The study of the ultrastructure of the blood stages of P.tropiduri Aragao & Neiva, 1909 shows close similarities between this species and P.floridense, another lizard malaria parasite. The absence of a rounded structure in the merozoites of P.tropiduri, and their tendency not to project beyond the original area occupiedby the schizont are the ultrastructural characteristics that differentiate the two species.

The close relationships of homology between P.tropiduri and the Plasmodium of birds are emphasized, and new evidence on their phylogenetic relationships is offered.

P.tropiduri has a mode of ingestion and digestion similar to that of P. elongatum. When the size of the cytostome, the size of the boluses produced by the phagotrophy, and the size of the digestive vacuoles are compared, they are seen to be nearly identical in the two species.

The comparative study of the structure of the cytostomes of the merozoites and the trophozoites, and of their relationships with the membranes that limit the parasites, gives new evidence that the external membrane of the trophozoites schizonts, and gametocytes is derived from the host cell.

An exflagellatory apparatus is described from the microgametocytes of P.tropiduri, and its relationships with similar structures described from the gametocytes of avian Plasmodium and from Coccidia are discussed.

The relationship between the age of infective O.cunictili larvae and the proportion that remained retarded in a subsequent infection was investigated. The proportion of retarded larvae recovered was found to depend on the length of time and the temperature at which infective larvae were stored prior to infection. At 4 °C the level of retardation was found to increase with increase in the length of storage until a maximum was reached. Further storage tended to decrease the proportion of larvae that remained retarded. These effects were found to be less marked when the larvae were stored at a higher temperature. At 17 °C the maximum level of retardation reached was lower and the process of the increase and the decrease in retardation was of shorter duration. An increase in the number of larvae from 200 to 6000/rabbit did not appreciably change the proportion that remained retarded.

We are most thankful for the advice and criticisms of Dr J. D. Dunsmore. Theskilled technical assistance of Mr Ted Eaton is gratefully acknowledged. This workwas supported by the Ontario Department of Agriculture and Food.

In the laboratory, transmission of Schistosoma mansoni to and from Biomphalaria glabrata can occur in artificial sea water at or below a concentration of 12-5%. Miracidia can also emerge and infect snails in 25 % sea water, but this concentration is inimical to the survival of the snails or their eggs. The parasite thus appears to be better adapted to brackish water than is the intermediate host. Epidemiological studies are needed to assess the limits imposed by brackish waters on the natural transmission of schistosomiasis.

An ascaridoid nematode Cotylascaris thylacini is described from the thylacine or Tasmanian marsupial wolf. Only female specimens were available. The new genus is distinguished by two cup-like structures on the anterior margin of each lip. It is suggested that these structures may comprise an adaptation to the unusual form of the intestinal wall in the thylacine. The significance of the ascaridoid nematodes of marsupials is discussed in relation to the possible route of entry of marsupials into Australia.

Financial assistance for this study was provided under grant No. 16 from the Australian Research Grants Committee.

Specimens of nematodes from the stomach, oesophagus and trachea of the cat, previously reported and described by Led et al. (1968), were found to resemble L. major more closely than other species in the genus. In the shape of the interlabia and the number of pits on the surface of the eggs, they conformed to this species. Accordingly they were identified as L. major although they differed from L. major in the width of the lips in relation to the postlabial collar and their more slender appearance, especially in the tail region. This is the third report of L. major and extends the host range and geographical distribution of this species.

The phylogenetic relationships of the Acanthocephala are reviewed in the light of recent ultrastructural evidence. Comparisons of spermatozoan ultrastructure suggest that the Acanthocephala cannot be closely related to the Platyhelminthes and that the similarities which exist between cestodes and acanthocephalans are the result of convergence. Similarities in the spermatozoan ultrastructure of acanthocephalans, rotifers, nematomorphs and nematodes probably have a phylogenetic significance and suggest that the acanthocephalans have a close common ancestor with some at least of the Aschelminthes. The case for this affinity is strengthened by ultrastructural evidence concerning the body walls and muscles of acanthocephalans, nematodes and rotifers.

I am indebted to Professor D. R. Arthur and Professor F. R. Whatley at King's College, London, for the provision of facilities. The work reported in this paper was undertaken during the tenure of a Research Fellowship from the Nuffield Foundation.

Farm-reared Leuciscus idus were experimentally infected with the intestinal tapeworm Caryophyllaeus laticeps, and maintained at different temperatures. At 12 °C and lower temperatures there was an initial loss of parasites after infection, but the survivors persisted for up to 1 month. At 18 °C only temporary survival of the parasite was possible, and worms were killed and rejected after 3 days. Experimental transplants to fresh hosts after 2 days indicated that the factors responsible for the death of the parasites became effective within this period.Neither inhibition of peristalsis nor host starvation affected the parasite's survival. The speed of the response to the parasite indicates that circulating antibodies produced in response to C. laticeps and directed specifically against it cannot be involved in the rejection. Other possible mechanisms of rejection are discussed, together with the significance of the results to the interpretation of data based on field observations.

I should like to thank Mr J. Harris for allowing me to refer to some of his unpublished data and for helpful discussions.

Two strains of T. congolense and two strains of T. vivax, resistant to normal therapeutic doses of trypanocidal drugs, were transmitted by G. morsitans and G. tachinoides through series of cattle in the absence of drugs to assess the transmissibility of the trypanosomes by tsetse flies and the stability of drug resistance in the organisms during cyclical development.

The characteristics of both strains of T. vivax and one strain of T. congolense were typical of the species, but the infectivity of the second strain of T. congolense for laboratory animals was abnormally low.

The transmissibility of the trypanosomes was assessed on the bases of the infection rates in tsetse flies used to transmit the strains and the periods required for the development of the trypanosomes in the flies and bovine hosts. The transmissibilities of both strains of T. vivax and the first strain of T. congolense were normal by comparison with data obtained by cyclically transmitting non-resistant trypanosomes of the same species, but the period required for the development of the second strain of T. congolense in tsetse flies was abnormally long.

There was no loss of drug resistance in any of the strains examined in the experiments. A strain of T. congolense retained resistance to diminazene aceturate at a dose rate of 7-0 mg/kg, homidium chloride at 2-0 mg/kg, quinapyramine sulphate at 4-4 mg/kg and isometamidium chloride at 0-5 mg/kg during a series of eight transmissions by tsetse flies in a period of 376 days.

Some species in Subuluridae, Thelaziidae and Ascaridiidae are discussed. Subulara helicospicula sp.nov. from malcohas and a bulbul from Borneo is described. It differs from other species in having long, unequal spicules which are helical throughout their distal half. Allodapa suctoria (Molin, 1860) Seurat, 1914, is reported from several species of birds from Taiwan, Borneo and Palawan. A. turnicis (Maplestone, 1931) Inglis, 1958, was found in the button quail, Turnix sustator on Palawan.

Thelazia digiticauda Schuurmans-Steckhoven, 1937, is redescribed from specimens recovered from the eyes of a ruddy kingfisher, Halcyon coromanda from Taiwan. Ceratospira inglisi sp.nov. from a cockatoo in Palawan and a pigeon in Borneo differs in spicule size from the three species so far described. Oxyspirura {Y.) mansoni (Cobbold, 1879) Skrjabin, 1931, was recovered from bamboo partridges, Bambusicola thoracica, from Taiwan, and Oxyspirura (0.) popawi Skrjabin, 1929, was found in the eyes of a swallow, Hirundo daurica in Taiwan.

Ascaridia columbae (Gmelin, 1790) Travassos, 1913, was common in a variety of pigeons in Borneo and Palawan, and A. subaequalis (Wehr, 1930) was found in the lesser coucal, Centropus bengalensis in Taiwan and in a common coucal, C. sinensis in Palawan.

An account is given of the morphological characteristics of the larvae of Hyalomma anatolicum anatolicum and H. a. excavatum. Emphasis is placed on the importance of chaetotaxy in the identification of the larvae.

I am most grateful to Professor Don R. Arthur for reading the manuscript and for his most useful advice and encouragement. Also I wish to express my sincere thanks to Dr R. P. Chaudhuri, Dr C. M. Clifford, Dr V. Dhanda, Dr H. Hoogstraal and Profesor B. Feldman-Muhsam for supplying me with specimens.

Two behaviour patterns which could bring about clumping in N. brasiliensis, sexual attraction and thigmokinesis, were investigated. A T-tube choice-chamber to facilitate an in vitro study of sexual attraction is described. A method for studying aggregation is also outlined. Male N. brasiliensis were shown to be attracted to female worms, positive sexual attraction. Both sexes were shown to exhibit strong thigmokinetic responses, and possible functions of such behaviour are discussed.

I am grateful to Professors J. Brough and D. Bellamy for providing the facilities during the course of this study. I should also like to thank Dr E.. A. Hammond for his supervision, help and criticism throughout the project. This work was carried out during the tenure of a Scientific Research Council Studentship.

1. The distribution of larvae within the dorsal longitudinal muscle fibre groups of A. aegypti infected with subperiodic B. malayi or B. pahangi and of M. uniformis infected with the former parasite was studied with the aid of serial sections.

2. In all experiments the top bundles of fibres contained fewer parasites than either the middle or lowest bundles, but whereas most parasites were concentrated in the middle bundles in A. aegypti there was no significant difference in M. uniformis between the bottom and middle fibre bundles. The lowermost fibre was not the most heavily parasitized of the dorsal longitudinal group.

3. Nothing is known of what determines whether or not a particular fibre becomes parasitized, but it was concluded that the distance between the fibre and the gut (from which the parasites emerge to travel to the muscle) is not the principal factor.

I am very much indebted to Mr Cheong Weng Hooi and his colleagues for collecting and processing mosquitoes in Kuala Lumpur and to Mrs J. Oliver who finished the processing in Liverpool. I am grateful also to Dr W. W. Macdonald for his advice and help in all stages of this work and to Miss M. A. Johnson who assisted with the care of the A. aegypti colony and prepared the diagrams for this paper.

Active digenean larvae in Bithynia tentaculata were found to utilize both host carbohydrates and lipids, and these were incorporated into the bodies of developing cercariae. Although rediae obtain most of their nutrients by direct ingestion, a decrease in the glycogen and neutral fat content of intact digestive gland cells of snails infected with rediae suggests that these larvae may also absorb nutrients through their body walls, like sporocysts. In sporocyst infections, host glycogen is believed to be hydrolysed to glucose and other monosaccharides by the host's own enzymes, possibly phosphatases, which have been stimulated to greater activity by the parasites. These simple sugars, due to their small size, can then be absorbed through the sporocyst body wall. Similarly, host fats are probably degraded to fatty acids, possibly by a parasite-secreted lipase, prior to absorption by the parasites. Encysted metacercariae do not appear to feed, but they do have severe pressure effects which disrupt some of the digestive gland tubules. Such tubules show an increase in their neutral fat content and a decrease in phosphatase activity. Aminopeptidase enzymes secreted by active digenean larvae may be responsible for some lysis of host cells, and may also be involved in the breakdown of lymph proteins to amino acids.

I am indebted to Dr F. R. Stranack for her supervision and guidance throughout this investigation, and to Dr G. Charles and Dr T. Jenkins for their continued help and advice. This study was carried out during the tenure of a research assistantship awarded by the governors of Portsmouth Polytechnic.

The tetrad and companion cells, in association with the oesophagus and dorsal tract of tissue, of Tetradonema plicans, have been investigated in the host Bradysia paupera. The companion cells contain glycogen and mucopolysaccharide. The tetrad cells are rich in RNA and probably synthesize protein and secrete some as enzymes. The gut is functional and the oesophagus is in connexion with the tetrad. The dorsal tract is not connected with the oesophagus or tetrad and contains lipid with characteristics similar to that found in the eggs.

It is suggested that the tetrad and companion cells are homologous with the stichocytes present in the Trichuroidea and some members of the Mermithoidea.

I am grateful to Professor T. R. E. Southwood in whose department this work was carried out. I should like to thank Dr N. A. Croll for supervising and encouraging this work and Dr T. Jenkins for his helpful criticism and advice. The work was supported by the Agricultural Research Council.

Methotrexate inhibited the growth of D. viviparus by acting either on the moult from the third- to the fourth-stage larva or on the growth of fourth-stage larva itself. Despite these effects of the drug, the early developmental stages of the worms primed the immune mechanism of the host; the inductive phase of antibody production was completed in less than 5 days. An effective immune response, however, was blocked by the continued presence of methotrexate which resulted in delayed elimination of a superinfection and the absence of anaphylactic antibody in the serum. Immune elimination appeared to require a quantitative threshold of worm antigen and host antibody.

This work was made possible by the generous supply of infective helminth material from Messrs Allen and Hanbury Ltd, Ware, Herts. I should also like to thank Dr J. H. Humphrey and Dr S. R. Smithers of N.I.M.R. for their helpful discussions.

Haliotrema balisticus attaches itself between the secondary gill lamellae of its host, Balistes caprisctis (= carolinensis), by means of a pair of ventral hamuli, which point ventrolaterally, and a pair of dorsal hamuli which point in a dorsal direction. The ventral hamuli, each of which pivots at the end of a ventral connecting bar, are operated by a pair of extrinsic muscles. The tendinous regions of these muscles pass through fibrous loops attached to the ventral hamuli and then join each other in the mid-line. Such an arrangement has a high mechanical advantage which may reduce the expenditure of energy required to maintain the hamuli in position in the gill tissue in the face of a strong gillventilating current and may also preserve the mobility of the free region of the body.

The dorsal hamuli are not operated by the muscles and tendons associated with the ventral hamuli.

Marginal hooklets and glands play subsidiary roles in attachment.

I wish to express my thanks to Commander J. Parreira, Director of the Aquario Vasco da Gama in Lisbon, and to the Staff of the Aquario, in particular to Mr A. Ferreira, for their kind assistance and hospitality. I am also grateful to Dr Maria Monteiro and to Dr Maria Baptista. I am indebted to the Calouste Gulbenkian Foundation for a grant which made my visit to Portugal possible.

A drug-resistant strain of T. vivax was transmitted through a series of four duikers (S. grimmia) and a gazelle (G. rufifrons) by G. palpalis and G. tachinoides. The cyclical transmission of the strain in cattle, started in earlier work, was continued for control purposes. Infections with the drug-resistant strain in the antelope were of the same type as those produced by field strains of T. vivax in the same host species, and infection rates in tsetse flies fed at early stages of infection on the antelope were similar to those found in flies fed on cattle infected with the drug-resistant strain. The strain retained resistance to diminazene aceturate at a dose rate of 7-0 mg/kg and quinapyramine sulphate at 5'0 mg/kg in the absence of the drugs for 7 months in tsetse flies and antelope, and for a total period of 29 months in tsetse flies and cattle.

We wish to thank Dr A. A. Amodu, Director of the Nigerian Institute for Trypanosomiasis Research, for permission to publish this paper.