Single receptors seen with the electron microscope to consist of a terminal cilium embedded in a nerve bulb have been found in the skin parasitic monogenean Gyrodactylus sp., adult and larval Entobdella soleae, Leptocotyle minor (adult) and in the endoparasitic juvenile of Amphibdella flaviolineata. Their distribution in Gyrodactylus sp. has been mapped using phase-contrast microscopy, and staining with the indoxyl acetate method for non-specific esterases revealed a nervous connexion between the lateral ventral nerve cord and the ‘tangoreceptor’. The general relationships of the nervous system in Gyrodactylus were investigated by staining with the thiocholine method for cholinesterase. The ‘sensory’ neurone contains vesicles, microtubules and mitochondria and the whole nerve bulb is sealed into the epidermis by means of septate desmosomes. The basal body of the cilium is not greatly modified and there is no obvious rootlet system. The terminal cilium of single receptors in Entobdella soleae has a 9 + 2 structure. A compound sense organ from the head of Entobdella soleae has also been described. The significance of the fibre arrangement in the terminal cilia and the possible roles of these sense organs have been discussed.

I should like to thank Dr D. L. Lee for training in electron microscope techniques and for his encouragement, Dr P. Tate for his continued interest and the staff of the Marine Biological Laboratory, Plymouth, especially Mr G. Best and Mr J. E. Green, for their assistance. This work was conducted during tenure of a Science Research Council Fellowship and the Tucker-Price Research Fellowship of Girton College.

The morphology and biology of the various stages of the life-cycle of Trypanosoma evotomys has been studied in laboratory bred vertebrate and invertebrate hosts. The morphology of T. evotomys in the blood of the bank-vole, Clethrionomys glareolus, is typical of that of a trypanosome of the subgenus Herpetosoma, but the shape and size of the trypanosome in C. glareolus is more variable during the reproductive phase of the infection than in the adult phase. The duration of the infection of T. evotomys in C. glareolus varies from 22 to 31 days in experimental infections and the incubation period of the infection initiated by metatrypanosomes is 5–6 days. The peak of the parasitaemia varied from 15000 to 55000 parasites per mm3 of blood. The reproductive stages of T. evotomys are found in the spleen and lymphoid tissue (appendix) of the bank-vole, and reproduction is by equal, binary or multiple fission in the amastigote stage. T. evotomys was not transmissible to M. agrestis, mice or rats. Cyclical development of T. evotomys took place in the flea, Nosopsyllus fasciatus, and metatrypanosomes developed in the intestine of the flea. No intracellular forms of T. evotomys were found in the mid-gut of the flea.

The encouragement, help and advice of Dr P. Tate is gratefully acknowledged. Financial support was received from the Agricultural Research Council to whom thanks are also due.

Sera pooled from rats which were immune to Babesia rodhaini infection had demonstrable protective activity against the piroplasm in non-immune rats. When inoculated with parasites or close to the time of infection, antiserum caused a delay in the onset of patency but did not usually affect the course and severity of the patent parasitaemia in young rats. There were more survivors when antiserum was given in a series of small doses over a period rather than a single large inoculum at the time of infection. Serum from rats previously reinfected several times, when injected into infected rats during the early stages of patency, was followed by a rapid fall in parasite numbers. A severe secondary parasitaemia eventually ensued in these animals.

I would like to thank Dr S. F. Barnett for his advice during the work and Dr F. Hawking and my colleagues at Mill Hill for criticizing the manuscript. This work was carried out during the tenure of an Agricultural Research Council Studentship.

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.

Dense larval suspensions have a higher Qo2 than dilute ones, probably due to an oxygen debt acquired during preliminary washings. Ammonia (0·1M) did not affect the respiratory rate of the infective larvae of S. ratti. Mechanical agitation of larvae reduced their rate of respiration.

The partial pressure: respiration curve of the infective larvae was sigmoidal, that of larval homogenates was a rectangular hyperbola. At low partial pressures of oxygen the respiratory rate depends to a certain extent on the rate of change of the partial pressure.

In 100% oxygen the respiratory rate of the larvae is at first stimulated and then falls to its initial level. On returning to air after 100% oxygen, the respiratory rate is at first depressed and then recovers.

The infective larvae of S. ratti develop an oxygen debt when kept under anaerobic conditions. During the recovery period, which lasts for about 1 h, the Qo2 may increase by up to 100%.

The respiration of the infective larvae increases as the temperature is increased. When larvae are maintained at 37 °C there is a gradual decrease in their rate of respiration and this change is apparently irreversible.

The rate of respiration of the larvae is slightly inhibited at low pH values and slightly stimulated at high pH values.

Moderate levels of carbon dioxide (5%) have no effect on the respiratory rate of the infective larvae of S. ratti, high levels of carbon dioxide (10–20%), have a definite narcotic effect on the larvae.

The respiration of the infective larvae is unaffected by osmotic pressures up to an equivalent of 1% sodium chloride. Above this level the respiratory rate is at first stimulated then decreases.

This work was carried out during the tenure of a Medical Research Council Scholarship.

At present our knowledge of the host-parasite relationships of the Schistosomatidae and their snail intermediate hosts is meagre. Because this knowledge is necessary in the search for effective therapeutic and control measures it would be useful to find an in vitro procedure facilitating continuous progenesis of schistosomes without the use of a snail host.

The fine structure of two kinds of compound presumed sense organs from the heads of three skin parasitic monogeneans Gyrodactylus sp. Entobdella soleae (larva only) and Acanthocotyle elegans is described. One kind of compound receptor consists of a number of associated sensilla, each ending in a single cilium (the spike sensilla of Gyrodactylus and the cone sensilla of E. soleae oncomiracidium).

The other kind of compound organ is made up of one or a few neurones only, each of which bears many cilia (pit organs of E. soleae oncomiracidium and feeding organ sensilla of Acanthocotyle elegans). The spike sensilla of Gyrodactylus have also been studied using a Cambridge Instrument Co. Stereoscan electron microscope and by phase-contrast microscopy. The ciliary endings of all these sense organs are highly modified and have lost the 9 + 2 structure, being packed with many fibres. The fibre arrangement in the cilia of the cone sensillae of E. soleae oncomiracidium and the feeding organ sensilla of A. elegans has been compared with that in the ciliary endings of other invertebrate mechano- and chemoreceptors. The possibility that the spike sensilla of Gyrodactylus may be chemoreceptors has been discussed but it is considered premature to attempt to assign functions to the other sense organs studied. Electron dense membrane-bound inclusions occurring specifically in the nerves supplying the spike sensilla of Gyrodactylus may be neurosecretory.

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.

The ultrastrcuture and development of the protozoan Histomonas meleagridis in the reproductive system of the female nematode, Heterakis gallinarum, have been described. It has been shown that there is a distinct cycle of events in the reproductive system. The stage of Histomonas in the germinal zone of the ovary is extracellular; the protozoan feeds and multiplies here. The histomonads move down the ovary with the oogonia and become intracellular when they penetrate the developing oocytes in the growth zone of the ovary. This is apparently an active penetration of the oocyte by the parasite. Feeding and division of the histomonad occurs in the oocyte and also in the newly formed egg; apparently a gradual reduction in size occurs along the reproductive system. These stages in the parasite are similar to the tissue-inhabiting stages in the bird but are much smaller. There are no mitochondria present.

There is strong circumstantial evidence that the nematode is able to repair damage to the wall of the reproductive system and to the oolemma caused by the protozoan.

I wish to thank Dr E. E. Lund of Beltsville Parasitological Laboratory, Maryland, U.S.A., who very kindly sent me some of his Histomonas-free strain of Heterakis and Dr L. P. Joyner and Mr C. C. Norton of the Central Veterinary Laboratory, Weybridge, for the strain of Heterakis which was found to transmit Histomonas wenrichii. I also wish to thank Mrs B. Fisher, Mr B. Millard and Mr M. Shirley for technical assistance and Mr P. Rogers for assistance with the photography.

In the system investigated the spleen was essential in most rats for the acquisition of immunity and its maintenance in the presence of the piroplasm.

It is stressed that model immunological systems involving the role of the spleen do not simulate the complexity of a parasitic infection. The roles of the spleen as an antibody producer, and as a site of phagocytosis and haemopoiesis are discussed in relation to the control of Babesia rodhaini by the rat. The possibility that the piroplasm may be able to undergo antigenic variation is considered.

The ultrastructure and development of the body wall of Polymorphus minutus is described during the acanthella stages in the intermediate host Gammarus pulex.

The cortex of the early acanthella is simple in structure, consisting of a syncytium containing giant nuclei, mitochondria, Golgi bodies, endoplasmic reticulum and a few lipid bodies. The cuticle is penetrated by pores which open into vesicles in a vesicular region.

The cortex gives rise to the body wall, neck and lemnisci of the adult, and may be considered as divided initially into two and later three functionally different regions: an outer absorptive region, an inner storage region and a layer between which develops to provide firm, resistant skeletal protection.

Absorption is facilitated by the increase in surface area due to the pores which penetrate the surface. This region is functional throughout development and the pores increase in number, possibly by division, as the animal grows. Beneath the pores a pore canal layer develops towards the end of the late acanthella stage.

During development supporting fibres are deposited throughout the cortex. Beneath the vesicular layer a concentration of mitochondria, Golgi bodies and endoplasmic reticulum deposit the fibres of the felt layer. In the anterior meta-soma no felt layer is found, but the body spines originate from this region.

The inner region of the cortex, or radial layer, contains the giant nuclei, which are arranged in the early acanthella in 5 rings of 4, 6, 5 or 6, 6 and 5 nuclei respectively. During development they produce dendritic process and finally fragment. They are considered here to be concerned with the formation of the felt layer and lemnisci rather than the ‘lacunar channels’. The inner cortical region becomes ensely packed with lipid bodies, and it is suggested that these may be responsible, owing to fixation artifact, for the so-called ‘lacunar channels’ previously described in these regions, the extensive cavities seen in some light microscope preparations being filled with a fluid lipid rather than an aqueous solution.

The lemnisci are similar in appearance to the cortex except for regions of concentrated endoplasmic reticulum.

The mitochondria in the cortex of the acanthella appear to be aerobic rather than anaerobic, as they possess many cristae and are larger than the mitochondria of the adults.

My thanks are due to Dr D. W. T. Crompton for advice and encouragement, and to Dr D. H. Northcote and Dr F. B. P. Wooding for time on the Phillips Electron Microscope 100. This work was carried out during the tenure of an S.R.C. studentship.

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.

Foleyella confusa sp.nov. is described from Rana limnocharis on Luzon, near Manila. It differs from F. duboisi (Gedoelst) in arrangement and number of anal papillae, in spicule size in relation to length of body, and in overall size. Icosiella hoogstraali sp.nov. from Rana macrodon on Palawan differs from others in the genus in having a left spicule 1·955 mm long and a single preanal papilla. Gnathostoma spinigerum and Cosmocerca japonica are also reported.

We thank Dr Roy C. Anderson, University of Guelph, for reading the manuscript and offering his comments on it.

The authors wish to acknowledge the field support by Dr D. S. Rabor, Department of Biology, Silliman University, Dumaguete City, Negros Oriental, Republic of the Philippines, and the technicians of the Parasitology Department of Naval Medical Research Unit no. 2 for general assistance in obtaining and examining hosts.

Initial work for this study was supported by funding under Public Law 480, Section 104 (c), by funds provided by the Bureau of Medicine and Surgery, Navy Department Work Unit MR 005·20–0098, and by Contract No. NR 103–690/N0014–66-C 0094, between the Office of Naval Research, Department of the Navy, and the Southwest Foundation for Research and Education.

Final efforts were sponsored by Department of the Army, Contract No. DADA 17–68–C–8094.

Most epidemiological models assume that disease is the inevitable outcome of infection (see Bailey, 1957). Yet as Dubos (1965) has said; ‘Throughout nature, infection without disease is the rule rather than the exception’. There are, in fact, many diseases whose distribution cannot be explained solely by a consideration of the probabilities of host parasite encounters. In these cases, a diseased state is only one possible outcome of an interaction between parasite phenotypes, host phenotypes and the external environment. Haemonchosis is an example of such a disease and has been studied extensively in quantitative terms (see Whitlock & Georgi, 1968).

Camallanus marinus sp.nov. is described from eleven specimens recovered from a variety of marine fishes. It is the only species in Camallanus s.s. with a single spicule. Spinitectus palawanensis sp.nov. is described from two males and one female found in a scombrid fish. It is characterized by having 12–17 rings of 60–65 spines, a spicule ratio of 5:1, and ten pairs of caudal papillae. Cucullanus lutjani, sp.nov. is described from two specimens from lutjanid fishes. It is distinct in lacking a preanal sucker, in possessing a gubernaculum and eight pairs of cloacal papillae.Oceanicucullanus padfica gen. et sp.nov. is described from two specimens found in cyprinid fish. It is most similar to Indocucullanus, differing from it in the absence of a gubernaculum, and in possessing three pairs of teeth in the radii of the oesophagus. Philometra (Philometra) lateolabracis Yamaguti, 1935; Terranova sp.; and Procamallanus spp. are reported for the first time in the Philippines.

We wish to thank Dr D. S. Rabor, Silliman University, Damageute, Negros Oriental, Republic of the Philippines, for general support in field activities, and Dr Loren Wood, Curator of Fishes, and Dr Robert F. Inger, Curator of Reptiles, Chicago Museum of Natural History, for identification of hosts. Acknowledgement is also made to Dr Betty June Myers for technical assistance and advice in handling materials.

Cotugnia columbae sp.nov. from the domestic pigeon is described and compared with other species of the genus. The tapeworm measures 108–130 mm in length and 2·236 mm in maximum breadth. The rostellum bears 200 hooks, 7·7–10·6 μm. long arranged in two rows. Testes number 105–130 and the cirrus pouch measures 0·354–0·376 x 0·054–0·065 mm.

We are grateful to Dr C. M. Singh, Director, Indian Veterinary Research Institute, and to Dr H. D. Srivastava, erstwhile Assistant Director and Head of the Division of Parasitology, for their interest in these studies and for the provision of the necesssary facilities.

The normal position of attachment of the scolex of H. diminuta from day 6 to day 27 in single worm infections in male Wistar rats was determined. A migration forward from a position of attachment 30–40% down the length of the small intestine to the 10–20% region during day 7 to day 14 was confirmed. A concomitant and slightly faster movement backwards of the posterior of the worm led to the mid-point of the strobila changing from 35 to 50% down the intestine. By day 18 the worms were mature and occupied the whole small intestine except the anterior 10% and the posterior 5–10%. The worms weighed approximately 350 mg dry weight (= 1550 mg fresh weight) at maturity, no significant weight increase occurred thereafter.

6½ and 7½ day-old worms were transplanted from donor rats into the duodenum or posterior ileum. Nearly 100% recovery of the single worm transplants in recipient rats was obtained. Both anterior and posterior implanted worms returned to the region of the intestine from which they had been removed. Migration was measured over periods of 13–72 h and was found to be complete within 24 h. Worms transplanted into the duodenum continued to grow at nearly the normal rate but worm transplants into the posterior ileum lost nearly a day's growth. The significance of this point is discussed.

Opium was found to prevent migration of the tapeworm, but ligation of the bile duct did not prevent the tapeworm migrating forward from a point of insertion near the ileo-caecal junction.

The evidence confirms the concept that a tapeworm is a dynamic organism capable of recognizing a specific region of the intestine, of detecting when it is not in this region, of being able to interpret one or more stimuli to indicate direction and of having a motor system sufficiently coordinated to overcome peristalsis.

We gratefully acknowledge financial support by the Medical Research Council (London) and technical assistance from Miss Helen Stallard, B.Sc., Miss Pat Grant and Miss Gillian Moore. One of us (Trond Braten) wishes also to thank the University of Glasgow for the award of a Visiting Fellowship.

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.

1. Three new species of eugregarine are described from the mid-gut of the larva of Costelytra zealandica (White).

2. Euspora zealandica occurs in the anterior region of the mid-gut near the gut caeca. Slictospora costelytrae occurs just posteriorly to E. zealandica. Euspora sp. occurs posteriorly to the malpighian tubules, but was found only rarely.

3. A description of the species and the life-cycle of two species are given.

4. The life-cycle of S. costelytrae takes about 8 weeks and that of E. zealandica 6 weeks.

5. Gametocysts will develop only at a humidity of 95–100 %.

6. The bi-associative species, E. zealandica, is more abundant in the 1st and 2nd instars than the large species, S. costelytrae. Both species are equally abundant in the 3rd instars, but the peak for incidence was in May for E. zealandica and June for S. costelytrae. This is related to the longer period of time taken for the life-history of S. costelytrae.

7. The incidence of gregarines builds up in each instar to over 90% then falls off, the time of fall off corresponding with the production of gametocysts which pass out with the faeces of the grub.

8. The gregarines mostly complete their cycle before the instars moult. This is borne out by the observation that cysts were not found in the smallest size group of each instar. There were many cases in the larger size group of all three instars where cysts only occurred.

9. Gametocysts are produced mainly in the autumn.

10. Gregarines were not found in grubs approaching metamorphosis (prepupae) nor in pupae nor adults.

11. It seems unlikely from the results that the gregarines have an adverse effect on the grubs as, in general, it was found that the larger grubs had the greater number of gregarines present, but further work on this is needed.

12. The life-history of the gregarines is closely correlated with the life-history of the beetle.

13. The presence of the gregarines only in the larval stages is related to the different habitat and behaviour of larva and adult.

I am grateful to the University Grants Committee for financial support.