Recent experimental work has indicated that species of Archigetes are capable of infecting and maturing in fishes in addition to tubificids.

The genus Archigetes is re-defined on the basis of morphological and biological characters, with particular emphasis on recent life history studies. All species capable of neotenic development have been re-united in a single genus.

A description of all species of Archigetes together with a key for their identification are included. New synonomies are discussed.

The relationship of Archigetes to other genera of the family is briefly considered, and it is concluded that it forms the terminal stage in a series showing the attainment of neotenic development.

I wish to thank Professor R. J. Pumphrey in whose Department the work was carried out, and Dr J. C. Chubb for advice and help in the preparation of this manuscript. I am also grateful to Professor K. Berg and Dr R. L. Calentine for the loan of specimens. The work was carried out during the tenure of a Nature Conservancy Research Studentship.

The effects of parasitism by five species of larval Digenea on the digestive gland cells of Littorina saxatilis (Olivi) subsp. tenebrosa (Montagu) were investigated by comparing histochemically stained cells in healthy, starved and parasitized animals.

Two distinct effects were apparent: (a) The germinal sacs compress the walls of the digestive gland tubules and block the lumen. Since food cannot pass this block, digestive gland cells behind undergo starvation autolysis. (b) In digestive gland tubules not cut off from their food supply, glucose, glycogen, glycoproteins and lipid food storage globules are reduced and there is a compensatory increase in the number of food vacuoles in the digestive cells; an increase in glucose but a decrease in glycogen in the visceral haemocoel; and an increase in the number of secretory cells, perhaps in order to eliminate the parasite's excretory products from the visceral haemocoel.

Schistocephalus plerocercoids in the weight range 2–200mg F.W. recovered from the perivisceral cavity of Gasterosteus aculeatus were cultured in various media. In a medium composed of 25% horse serum, 0·5% yeast extract, 0·65% glucose and Hanks's saline at pH 7·1, 21°C, 95% air +5% CO2, dry weight increases of up to 500% were recorded in 8 days. The specific growth rate of large plerocercoids was only one-tenth of the rate observed in small plerocercoids. A plerocercoid of double the weight of another had approximately half the specific growth rate.

Worms after 8 days cultivation were found to have only slightly higher than normal glycogen and water content, and to be able to mature when heated to 40°C. However, the rate of growth slowed to zero by the 24th day in culture at 21°C. Electron microscopic examination showed a ‘deposit’ formed over the microvilli, thin at 8 days but dense after 21 days.

The in vivo glycogen and water content of plerocercoids from 3–300 mg F.W. was determined. Glycogen rose from 24% in plerocercoids of 10mg F. W. to 50–55% in plerocercoids over 80mg F. W. The water content was found to mimic precisely this change, falling from 82% to a plateau of 67–69%.

We wish to thank Professor Gareth Owen for permission to use the photograph shown in the Plate and for his help while using the electron microscope. It is also a pleasure to thank Miss Patricia Grant for her technical assistance.

In the course of work designed to measure the extent to which methionine absorbed in one region of a tapeworm became distributed throughout the strobila, it was observed that, whereas, in saline, Hymenolepis diminuta lost previously absorbed methionine slowly, in the intestine of a rat the methionine was lost very rapidly. The fact that two worms containing initially the same amount of free methionine should, after a short period in different environments, contain utterly different quantities of methionine indicated that the quantity of a free amino acid present at any time is not simply dependent on the amount previously absorbed less the amount metabolized.

This observation has a bearing on several aspects of tapeworm physiology. Do tapeworms normally absorb amino acids from the intestine during periods of high concentration and release them when the concentration falls? If they do, the presence of tapeworms in the intestine could be beneficial to the host by extending the period over which an amino acid is available to the host, an important point as a mammal is unable to store amino acids for more than a few hours (Gitler, 1964). A knowledge of the environmental conditions which influence the level of free amino acids in a tapeworm, and thereby its ability to synthesize proteins, is obviously also of critical importance to workers attempting to grow worms in vitro.

The simple cylindroid pharynx of Ascaris lumbricoides has been shown to be characterized by a pharyngo-intestinal valve which is composed of two sets of valves. This is the only valve structure in the pharynx of this nematode. The pharynges of Oxyuris equi, Aplectana brevicaudata and Panagrellus silusiae have a series of valve structures along the lengths of the organs. All these types have a valve at the anterior end of the corpus, corrugated ‘valves’ in the posterior bulb, and pharyngo-intestinal valves. A valve between the corpus and the isthmus has been shown in Oxyuris.

In all the nematodes studied the pharyngo-intestinal valve is characterized by an overlapping of the muscle fibres associated with the valve and the main musculature of the pharynx that immediately precedes them. It has been suggested that the pharyngo-intestinal valves of the types studied may have evolved from the type of valve found in the Enoplina.

The marginal tissues of the pharynx of Ascaris lumbricoides have been shown to be composed of three types of tissues. It has been suggested that the marginal tissues have a suspensory and skeletal function. The suspensory strands subserve the first function, while the investing strands and the associated fibrous layer provide a longitudinal skeletal structure in the pharynx. In Oxyuris no fibrous layer is associated with the marginal tissues.

I would like to thank Professor J. E. Harris and Dr H. D. Crofton for their help and criticism during the course of this work, which was carried out during the tenure of a Postgraduate Studentship awarded by the Agricultural Research Council.

While some variation of detail may be found in different species of the hermaphroditic Digenea, the process of spermatogenesis conforms to a common plan throughout the group. There is divergence of opinion concerning the interpretation of the structure of the ripe spermatozoon.

In the testis of a young animal, primary, secondary and tertiary spermatogonia are present. The tertiary spermatogonia divide to give a group of eight primary spermatocytes that undergo the first division of the meiotic phase to form sixteen secondary spermatocytes. As a result of the second division of the meiotic phase, the sixteen secondary spermatocytes give a group of thirty-two spermatids that through a complicated series of changes are transformed into spermatozoa. During spermateleosis much of the cytoplasm of the late spermatid is sloughed off.

Recent research, including the application of the Feulgen technique, the examination of living material, and electron microscopy, supports the view that the ripe spermatozoon is composed of an elongate head, and a flagellum. The head, with the exception of its external sheaths, consists of nuclear material.

The flagellum of the sperm of Fasciola hepatica is Feulgen-negative and is composed of a proximal and a distal region. Its proximal part is provided with two axial filaments, a middle fibril and an external sheath. The axial filaments and the middle fibril are continued into the distal region where the external sheath appears to be absent. The external sheath of the flagellum of the sperm of Haematoloechus ( = Ostiolum) medioplexus is said to be composed of thirty-six fibrils. Claims that the flagellum of some digeneans contains a centriole, a middle-piece, and mitochondria have not so far been substantiated by means of electron microscopy.

Relatively little is known of the way in which the populations of monogenean parasites are related to the behaviour and physiology of their hosts or how such populations vary in composition over the year. Bychowsky (1957, trans. Hargis, 1961, p. 108) has described the results of studies upon six different fish hosts and parasites, while Llewellyn (1962) has studied two other species of Monogenea living on one host species. In all these investigations, the various ecological factors which influence the life cycles of the parasites have been determined.

Dogiel, Petrushevski & Polyanski (1961, p. 14) referred to a previous population study of Discocotyle sagittata on a host Coregonus lavaretus in Lake Ladoga, Russia. The parasite was said to breed in July and August, while in September broods of young and old parasites overlapped. The large individuals disappeared in October and November and this ‘death of the older generation’ was taken to indicate that the parasites had a life span of only one year.

The only relevant work available when the present study was commenced was the population study of the monogeneans Gastrocotyle trachuri and Pseudaxine trachuri by Llewellyn and as this concerned a marine host, it was of obvious interest to investigate the population dynamics of a parasite of a freshwater fish. Discocotyle sagittata was chosen because the biology of the British host, the brown trout (a different host from that in the Russian study of Discocotyle), has been intensively studied for many years. It was thought that this would be of advantage in attempting to relate any seasonal fluctuations in the parasite population to the biology of the host.

The dry weight and nitrogen content of Hymenolepis diminuta were studied after a period of 15 days‘ growth in rats receiving different diets. The special diets were fed during the last 7 days of this period, that is, while the major part of growth is occurring but before shedding proglottids.

Replacement of all the protein (casein) of the diet by carbohydrate resulted in greater development of the worm. However, replacement of casein by fat had no effect on worm development, indicating that removal of protein is not a significant factor, whereas addition of carbohydrate is effective.

Since exchange of casein by the nutritionally incomplete protein zein did not affect worm development significantly, protein quality appears not to be a factor in growth of H. diminuta.

The addition of tryptophan and lysine to the meal containing zein, in order to raise its nutritional value to that of casein, did not influence the growth of the parasite. However, the feeding of the same amounts of these two amino acids at times other than those of giving zein depressed the growth of the parasite. This was not due to type of protein (zein) fed in the rest of the diet, since a similar phenomenon occurs when casein is the dietary protein, or when a protein-free diet rich in carbohydrate is given. Addition of these amino acids, singly, either to diets containing protein or deficient in protein, had significant actions on worm growth, which was sometimes stimulated and in other circumstances reduced.

The nitrogen balance of the host was unaffected by the worm burden.

We are deeply indebted to Dr C. A. Hopkins for providing facilities, for infection of rats with the cestode, and for his helpful advice. Miss M. Cowan provided skilled technical assistance during the feeding of the animals and analysis of the host and parasite tissues. One of us (D.F.M.) gratefully acknowledges the Award of a Commonwealth Bursary from the Royal Society and the Nuffield Foundation.

The tegument of Proteocephalus pollanicoli is organized at two levels. The external level is a syncytial layer of protoplasm composed of vesicles in a granular matrix. The surface of this level has microvillus-like structure, microtriches, with sharp electron dense tips and less dense bases. Pinocytotic vesicles, large vacuoles, and evaginations occur at the surface of the external level, whereas mitochondria and lipid droplets tend to be concentrated basally. The basal plasma membrane has in some places small invaginations and at others large evaginations forming protoplasmic tubes which connect the external and internal levels.

The internal level is composed of individual areas of dense, granular protoplasm, each of which has a nucleus. These areas, or cuticular cells, contain lipid droplets, mitochondria, short lengths of agranular endoplasmic reticulum and phospholipid bodies. Two cuticular cells lying close together may be joined by a protoplasmic bridge and thereby form a limited syncytium.

The morphology of the tegument of this proteocephalid is compared with that of the cyclophyllidean species and the function of the tegument in parasitic helminths is discussed.

I should like to acknowledge the technical assistance of Mr W. Ferguson and Mr A. Lyness. My thanks are also due to the Wellcome Trust for a grant to purchase the Akashi Transcope and ancillary equipment.

Four species of the genus Capillaria—C. anatis, C. bursata, C. caudinflata and C. obsignata—are described from the domestic fowl. C. bursata constitutes a new species record for Britain. Taxonomic problems concerning these species in particular, and of Capillaria species in general, are discussed in the light of experience gained from the present study.

This work was carried out during the tenure of a British Egg Marketing Board research studentship and is part of a thesis accepted by the University of London for the Ph.D. degree. My thanks are due to the director of the Houghton Poultry Research Station for granting facilities for work and to the staff for their help and encouragement. Mr R. A. Avery kindly supplied me with material collected from the Wildfowl Trust, Slimbridge.

Polycercus lumbrici, previously described only by Metchnikov (1868) in Russia, has been found in Scotland in the earthworm Allolobophora terrestris.

Experimental infestation of earthworms with Polycercus lumbrici as a result of feeding them with the eggs of Paricterotaenia paradoxa shows that the proliferating cysts are the larvae of the latter as suggested by Joyeux and Baer (1939).

There is evidence that Polycercus may induce parasitic castration of the earthworm in which it occurs.

Metchnikov's description of the development of the larva is confirmed for the most part but is modified to include a retroversion of the larva in its early stages of development. The development of the scolex is shown to follow a pattern similar to that of Cysticercus fasciolaris in that the hooks are borne on a ‘prebulb’ which later invests a ‘bulb’, the former giving rise to the muscles of the walls of the double sac of the rostellum, the latter to the hook-elevator muscles.

The rostellar hooks of P. paradoxa are elevated by the action of a double layer of muscles which radiate from a conical invagination of the bulb. These muscles, in association with the muscles of the wall of the rostellum, elevate the hooks by modifying the shape of the rostellum rather than by direct action on the hooks.

The nervous system and the excretory system in the scolex of P. paradoxa are briefly described and the nature of the ‘glandular’ structure of the rostellum is discussed.

I wish to record my gratitude to Professor H. G. Callan for affording me the facilities for my work and to the many persons, too numerous to mention by name, who helped me to obtain my material. I am particularly indebted to Mr D. R. R. Burt for guidance and constructive criticism at all stages of my work and to Mr I. M. Sandeman for much helpful advice.

The work was carried out during the tenure of Research Grants from Madras College, St Andrews, and from the D.S.I.R., to whom I would express my appreciation for their aid.

At present many laboratories throughout the world are studying the chemotherapy and immunology of Schistosoma mansoni in laboratory hosts. Many workers judge the success or failure of their attempts to cure or immunize these hosts from the ratio of the number of living adult worms recovered to the number of infecting cercariae. This ratio is affected, however, not only by the efficacy of any treatment, but also by the methods used to infect the animals and to recover the worms. If these methods result in widely varying worm recoveries amongst the animals in any experimental group, then small but significant effects of treatment might well be missed. Alternatively, such large experimental groups must be used that the work becomes tedious to perform and depends upon the availability of a great deal of technical assistance. This paper describes techniques which are rapid and do not require great skill in their performance. More important, in our hands they have given very consistent results. In this respect, particularly, we believe that these techniques have advantages over others which are currently practised.

The techniques described here are those which were used in other investigations reported in this journal (Smithers & Terry, 1965a, b).

The strain of S. mansoni used throughout this work was isolated in Puerto Rico and was obtained through the courtesy of Dr W. B. DeWitt of the National Institutes of Health. The parasite is maintained in an albino strain of Australorbis glabratus (Newton, 1955). Snails are exposed individually to ten miracidia overnight at 27 °C.

Leptocotyle minor is a monogenean parasite specialized for living on the denticlecovered skin of the dogfish Scyliorhinus canicula. Each adult parasite attaches itself to the outer surface of a single denticle. Small postlarvae are found on the inner surfaces of the denticles. The denticles have hard enamel on both inner and outer exposed surfaces. Haptoral hooks, such as are present in other monogenean oncomiracidia, would be unable to penetrate this hard surface and are absent in postlarval and adult parasites which attach themselves by cement. The oncomiracidium possesses six haptoral spicules which may be homologues of the marginal hooks of other monogeneans, but these spicules disappear early in postoncomiracidial development.

Adult parasites feed on the host's epidermis which lies between the denticles. The position of small postlarvae on the inner surfaces of the denticles enables them to reach the epidermis for feeding purposes. Growth then permits young parasites to migrate away from the epidermis on to the outer surfaces of the denticles.

Adult parasites move from one denticle to another in a leech-like manner, and copulation occurs between parasites which meet as a result of these movements.

I am grateful to the Director and Staff of the Plymouth Laboratory for their hospitality and particularly to Mr J. E. Green for his interest and co-operation.

Male rats were more susceptible than female rats to experimental infection with Amplicaecum robertsi. This sex difference first appeared at the time of puberty, i.e. about 50 days of age.

Suckling rats were less susceptible to infection than weanlings. This was attributed to the cumulative effects of intestinal structure, the indirect effects of milk on the physio-chemical factors in the gut, and the direct effects of milk on the worms themselves.

The rat showed an age resistance to A. robertsi: this was greatest in the female host.

The larvae recovered from females were shorter than those recovered from male rats except in immature animals.

The longest larvae were recovered from suckling rats: in old rats the length of the larvae recovered became progressively shorter as the animals aged.

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

In recent years there has been a steadily growing interest in the immunology of schistosomiasis. Much work has been carried out using a variety of experimental hosts and many attempts have been made to vaccinate these hosts against schistosomes, both by exposing them to irradiated cercariae and by injecting them with various antigens derived from the parasites. These attempts have been attended by varying degrees of success. The ultimate object of much of this work has been its possible application to the disease in man. We believe, however, that work of this kind cannot really be profitably undertaken unless there already exists a sound understanding of the basic host-parasite relations between the schistosomes and their hosts. This knowledge is essential in order to decide whether a finding in one host is likely to be applicable in others. With this idea in mind, we report here our findings on naturally acquired resistance to Schistosoma mansoni in the rhesus monkey.

A large number of nematodes from Protemnodon rufogrisea, P. bicolor, P. dorsalis, and Macropus canguru, from southern Queensland, M. robustus from Western Australia, and Megaleia rufa (? locality) have been examined. The species described in this paper are: Pharyngostrongylus macropodis Yorke & Maplestone, P. kappa, sp.nov. (syn. P. macropodis in part), P. eta Johnston & Mawson, P. lambda, sp.nov., Rugopharynx epsilon (J. & M.), R. delta (J. & M.), R. zeta (J. & M.) (syn. R. longibursaris (Kung)), R. theta (J. & M.), Coronostrongylus coronatus J. & M.; a new genus is proposed, Oesophagonastes, with type O. kartana syn. Spirostrongylus kartana Mawson to include O. parma syn. Pharyngostrongylus parma J. & M., O. gallardi syn. P. gallardi J. & M., and O. leptos, sp.nov.

Most of the parasites were sent for examination by Dr M. J. Mackerras, then of the Queensland Institute of Medical Research. Those from Western Australia were from Dr T. Ealey of the Zoology Department, Monash University, Victoria, who was working on Macropus robustus in the north of that state. I am most grateful to both of these helpers.

The small intestine of rats was divided into twenty sections in a reproducible manner in order to study the distribution of Nippostrongylus brasiliensis by stretching it under a tension of 5g in adrenaline saline.

A small but significant difference between the distribution of parasites in male and female rats was observed.

As larvae had virtually ceased to reach the intestine by the fifth day all changes in distribution after that day were due to movements of the established adult population.

Up to the twelfth day of a primary infestation the majority of the worms were found between the third and tenth sections, the population mode being in the fifth or sixth section. After the thirteenth day the number of worms in this region fell sharply.

The female worms had not ceased from egg-laying by the time most of the worms were being rejected.

The posterior half of the small intestine, i.e. the eleventh to twentieth sections, was not heavily parasitized, many of the worms seen being in passage to the anus.

The first section was not parasitized until the seventh day, but thereafter remained parasitized until long after worms had disappeared from the more posterior sections.

The relative number of male worms present increased as the infestation aged.

Throughout the experiment the relative number of male worms present at the anterior end of the smaller intestine was higher than that at the posterior.

Fourth-stage larvae were found chiefly in the sections that were later most heavily parasitized by adult worms.

The present studies were designed to characterize the response of sheep infected with N. spathiger to challenge infection with the same species.

The results showed that: (1) the level of worm counts attained on secondary infection with the parasite was much reduced compared with the counts recorded following initial infection; (2) the relative fecundities of parasite populations which developed in individual hosts following challenge infection, ranked in magnitude in an order which was similar to that established following initial infection; (3) as fecundity was highly correlated with the number of egg-laying females and the number of males and females was nearly equal in individual specimens, it was concluded that worm burdens were also ranked for individual animals on primary infection and following challenge; and (4) the animals of the control group remained as susceptible as the animals of groups 1 and 3 at the time of their primary infection; consequently, in the absence of previous immunological experience due to infection, there was no apparent change in susceptibility of the animals due to an effect of age during the course of the 17 weeks of the experiment.

We are greatly indebted to Dr Michael Tallis, Division of Mathematical Statistics, C.S.I.R.O., for advice and assistance with the statistical analyses, and to Dr D. F. Stewart for his interest and constructive criticism during the course of these studies. Our thanks are also due to Mr E. Teleki and Misses Helen Giller and Lindy Stothart for their valuable technical assistance.

The host–parasite relationships of Schistosoma mansoni in the albino rat have been studied recently by Ritchie, Garson & Knight (1963) and by Sadun & Bruce (1964). The general findings of these and other workers are that, in rats, the worms are always stunted, although they may pair and non-viable eggs may be found in the liver. The worms remain in the liver and seldom, if ever, migrate to the mesenteries. Ritchie et al. (1963) found that the majority of the worms were eliminated between the 4th and 8th week after infection; this is much sooner than in other experimental hosts. Sadun & Bruce (1964) suggest that rats are particularly suitable and convenient for studies on acquired resistance to S. mansoni; this suggestion is based mainly on the grounds that acquired resistance is more easily demonstrated in animals which show a relatively great degree of innate resistance.

The present work has been undertaken in order to establish whether the immune response in the albino rat is at all similar to that in the rhesus monkey (Smithers & Terry, 1965b). Two features of the host–parasite system have been studied in detail; the elimination of the worms which follows an initial infection and a comparison of the development of acquired resistance following exposure to X-irradiated cercariae with that following exposure to normal cercariae.

A Puerto-Rican strain of S. mansoni, and Sprague–Dawley rats weighing 100–200 g, were used in all experiments. The methods of exposing the rats percutaneously to infection and the recovery of the worms by perfusion have been fully described (Smithers & Terry, 1965 a).

A short account of the biology and life-cycle of Diplostomum spathaceum is given.

The morphology of the forebody is described in some detail. The lappets are well developed and function as organs of attachment as well as being reservoirs for the secretion from the forebody gland cells. The adhesive organ is variable in form and the muscles facilitating the movements are described in detail. Pressure changes in the excretory system are thought to assist in evaginating the adhesive organ.

The histochemical tests gave the following results. The phosphatases present in the adhesive organ and the forebody gland cells are of the acid variety, the alkaline phosphatase is only present in the cuticle, traces of it occur in the lappets and forebody gland cells. No leucine aminopeptidase activity was observed. The esterase present is either a pseudocholinesterase or a non-specific B-esterase. The presence of RNA is confined to areas of enzyme formation.

The in vitro studies confirmed the absence of alkaline phosphatase and the presence of acid phosphatase and esterase in the forebody. They also showed a secretion of acid phosphatase and esterase to the exterior of the parasite.

Diplostomum spathaceum has very little effect on the host mucosa, but some lysis of the host tissues in contact with the forebody and especially the lappets, is observed. The response of the host to the parasite is likewise very small.

The source of nutrients for D. spathaceum is believed to be the contents of the host alimentary tract, host mucus secretions and to a small extent disintegrated host tissues.

I am very grateful to Dr D. A. Erasmus for advice and encouragement during this study and to Professor J. Brough for his interest and for provision of excellent working facilities. The work was conducted during the tenure of a Rotary Foundation Scholarship for International Understanding and a grant from Nylands Nation (Helsingfors, Finland).