The infective larvae of S. ratti had a high Qo2 (9·8 μl/mg dry weight/h at 20 °C). Their R.Q. was 0·74 indicating that lipid was the main metabolite.

As the infective larvae aged there was an exponential decrease in their infectivity and a corresponding decrease with age in the Qo2 rate of activity and rate of substrate utilization. The R.Q. of fresh and aged larvae was the same showing that there was no change in the substrate being utilized.

The infective larvae of S. ratti contained 25% lipid 3% glycogen and 0·25% free sugars (dry weight). The free sugars consisted of glucose and trehalose in approximately equal amounts. Both lipid and glycogen decreased as the larvae aged.

Analyses showed all the normal lipid components to be present, the major lipid fraction detected was free fatty acid. As the larvae aged there was a big decrease in the amounts of triglyceride and free fatty acid. There was, however, no difference in the amounts of cholesterol or in the phospholipid composition of fresh and aged larvae.

The fatty-acid composition of the larvae was analysed; 68% of the total fatty acids were unsaturated and the major fatty acid was C18:1. As the larvae aged, the fatty acids were not all catabolized at the same rate; the acid C18:1 was the most heavily catabolized decreasing by 75% in 8 days, whilst C20:1 decreased by only half this amount in the same time.

The distribution of lipid and glycogen was investigated histochemically. Lipid droplets were confined to the posterior two-thirds of the infective larvae and occurred in the lateral lines, non-contractile parts of the muscles and in the cells of the intestine. As the larvae aged the lipid in the body wall and lateral lines disappeared first. Dark granules, possibly excretory products, accumulated in old larvae. Glycogen was found in the lateral lines throughout the length of the larva.

I should like to thank Dr P. Tate for his constant advice and encouragement. This work was carried out during the tenure of a Medical Research Council scholarship.

Ultrastructural studies on adult Nippostrongylus brasiliensis removed at varying stages after infection from rats given a single dose of 2000 infective larvae have shown that changes in their cytology occur and that these can be correlated with the onset and development of immunity to this nematode in the host. The most noticeable changes are resorption of spermatozoa by the male reproductive tract, a drop in egg production, changes in the cytology of the intestinal cells, and the appearance of many large droplets, which look like droplets of lipid, in the cytoplasm of the hypodermis, muscles, excretory glands, reproductive system and the intestine. There are no obvious changes in the structure of the cuticle of the nematode during the course of infection. Possible reasons for the onset of these changes in the nematode are discussed.

Much of this work was carried out during the tenure of grants from the U.S. Public Health Service (grant number AI 04275) and from the Agricultural Research Council while I was working at the Molteno Institute in Cambridge. I wish to thank the late Professor J. D. Boyd and Dr C. C. D. Shute of the Department of Anatomy, Cambridge, for permission to use the electron microscopes, Mrs B. Fisher for technical assistance and Mr Paul Rogers for assistance with the photography.

The fine-structure of the epimastigote forms of Trypanosoma lewisi in the rectum of the flea, Nosopsyllus fasciatus is described.

The parasites are attached to the rectal wall by penetration of the flagella between the highly folded wall of the rectum, by the wedging of the expanded flagella in these folds and irregularities of the wall and by a zonula adherens functional complex between the flagellar membrane and the cuticle of the rectum.

The parasites remain associated with each other by their flagella. The daughter axonemes produced during division are retained within the parent flagellar membrane; this delayed division of the flagella results in up to four axonemes being found within a single flagellar membrane. The parasites accumulate together in masses in the lumen of the rectum and their surfaces interdigitate resulting in the mass of parasites being retained together.

The financial assistance of the Agricultural Research Council is gratefully acknowledged. My sincere thanks are due also to my supervisor Dr Tate for his help, encouragement and enthusiasm through the course of this work.

The structure of the sporozoite of Eimeria tenella has been studied with the aid of the electron microscope, using both sectioned fixed material and negatively stained or shadowed whole cells. An apical ring gives rise to a series of 24 fibrils which extend posteriorly under a double bilamellar membrane. Within the apical ring lies a conoid with spirally arranged tubules and an apical pore; under some conditions at least this conoid can be pushed forward beyond the apical ring. Originating within the conoid are a group of club-shaped organelles which extend backwards into the cell to lie among an extensive regular array of micronemes. The nucleus lies between two apparently structureless paranuclear bodies, and scattered throughout the remaining cytoplasm are mitochondria with tubular cristae, oval structures containing amylopectin, occasional lipid droplets, and vesicles containing nondescript material possibly undergoing digestion and which may have a possible connexion with a lateral micropyle.

I am indebted to Mrs Muriel Bentley for much technical collaboration and to Mrs McLaren for making her manuscript available before publication.

The value of metazoan parasites as models for the investigation of fundamental biological phenomena is discussed. The fields in which they are of particular value include: immunology, neuromuscular physiology, nucleo-cytoplasmic relationships, genetics, cyto-differentiation, comparative biochemistry.

After reviewing some examples of experimental organisms in trematodes and nematodes, the cestode, Echinococcus granulosus, in particular, is examined as a model.

Problems of particular interest in this organism are: the function of the rostellar gland, whose cells release a secretion which has its origin in the nuclei of these cells; the mode of action of the suckers, which in vivo are thought to remain in a state of sustained contraction; the mode of digestion at the host–parasite interface—membrane digestion appears to be involved; the various modes of membrane transport through the tegument; factors controlling cystic or strobilar differentiation, a problem viewed in the light of the Jacob–Monad model of gene action.

A hypothetical control circuit for differentiation of the stages in parasite life-cycles in general is also put forward.

Some preliminary experiments on the in vitro culture of Taenia serialis, are described. The factors controlling differentiation into cystic or strobilar directions appear to be generally similar to those which operate for Echinococcus granulosus.

It was stressed that many other species of parasites could also serve as models for investigating similar or related problems.

Some of the studies described here were supported by grants from the Australian Wool Board, the U.S. Public Health Service (Grant no. A 107452–02) and the Rural Credits Development Fund.

I am grateful to Mr D. Heath for the provision of the micrographs of Taenia pisiformis and T. serialis in P1. 2, figs. 2, 3.

The range of interactions between blood-sucking arthropods and their hosts is enormous and their importance to man would be difficult to overestimate. From the tundra to the tropics man, and animals in which he has a vital nutritional or economic interest, represent to the blood-sucking arthropod a source of animal protein. By their activities in obtaining a blood meal the host may be subjected to minimal inconvenience, acute irritation, or suffer severely from blood loss or from the effects of toxins introduced during feeding. Superimposed upon these situations is an enormous variety of disease agents which may be transmitted to the host by the parasite during or after feeding.

The durations of the various phases of the life-history of the ixodid tick H. anatolicum anatolicum are described with particular reference to the modifying influence of temperature and relative humidity. An increase in the ambient temperature was found to decrease the length of the feeding, preoviposition, oviposition, larval and nymphal moulting periods and the egg-incubation period. The effect of change in relative humidity was less marked, the only significant effect being on the oviposition period which was protracted when the relative humidity was increased.

Daily percentage water loss and longevity were found to be more or less inversely proportional. Thus at high temperatures and low relative humidities a large percentage of water was lost daily with consequent reduced life span.

I wish to thank Dr R. P. Chaudhuri for providing me with the specimens of H. anatolicum anatolicum from which the culture was started, and Professor D. R. Arthur for his advice and assistance. I am also grateful to the Science Research Council for the provision of a research grant.

Red blood cells from rats were labelled with 51Cr by in vitro incubation with labelled sodium chromate. After re-injection of autologous labelled red cells, rats, each infected with 5000 N. brasiliensis larvae, and non-infected controls were placed in individual metabolism cages. Twenty-four hour samples of faeces and urine from both groups were collected throughout the course of the infection. From a knowledge of the specific radioactivity of host blood, it was possible to express the radioactivity found in the faeces and urine in terms of volume of whole blood. A burden of approximately 1000 adult worms caused a blood loss of about 100 μl every 24 h during the period day 4 to day 14 after infection. This gastro-intestinal haemorrhage could provide a vehicle whereby antibody gains access to the worm. No radioactivity was found in adult worms isolated from infected rats killed at 2-day intervals throughout the course of the infection. Adult N. brasiliensis did not ingest an appreciable quantity of host blood.

Host records, geographical distribution, and the taxonomy and synonymy, of the ascaridoid nematodes recorded as occurring in the python family, are presented. The evidence indicates that, besides Amplicaecum robertsi, which is so far reported only from Australian pythons, and possibly Hexametra quadricornis, there are two other distinct species which can be regarded as natural parasites of pythons, namely Ophidascaris filaria and Polydelphis anoura; these two species are described from specimens from Queensland pythons and compared with specimens from Africa and India. Perhaps it may eventually be established that further distinct species, or subspecies can be differentiated among the ascaridoid nematodes of pythons in different geographic areas, in addition to the above-named species. But it is suggested that the existing criteria for differentiation of other previously named species are not valid. Accordingly, it is proposed that the following specific names be reduced to synonymy within these two species respectively. Under O. filaria: rubicunda (male), infundibulicola, baylisi, and amucronata; ajgaris is a doubtful species. Under P. anoura: attenuata rubicunda (female), oculata, and bicornuta; mucronata is a doubtful species.

Grateful acknowledgement is made of help, advice and co-operation on various aspects of this work which have been received from the following: Dr W. W. Becklund, Beltsville, Md.; Dr A. G. Chabaud, Museum National d'Histoire naturelle, Paris; Mr H. G. Cogger, The Australian Museum, Sydney; Dr G. Hartwich, Institut für Spezielle Zoologie und Zoologisches Museum, Der Humboldt-Universität zu Berlin; Dr W. G. Inglis, British Museum (Natural History); London; Dr P. Tate, University of Cambridge.

The invaluable technical assistance of Ann McKeown and Marian Hollis is also gratefully acknowledged.

This work was financed by a grant from the Australian Research Grants Committee.

Observations have been made on the distribution of faecal deposits on a number of small fields of varying size stocked with varying numbers of sheep. These studies were undertaken to examine certain hypotheses proposed in the construction of mathematical models for the distribution of strongyloid nematode infective larvae in pastures grazed by sheep.

The negative binomial distribution was found to provide a reasonable empirical description of the distribution of faeces, although there was evidence of departure from the negative binomial on the largest field.

Although, tentatively, the distributions were additive with respect to the number of sheep, there was evidence of departure from additivity in respect of field area and time. The implications of these findings for the model are discussed.

Significant positive correlations were found between successive time intervals in the number of faecal deposits present in individual subplots of the majority of fields. It is suggested that these arise from the tendency of sheep to use the same portion of a field for resting and rumination when heavy concentrations of faeces are deposited. The effects of this and other possible patterns of grazing and faecal deposition are discussed in relation to the intake of strongyloid infective larvae.

The average number of discrete deposits of faeces on pasture produced per sheep per day was greater in flocks producing faeces in pellet form than in flocks producing soft faecal masses. However, within faeces types, there was evidence that the expected number of deposits produced per sheep per day could reasonably be assumed constant.

The greater part of this work was carried out while one of us (A.D.D.) was the holder of a C.S.I.R.O. Overseas Research Studentship. For his advice and constructive criticism I am much indebted to Dr H. D. Crofton, under whose supervision this study was undertaken. My thanks go also to Professor J. E. Harris, C.B.E., F.R.S., for the provision of facilities in the Department of Zoology, and to Professor T. K. Ewer and Mr M. R. McCrea for their willing co-operation in the provision and handling of pastures and sheep at the School of Veterinary Science, Langford.

Mr David Culpin and Mrs Jean Williams, of the Division of Mathematical Statistics, were responsible for programming and organizing the calculations on the CDC 3200 Computer, and were consistently helpful in proposing and criticizing tests and procedures.

Saurocytozoon tupinambi gen.nov., sp.nov. (Haemosporidiidea, Leucocytozoidae) is described in the teiid lizard, Tupinambus nigropunctatus, from Pará State, Brazil. This would appear to be the first record of a member of this family in a reptile; the existing genera Leucocytozoon and Akiba are known only in birds.

The gametocytes of S. tupinambi develop within lymphocytes and immature erythrocytes. Only developing and mature gametocytes are found in the peripheral blood: morphologically they are very similar to the gametocytes of Leucocytozoon and Akiba. No mega-schizonts or other tissue stages have yet been detected.

Aedes spp., have been trapped while feeding on Tupinambus in areas where infected lizards have been caught, and may possibly be the vectors.

We are indebted to Professor P. C. C. Garnham, the London School of Hygiene and Tropical Medicine, London, who has examined our material and offered his invaluable opinions.

The cost of the colour plate was defrayed by the Wellcome Parasitology Unit, Instituto Evandro Chagas, Fundaçao Serviço Especial de Saúde Pública, Belem, Pará, Brazil.

Plasmodium (Sauramoeba) diploglossi Aragão & Neiva, 1909, has been rediscovered in the skink Mabuya mabouya (Scincidae). This appears to be the first time that this malaria parasite has been re-encountered since its original description in the lizard Diploglossus fasciatus (Anguidae). Six out of 20 skinks were infected, all from the Utinga Forest, Belém, Pará, north Brazil.

P. (Sauramoeba) tropiduri Aragao & Neiva, 1909, is redescribed in the samelizard, Mabuya mabouya, representing a new host record. Many of the infections were concomitant with P. diploglossi. Exo-erythrocytic schizonts were encountered in blood and tissue smears but it remains uncertain to which parasite they belong.

P. (Sauramoeba) cnemidophori Carini, 1941, has been found in the teiid lizard Ameiva ameiva, a new host record. Exo-erythrocytic schizonts were abundant in lymphocytes and thrombocytes of the peripheral blood.

The cost of the colour plate was defrayed by the Wellcome Parasitology Unit, Institute Evandro Chagas, Fundacao Servico Especial de Saúde Pública, Belem, Pará, Brazil.

The structure, method of feeding and relationship with the host cells of the tissue parasitizing stages of Histomonas meleagridis have been studied by means of light microscopy, cytochemistry and electron microscopy. The invasive stage is amoeboid, lacks flagella but has what appears to be remnants of the flagella apparatus, and feeds by phagocytosis. The vegetative stage is a round quiescent form. It apparently feeds by secreting proteolytic enzymes, which carry out extra-corporeal digestion of the host's tissues, and then taking up small particles of disrupted host cells by means of pinocytosis and probably by diffusion. The vegetative stage also contains remnants of flagella apparatus. Neither the invasive stage nor the vegetative stage contains mitochondria and succinic dehydrogenase is absent. It is concluded that there is no resistant stage.

The reaction of the host's tissues to the parasites has been briefly described.

The authors are grateful to Dr C. C. D. Shute for permission to use the electron microscope, to Mrs B. Fisher and Mr M. Shirley for technical assistance and to Mr P. Rogers for assistance with the photography.

The effect of temperature on the hatching and viability of the eggs of Bunostomum trigonocephalum in a tropical climate is described. All experiments were performed with 500 eggs in each dish of saline medium. Experiments on hatching of eggs at any one temperature were repeated twelve times and on viability of eggs eight times.

The range of temperature for hatching was found to be 10–34 °C. The time for commencement and for termination of hatching of eggs (in hours) was 18 and 48 at 34 °C, 21 and 51 at 30 °C, 24 and 54 at 25 °C, 45 and 78 at 20 °C, 77 and 119 at 15 °C, and 264 and 360 at 10 °C.

The rate of hatching of eggs was observed at temperatures of 10, 15, 20, 25, 30 and 34 °C. Maximum hatching took place at 34 °C, which is also the optimum temperature for hatching. At a temperature of 10 °C, 98–99% of the eggs did not develop.

The viability of eggs was observed at temperatures above and below the hatching range. Eggs remained viable up to 7 h at – 10 °C, 5 days at 0 °C, 7 days at 5 °C, 10 h at 35 °C, 6 h at 40 °C, 1 h at 45 °C, and 20 min at 50 °C.

The authors wish to acknowledge a research grant from the Scientific Research Committee, Uttar Pradesh, India. We also express our thanks to Professor M. B. Lal for his kind encouragement and interest throughout this study.

The ultrastructure of the lappets of Apatemon gracilis minor Yamaguti, 1933, has been described. The tegument covering the lappets is characterized by the presence of long seta-like structures and unicellular gland cells which exhibit a holocrine type of secretion. The setae contain extensions of the tegument cytoplasm and are supported by a bundle of fibrils. The unicellular gland cells are pear-shaped with the tapering neck supported by a ring of 50–60 microtubules. The secretion consists of relatively large, membrane bounded bodies containing a fine granular material.

The lappets are regarded as representing a specialized host–parasite interface in which adaptation exists for attachment to the host tissue and the discharge of secretion from the parasite at this region. The concept of the specialized host–parasite interface is discussed and extended to include Apatemon gracilis minor.

The author wishes to acknowledge the research grant provided by the S.R.C. for the purchase of a vacuum coating unit and an AEI EM 6 electron microscope. The progress of this study was greatly facilitated by the excellent assistance of Mr T. Davies and Miss C. Green. The stereoscan micrograph (P1. 1, fig. 2) is published by permission of the Cambridge Instrument Company.

The establishment of N. dubius juveniles was more successful in mice maintained on a diet inadequate to support the full growth potential of the animals than in mice maintained on an adequate diet. The distribution of encysted juveniles along the duodenum was considerably more extensive in the former group of mice.

More juveniles were established in male than in female mice within the respective dietary groups, although the pattern of distribution was similar.

The distribution of adult nematodes at 10 days after infection was confined to a smaller area of the duodenum than that of the encysted juveniles at 5 days.

The differences in distribution of nematodes after 5 and 10 days between the HP and LP mice was not evident after 21 days. The survival of adult nematodes in the HP mice was more successful than in the LP animals.

The female nematodes in the LP mice achieved a greater length but were less prolific egg producers than those in the HP mice.

A diet inadequate to support the full growth potential of mice was therefore associated with the increased ability of Nematospiroides dubius juveniles to establish themselves in mice compared with the situation in adequately fed animals. The low plane diet, on the other hand, was associated with a decreased ability of the adult nematodes to survive.

This work was supported by grants from the Commonwealth Scholarship and Fellowship Plan and the Australian Wool Board. I should like to express my appreciation for the supervision of this work by Professor J. F. A. Sprent and to thank Miss Sharon McFeeter and Mrs Anne McKeown for their most competent technical assistance.

The development of the exsheathed third-stage larva of Haemonchus contortus to the fourth stage in vitro has been investigated in half-strength Ringer's solution and in solutions with ionic concentrations similar to those found in the abomasum during normal secretion of acid and during heavy infestations of Haemonchus. The concentration of carbon dioxide required for the optimum development of the larva has been shown to be dependent on the pH of the medium. At a low pH the optimum pC02 is low and the larva is less sensitive to a range of carbon dioxide concentrations than at a pH near neutrality. Gastric mucin and histamine have been shown to have an inhibiting and a potentiating effect, respectively, on development, in media at pH 6.