall from cuticular pores.

Structurally the wall of the trunk and that of the praesoma are similar. The lemnisci resemble the ‘inner layer’ of the praesoma wall. However, it is suggested that the wall of the trunk differs physiologically from that of the praesoma, and from the lemnisci. The possible roles of the wall of the praesoma and the lemnisci in fat excretion or uptake have been discussed.

The body wall of A. ranae has been compared with that of the other acantho-cephalans studied with the electron microscope.

Grateful acknowledgement is made to D.S.I.R. (now S.R.C.) for a research grant to the Department of Zoology for the purchase of a Huxley ultramicrotome, a vacuum coating unit, and an AEI EM 6 electron microscope.

I am grateful to Dr D. A. Erasmus for reading and criticizing the manuscript, and to Mr T. Davies for valuable technical assistance.

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.

The effects of parasitism by the daughter sporocyst of Cercaria bucephalopsis haimaena Lacaze-Duthiers, 1854, on the digestive cells of Cardium edule L. was investigated by comparing histochemically stained cells in healthy and parasitized animals. Two effects were apparent: (a) the branches of the daughter sporocysts compress the walls of the digestive gland ducts and tubules, and cut off the more distal tubule cells from their food supply, inducing starvation autolysis, (b) in digestive tubule cells not so cut off, glucose, glycogen, glycoproteins, phospholipid and proteolipid food storage globules and acid mucopolysaccharide are reduced. Neutral lipids, fatty acids, alkaline phosphatase, acid phosphatase and non-specific esterase are increased and there is also a compensatory increase in the number of food vacuoles. In the visceral haemocoel, in the neighbourhood of the parasite, glucose, fatty acids, neutral lipids and acid mucopolysaccharide are increased and glycogen reduced.

We are grateful to Professor E. W. Knight-Jones, for helpful comments and good working facilities, and to the Science Research Council, for the grant of a research studentships to one of us (E.A.B.).

Sepedon h. hispanica was collected in freshwater marshes in southern Spain and reared through its entire life-cycle in the laboratory.

Larvae of this species differ from those of the twenty-one other reared species of Sepedon in not normally invading water, feeding as parasitoids throughout the first half of larval life (that is, permitting their food snails to live for several days), and in restricting their diet to the genus Succinea during that time.

Each newly hatched larva of S. h. hispanica crawls between the mantle and foot of a living Succinea elegans, S. pfeifferi or S. putris and remains there while developing into or completely through the second larval instar.

After the death of the first food snail the larva becomes predatory and loses its host specificity. Then it attacks a wide range of pulmonate snails, kills each victim quickly, and destroys a series of individuals belonging to species of Succinea, Lymnaea and Physa.

Adults of S. ruficeps and S. scapularis were reared from larvae and pupae collected in Ethiopia.

The larvae of both African species are aquatic predators which kill and consume various pulmonate snails unselectively. They are equivalent ecologically to all other larvae of Sepedon previously described.

The egg, three larval instars, and puparium of S. h. hispanica, and the mature larvae and puparia of S. ruficeps and S. scapularis, are described and figured.

A key is presented to immature stages of the Palaearctic species of Sepedon.

We wish to thank Dr W. C. Frohne, Alaska Methodist University, Anchorage, Alaska for collecting and rearing Sepedon ruficeps and S. scapularis. Dr E. Morales Agacino, Instituto Espanol de Entomologia, Madrid; Dr M. Mendizabol, Dr J. Verdejo and D. J. Paris Lopez, Instituto de Acclimatacion, Almeria; and Professor Dr I. Docavo Alberti and Dr B. Llopis Minguez, University of Valencia, aided during the senior author's field-work in Spain. We thank Dr J. Verbeke, Institut royal des Sciences naturelles de Belgique, for determinations and distributional data; Mr L. W. Stratton, Harpenden, England, and Dr H. J. Walter, Dayton, Ohio, Museum of Natural History, for determinations of Gastropoda; and Mrs L. Lyneborg, Zoological Museum, Copenhagen, for Fig. 25.

Large number of fishes of the family Cryprinodontidae collected from a stream at Hastings, Sierra Leone, were found with a diplostomulum encysting on the skin and in the musculature.

Infected pieces of fish were fed to day-old chicks and cattle-egrets, and adult stages of a worm, considered to be new species and named Diplostomum (Dolichorchis) leonensis, were obtained.

Morphological details of the diplostomulum and the adult worm are described.

The author wishes to express his gratitude to Dr G. Dubois for his interest and advice in connexion with the study, to Miss M. R. Coker who prepared the diagrams; and to Messrs F. Deen, T. Alhassan and M. Seisay for technical help.

Cercariae of Psilotrema oligoon and Notocotylus attenuatus encyst free to form multilayered, hemispherical cysts. The former consists of four layers in which the innermost (layer 4) is divided into two, chemically and structurally different, areas. Cysts of the latter have three layers in which the innermost (layer 3) is again divided into two chemically different parts. A third cyst, produced by the encystment of Cercaria tarda in the tissues of Limnophilus flavicornis, is spherical and contains three layers (of which the outermost was not studied).

All of the cysts studied contain a high proportion of protein, which in some layers occurs combined with carbohydrates. In both free-encysting species the two outer layers are produced by expansion of a non-cellular layer covering the integument and the innermost layer is produced by secretion of bâtonnets from a layer of cells lying beneath the body wall. Cysts of P. oligoon are divided into two separate parts, each having two layers, whereas those of N. attenuates consist of a single three-layered wall. The innermost layer in both consists of a protein with disulphide and sulphydryl linkages, and in the cyst of P. oligoon it has a circular ventral area containing carbohydrates. A similar carbohydrate-containing area occurs in the innermost layer of the cyst of N. attenuatus but as a ventro-lateral ring of material. The cyst of Cercaria tarda is also predominantly proteinaceous and stains intensely for RNA.

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.

Adelphocystis aeikineta, a new species of monocystid gregarine, is found in the coelom of the earthworm Keffia variabilis in which the trophozoites occur in large numbers in permanently associated pairs. They are large, measuring 750–1500, μm in length, and exhibit continual peristaltic motion. The ectosarc of the trophozoite is markedly striated and there is no evidence of any anterior differentiation. The gametocytes are rounded and the presumed bipolar sporocysts measure 12–14·5 μm in length. Some associated pairs occur in groups enclosed in a membrane and there is evidence that gametocytes may form under these conditions. A. aeikineta is placed in the subfamily Zygocystinae and separated from Zygocystis on its elongate shape. The site of infection of this gregarine, and others recently described, suggests that the morphological basis on which the family Monocystidae is divided into subfamilies and genera is inadequate and a sounder classification should take into account these sites of infection.

The material on which this paper is based was collected while I was in receipt of a Wellcome Trust Travelling Grant, and I wish to thank the Trust for this support. It is also a pleasure to thank Mr R. W. Sims of the British Museum (Natural History) who identified the earthworms, Dr Keith Vickerman who helped me to collect them and Mr R. B. Freeman who advised me on the nomenclature.

The distribution of carbohydrates, lipids and enzymes in the daughter sporocyst and contained cercariae of Cercaria bucephalopsis haimaena Lacaze-Duthiers, 1854, is mapped and used as an indication of metabolism. It is concluded that host glucose, glycogen, fatty acids and neutral lipids are absorbed and pass through the syncytial tegument into the cellular sub-tegument. They are then either metabolized or passed into the body cavity and contained cercariae. Glucose seems to be either converted into glycogen and glycoproteins or metabolized anaerobically. The end products of anaerobic respiration are fatty acids and neutral lipids. These pass into the excretory system, which flushes them into the body cavity of the daughter sporocyst, from where they enter the body of the developing cercariae. In the cercariae they are deposited, together with host lipids, possibly as less toxic higher fatty acids or other stored lipids. The stored glycogen and lipids in the cercariae are probably utilized aerobically during the free-living existence. The transport of nutrients and excretory products across the cell membranes may be aided by alkaline phosphatase and esterase activity, as is the metabolism of carbohydrates and lipids. Acid mucopolysaccharide precursors appear to be synthesized in the dense cytoplasm of the mid-tegument of the daughter sporocyst and secreted into the host's visceral haemocoel. Acid phosphatase and esterase found in this region may be involved in the synthesis and also in the breakdown of complex incoming nutrients.

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

The cyst wall of Parorchis acanthus Nicoll, consists of an outer wall of three layers and an inner wall of two layers. The outer wall is hemispherical, complete, attached to the substratum and extends as a flange around the margin. The composition of its three layers differ dorsally and ventrally. Dorsally the layers are: (1) acid mucopolysaccharide and neutral mucopolysaccharide, (2) protein and lipoprotein, (3) acid mucopolysaccharide, neutral mucopolysaccharide and glyco-protein. Ventrally the layers are: (1) acid mucopolysaccharide, protein and lipoprotein, (2) acid mucopolysaccharide. The middle layer in both is secreted by the ventral granular cystogenous glands and the outer and inner layers, dorsally and ventrally, by the dorsal and ventral agranular cystogenous glands respectively.

The inner cyst wall is oval in surface view. It consists of two layers, both of neutral mucopolysaccharide, the outer giving a more pronounced reaction. Both are secreted by the dorsal granular cystogenous glands. The ventral wall is strengthened by a secretion from the plug-forming gland cells and consists of neutral mucopolysaccharide. All the layers of the cyst wall show the same histochemical reactions as the glands which secrete them.

Studies on the fine structure of the cercaria have revealed mammalian-type synapses in the neuropile of the central nervous system, rind cells around the ganglia, an elaborate arrangement of muscles in the wall of the oral sucker, the flame cell structure and the structure of the granules contributing to the formation of the cyst wall.

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.

The present material of buffalo–dog origin could not be transmitted to the laboratory animals, such as white rats, white mice, rabbits, golden hamsters, guinea-pigs and chicks.

Secondary development took place only in mice.

The strobilar phase was infective to a cat but did not show sexual differentiation even at 30 days, and worms were further observed to be at a degenerative stage.

The buffalo was highly suited to the perpetuation of the parasite, forming larger cysts and massive production of scoleces; nearly 90% of the cysts were fertile.

The strobila had mostly two segments and not more than three.

The number and distribution of testes were more posterior to the genital pore and were also observed posterior to the vitellaria, occasionally.

The position of the genital pore in the gravid as well as mature segments was both anterior and posterior but with an average of 52.8%.

Uterine sacculations were observed to range from 28 to 54.

Echinococcus material of buffalo–dog origin does not appear to fall into a definite ‘subspecies’ pattern. It is suggested that it be merely recognized as a further ‘mutant’, the existence of which can be accounted for on theoretical grounds.

The authors are thankful to the Director, Indian Veterinary Research Institute, and to Dr H. D. Srivastava, Head of Division of Parasitology, I.V.R.I., for providing the necessary laboratory facilities.

After infection with the nematode Trichuris muris 70–75% of mice of the Schofield strain developed an immunity to the parasite and eliminated the worms between the 16th and 19th days after infection. In these mice the acquired immunity persisted for at least 3 months and prevented the establishment of subsequent infections.

In 25–30% of the mice immunity was not produced and infections developed into mature worms. These non-resistant mice remained susceptible to further infection.

The development and action of the immune response were suppressed completely by the administration of cortisone acetate.

The ecology of the tick Ixodes trianguliceps Bir. is described from material collected at two localities in Wytham Estate near Oxford. The hosts examined were bank voles, Clethrionomys glareolus Schr., field voles, Microtus agrestis (L.), and field mice, Apodemus sylvaticus (L.).

The seasonal distribution of the tick is biannual, with spring and autumnal peaks as found in Ixodes ricinus (L.). Larvae occur throughout the year but principally in spring and autumn. Nymphs are found only from late spring until the autumn, and adults principally in the spring with a small peak in autumn.

Larvae and nymphs of I. trianguliceps were more abundant on male than on female bank voles, but there was no significant difference in the infestation of males and females by adult ticks. Similarly there was no significant difference in the infestation of adult and juvenile bank voles.

Larvae and nymphs occur principally on the ears, but the adult ticks are found mainly on the neck or body, although the ears were not excluded from infestation.

The seasonal distribution and life-history is discussed with reference to similar studies made in the Soviet Union and in Poland. Male ticks were found in copula on the host, and both males and nymphs were collected from the nests of voles.

A tentative summary of the life-history of I. trianguliceps is given based on known facts for the hedgehog tick, Ixodes hexagonus Leach, which has a similar seasonal occurrence.

We wish to thank Professor Don Arthur for identification of much of the tick material, especially in the early stages of this study, and for useful criticism of the draft manuscript. The work was carried out while one of us (M.J.C.) was a research student in the Hope Department of Entomology, Oxford, and the other (C.H.S.W.) a research student at the Bureau of Animal Population, Oxford. For facilities in both departments we wish to express our gratitude.

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.

The effect of an amino acid imbalance in the diet on the growth of Hymenolepis diminuta in rats was reinvestigated. The claims of earlier workers that growth of the tapeworm is retarded was not substantiated.

The size frequency distribution of 575, 13-day-old H. diminuta recovered from 9- and 10-worm infections was determined, to reveal the shape of the distribution and total variance. The total variance in worm weights was analysed to determine the amount attributable to variance ‘between rats’ (comparison of the mean weight of the worms recovered from rats) and variance between worms within individual rats. ‘Between rats’ variance was found to account for > 80% of the total variance.

Assuming variance equal to that in the experiments described, the degree to which experiments can determine a difference in effect between two diets on the growth of tapeworms is analysed, and the degree to which this is altered by increasing the number of rats in dietary groups and/or decreasing the ‘between rats’ variance is estimated.

The suitability of various procedures for host dietary experiments on tapeworms is discussed; these include the duration of the experiment, feeding regime, age of worm and the relative advantages of weighing worms singly or in groups.

We gratefully acknowledge the technical help we have received from Miss Patricia Grant and Mr Jack Keys. Our thanks are also due to Mr Robert Elton of the University of Glasgow Statistics Department who has advised us on statistical procedures and calculated Table 6.

Some factors controlling differentiation of the protoscoleces of Echinococcus into strobilate or cystic stages, in vitro, have been identified. Development in a strobilate direction requires (a) evagination of the scolex, (b) contact of the evaginated scolex with a suitable nutritive substrate. These conditions are satisfied by treating fresh protoscoleces with pepsin and by evaginating them with trypsin + pancreatin + dog bile and subsequently culturing in diphasic media.

The most satisfactory diphasic system so far developed consisted of a base of bovine serum (coagulated by heating) with a liquid phase of Parker 199 medium +20% hydatid fluid and a gas phase of 8% O2+5% CO2 in N2. In this diphasic system, protoscoleces have been cultured to the 3-proglottid strobilate stage with the anlagen of the male and female genitalia formed: the first proglottid appeared after 30 days culture in vitro compared with 16 days in vivo; three proglottids were formed after 60 days. Development in vitro stopped just short of mature egg-producing worms.

That an intimate contact of a ‘placental’ nature takes place between the parasite and the solid proteinaceous base is shown by the fact that strobilization never took place in cultures lacking bases or in those in which the serum base was separated from the protoscoleces by a cellulose membrane. Cytological and ultra-structure studies of an evaginated protoscolex revealed that fully developed microtriches were present only in the rostellar region and not in the post-sucker region. It is concluded that it is this anterior region which makes initial contact with the nutritive substrate—a contact which appears to provide the ‘strobilization stimulus’. It is not known if the ‘strobilization stimulus’ is stimulatory (i.e. nervous) or nutritive or both. Contact with a non-nutritive base (i.e. agar) does not induce strobilization.

In any one sample of protoscoleces, after a high initial evagination, only about 30–40% finally remained evaginated and continued to show sucker activity. The degree of evagination appears to be characteristic for any sample of protoscoleces and may be related to the source of the hydatid cysts. It may prove to be a genetically determined character.

Evaginated protoscoleces became inactive under certain conditions and the degree of activity seems to be related to stimulatory and nutritive factors. The activity of evaginated protoscoleces was used as a criterion for assessing the effectiveness of a culture medium or condition. Evaginated protoscoleces which lost their activity developed small posterior bladders as did the majority of unevaginated protoscoleces. In both cases, posterior bladders enlarged and secreted a PAS-positive layer which formed the laminated membrane of a miniature hydatid cyst.

The well-known appearance of vesicular forms in cultures appears to be related to unfavourable culture conditions in vitro, such as a low or high pH or a low or high O2 tension.

It is pointed out that since the morphogenesis of Echinococcus protoscoleces can now be controlled to a considerable extent in vitro and since the contents of any one cyst have the same genotype, this organism provides exceptionally interesting material for studies on morphogenesis.

Acknowledgement is made to the Australian Wool Board, the National Institutes of Health, U.S. Public Health Services Grant No. AI-04707–03 and the World Health Organizati on for generous financial assistance in support of this work.

The skilful technical assistance of Mr A. B. Howkins and Mrs Mary Barton is also acknowledged.