Scanning electron-micrographs have shown the covering of microvilli on the surface of the redia of Parorchis acanthus. In the contracted state the elongated microvilli with bulbous extremities seen in the surface grooves may be the result of compression. The surface of the epidermis of the cercaria is smooth on a large area of the ventral surface and lattice-like with microvilli, laterally, anteriorly, dorsally and on the tail. The spines on the body can be withdrawn into sheaths by the contraction of muscle fibres inserted into the basement lamina below each spine.

I would like to express my sincere gratitude to Dr I. ap Gwynn of this department for preparing the scanning electron-micrographs and the School of Engineering Science, University of North Wales, Bangor for the use of their stereoscan. I should also like to thank Mr M. C. Bibby for technical assistance and Professor E. G. Gray and Dr W. Sinclair for assistance with the transmission electron-micrographs.

The culture method developed by Geiman and co-workers has been modified to give average multiplication rates for Plasmodium knowlesi of at least six-fold in 24 h.

Individual modifications have been assessed on the basis of parasite multiplication and [3H]leucine incorporation into parasite protein. The second cycle of in vitro development which is particularly influenced by the conditions of culture was improved by: (i) culture of washed parasitized red cells in a medium containing dialysed homologous serum screened for its ability to support parasite growth; (ii) addition of ATP, Co-enzyme A (or pantothenate) and glutamine.

We thank Mr E. D. Dennis and Miss Susan Wanstall for technical assistance. This work was supported by grants from the Medical Research Council and the World Health Organization.

Many studies have been carried out on the structure and chemical composition of trematode and cestode egg envelopes. Among the nematodes, much work has been done on the eggs of ascarids but there have been few studies on the egg envelopes of species in the Superfamily Trichostrongyloidea. Zawadowsky et al. (1929) reported on the biology of five species of this family, namely Trichostrongylus instabilis (T. colubriformis), T. extenuatus (T. axei), T. probolurus, Cooperia pectinata and Ostertagia mentulata. They described the egg envelope as consisting of three or possibly four layers. The physical and chemical properties of these layers were investigated and these workers concluded that the structure of the egg envelope was similar to that of Ascaris. However, the solubility of the inner layer differed from that of Ascaris in dissolving only partially in absolute alcohol and very little in ether.

The present study includes five new and three known species of trematodes from marine fishes of India. Records of new hosts and new localities are given. The new species are: Plagioporus longicaudus, Opegaster paramacrorchis, Pseudopecoelina elongata, Pseudopecoeloides scomberi and Hamacreadium leiognathi. The known species are: Podocotyloides parupenei (Manter, 1963) Pritchard, 1966, Hamacreadium mutabile Linton, 1910 and H. krusadaiensis Gupta, 1956. Hamacreadium leiperi Gupta, 1956 is considered a synonym of H. mutabile.

The author wishes to express his gratefulness to Dr Ather H. Siddiqi for his guidance and to Professor H. W. Manter of the University of Nebraska, U.S.A. for his valuable comments. The present study was financially supported by the Council of Scientific and Industrial Research, New Delhi.

The cercaria of Parorchis acanthus swims by a series of spasmodic ventral flexions of the body moving with the dorsal surface of the body foremost. The wave of contraction continues along the tail which therefore moves dorso-ventrally. Creeping movements precede cyst formation. The structure of the tail is admirably suited to swimming movements and to rapid shortening and lengthening. The wall is provided with circular and longitudinal muscles and abundant mitochondria. The centre of the tail contains fluid in which is a network of large myoblasts the processes of which overlap and probably slide over one another during the tail movements. Glands opening into the terminal tail invagination secrete an adhesive substance consisting of mucoprotein and glycoprotein which together with the vesicular secretion around the aperture enables the tail tip to attach itself to the substratum.

I would like to express my sincere thanks to Dr I. ap Gwynn of this department for invaluable help in the preparation of the electron-micrographs and Mr R. A. Moore and Mr M. C. Bibby for technical assistance.

In an attempt to determine the mechanisms of resistance to schistosome infection operative in bulinid snails, an histological study was carried out on batches of a refractory strain of Bulinus truncatus and two highly susceptible strains of B. africanus killed at various intervals of time, ranging from 7 h to over 3 months, after exposure to infection with Schistosoma mattheei miracidia.

It was found that only a small fraction (less than 1%) of the miracidia penetrated B. truncatus, and that they penetrated only a few of the snails exposed. Much larger proportions (up to 26%) of the miracidia penetrated B. africanus and they penetrated nearly all the snails exposed.

In the susceptible B. africanus only miracidia which settled in the loose tissues of the head-foot proceeded to develop: those settling in dense tissue failed to develop and degenerated within about 2 days.

A strong infiltration reaction of amoebocytes rapidly destroyed the few miracidia that successfully penetrated B. truncatus. In contrast, B. africanus did not respond to the presence of miracidia in its tissues, nor to mother or to daughter sporocysts before the onset of cercarial shedding: it reacted only to cercariae.

It is concluded (a) that the combination of an effective surface barrier and an active defence mechanism due to amoebocytes were responsible for complete resistance to S. mattheei infection in B. truncatus (b) that successful establishment of S. mattheei infection in B. africanus depended on the failure of the snail to recognize the schistosome miracidia and sporocysts as foreign material.

I am indebted to Professor George S. Nelson for advice and encouragement in this work which was carried out at the London School of Hygiene and Tropical Medicine. I am most grateful to Dr R. J. Pitchford, Dr C. A. Wright and Dr F. Arfaa for providing material for schistosome and snail cultures and to the East African Community and the British Ministry of Overseas Development for financial support.

Trypanosoma avium bakeri ssp.nov. has been recorded from naturally infected red-whiskered bulbuls (Otocompsa jocosa Linn.). This trypanosome resembles T. avium Danilewsky, 1885 in general morphological features but it is larger in every respect. The parasite can also infect chicks, black-breasted quails and pigeons.

The division stages of the parasite are also very interesting. Considering the different morphological criteria, division stages and loose host specificity, the parasite is designated as subspecies of T. avium.

For the first time division stages of an avian trypanosome have been studied in detail in the peripheral circulation and bone marrow of the bird. The parasite multiplied by longitudinal binary fission and two types of plasmotomy.

Our thanks are due to Dr S. Mookerjee, Professor and Head of the Department of Zoology, Presidency College, Calcutta for providing the necessary laboratory facilities and constant encouragement and for going through the manuscript and to Dr J. R. Baker, Department of Medical Protozoology, London School of Hygiene and Tropical Medicine, for his valuable suggestions in the preparation of this article. The financial support provided by the Indian Council of Medical Research (for H.N.R.) and the University Grants Commission (for D.K.C.) is gratefully acknowledged.

Isoenzyme patterns of normal adult Nippostrongylus brasiliensis were compared with those of damaged worms permanently affected by immunity and adapted worms which show increased resistance to immunity.

Isoenzymes of acetylcholinesterase, found in glands of the head end of the worms, showed changes in damaged and adapted worms which can be related to the effects of immunity.

Aminopeptidases and non-specific esterases from the same glands were unchanged in worms affected by immunity.

Isoenzymes of acid phosphatase found in the gut of the worms were altered in damaged and adapted worms; these changes are probably secondary to the changes in acetylcholinesterases.

Of the enzymes studied, acetylcholinesterase is the enzyme most likely to be the antigen which stimulates host immunity, but final proof of this is required.

We are most grateful to Dr Valerie E. Jones for many helpful suggestions.

Sections of salivary glands from adult R. appendiculatus ticks and sections of the whole bodies of nymphal R. appendiculatus ticks were studied to obtain data on the incidence and development of T. parva during feeding. Since, in their previous instars, all the ticks had dropped engorged from a parasitaemic calf on the same day, it was possible to compare the resultant infection rates in nymphs and adults.

In the adult ticks, the highest percentage infection (45%) and the greatest number of infected acini (45 from 20 ticks) were observed after 3 days feeding. Mature parasites were first seen after 4 days. In the nymphs, the highest percentage infection (35%) was observed after 3 days feeding. Equal numbers of infected acini (14 from 20 ticks) were observed in unfed nymphs and in those which had fed for 2 or 3 days. Mature parasites were first seen after 2 days, and all were mature after 3 days.

It was concluded that a batch of nymphs would contain more parasites per gramme than a corresponding batch of adult ticks. This fact, together with the observed uniformity of maturation of the parasites in nymphs, may make nymphs a more suitable choice than adults for the preparation of ground tick stabilates.

We are most grateful to Mr M. P. Cunningham, who inoculated the calf on which the ticks fed, and to Mr A. Musoke who observed the subsequent clinical reaction of the calf.

This work was carried out as part of the programme of the Immunological Research on Tick-borne Cattle Diseases and Tick Control Project which is supported by the United Nations Development Programme/Special Fund, with F.A.O. as the Executing Agency, in collaboration with the East African Community. The assistance of our colleagues on the Project is gratefully acknowledged. This paper is published by kind permission of Mr A. Rashid, the Director of the East African Veterinary Research Organization.