Sarcocystis species have been described in a wide variety of mammals, birds and reptiles throughout temperate and tropical countries of the world. Although early workers (Alexeieff, 1913; Wenyon, 1926) considered it impossible to differentiate species on the morphology, there are such clear-cut differences in the structure of the cyst wall, morphology of spores and other details, that there at present seems every justification for giving specific rank to these different parasites in their various hosts. Furthermore, with one or two exceptions, attempts to infect laboratory animals have met with little or no success and it would appear that at least some species of Sarcocystis may be highly host-specific.

A new species of Trypanosoma, T. (Megatrypanum) hoarei, is described from shrews of the genus Sorex (S. trowbridgii, S. vagrans, S. ornatus, and S. palustris) in California. Four body types are found in adult populations of T. hoarei, two common and two rare, as well as intermediate stages, indicating a tendency to-ward polymorphism in this species. Multiplication is discontinuous, with a reproductive and growth period characterized by moderate numbers of diverse and bizarre developmental forms in the blood and high coefficients of variation for total length (20–30 %), succeeded by an adult phase with few trypanosomes in the blood and lower coefficients of variation for total length (7–12 %). The method and site of multiplication in T. hoarei is unknown but evidence indicates that reproduction by multiple fission or binary fission of amastigote forms may occur in tissue capillaries, with the production of large, rounded amastigote individuals. The latter develop, without intervention of epimastigote stages, into large, rounded trypomastigote individuals from which a wide variety of other developmental forms proceed, including certain distinctive, very long ribbon-like trypanosomes. A review of the literature on mole and shrew trypanosomes suggests that T. hoarei of Sorex, T. petrodromi from Petrodrorrms, and T. talpae from Talpa are related, primitive insectivore forms, clearly distinguishable from the lewisi-type trypanosomes also found in insectivores.

Two types of exflagellating bodies of the malaria parasite are described in the infected blood of the Himalayan flying squirrel (Petaurista magnificus). The first type, which is larger, is characterized by a blue-staining cytoplasm. These produce blue-microgametes with red chromatin dots. The second type, which is smaller, stains red, and also produces red microgametes. Certain large schizonts occurring in the peripheral blood are described. It is speculated that these might represent the primary tissue stage of Rayella rayi.

An artificial skin of hardened gelatine was used to examine the factors affecting penetration of skin by cercariae of the bird schistosome, Austrobilharzia terrigalensis.

The percentage of cercariae able to penetrate through a gelatine membrane was increased by a factor of 3–4 by coating the membrane with a thin layer of lipid collected from the surface of chicken skin.

The free sterol fraction, isolated from chicken skin surface lipid by thin-layer chromatography, stimulated penetration to the same extent as whole skin lipid.

Cholesterol was detected in the sterol fraction by mass spectrometry and pure cholesterol had the full stimulating effect on cercarial penetration.

Skin lipid, from which free sterols had been removed, lost the stimulatory effect on cercariae, but full activity was recovered by adding cholesterol to the sterol-free lipid. Fractions of skin lipid containing free fatty acids or triglycerides, wax esters and sterol esters similarly failed to stimulate penetration.

These results establish that penetration of A. terrigalensis cercariae is greatly stimulated by the free sterols present in the surface lipid of chicken skin but cholesterol may not be the only active sterol. Cholestanol and the plant sterols campesterol and β-sitosterol were also detected in chicken skin surface lipid. These sterols were not tested for activity on cercariae because samples free from cholesterol could not be obtained.

Some cercariae were able to penetrate plain gelatine membranes not coated with cholesterol but small amounts of free sterol were detected in the gelatine itself. This sterol could not be completely removed by prolonged solvent extraction and consequently it is not known whether any cercariae are able to penetrate in the complete absence of sterols.

Temperature had a marked effect on penetration of cercariae; lowering the temperature from 40 to 25 °C reduced the number of successful penetrants by a factor of 4.

I am grateful to Miss V. Bowen for excellent technical assistance and to Dr J. McLeod of the Research School of Chemistry, Australian National University, who performed and interpreted the mass spectrometry. This investigation was partly supported by a research grant from the World Health Organization.

The adhesive organ of Apatemon gracilis minor Yamaguti, 1933, consists of two lobes lying in a cup-shaped fore-body. The cytoplasmic tegument covering the apposing faces of the lobes is different from that covering their outer surfaces. The covering of the outer surface corresponds to the general tegument present on the rest of the body, whereas that on the inner surfaces is finely pitted and in certain regions is elevated to form a coarse reticulum. This specialized surface is covered externally by a plasma membrane and is in continuity with nucleated cell bodies lying below the basement layer. The cell bodies contain large quantities of granular endoplasmic reticulum as well as several Golgi complexes and numerous mitochondria. Masses of secretion bodies are present and these also occur in the extensions to the external tegument as well as within the external tegument of the lobes. When the parasite is attached the inner faces of the lobes come into contact with the vascular lamina propria of the host. The possible biological role of this specialized host–parasite interface is discussed and it is suggested that this surface specialization may form a morphological basis for the ‘placental’ function suggested for the adhesive organ by earlier workers.

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 micrographs are published by permission of the Cambridge Instrument Company.

The parasitaemia and associated blood picture following infections of Nuttallia rodhaini in rats and mice, and N. microti in rats are described. After inoculation of the blood stages of these parasites, it is evident that only further blood stages occur in the host animal. Infections with both piroplasms follow a similar course, there being a prepatent period followed by a patent infection which reaches a peak and then disappears. The prepatent period is an artifact due to the rarity of the parasites in the peripheral blood. The patent infection of N. rodhaini in rats and mice rapidly reaches a peak of up to 90% R.B.C. infected. That of N. microti develops more slowly and rarely reaches 20%. Rats show age resistance to both piroplasmoses but none is shown by mice aged up to 1 year against N. rodhaini infection. Increase of the parasite inoculum dose decreases the prepatent period and also the time from infection to the peak. In rats there is a corresponding increase of the peak but in mice this remains constant. N. microti infections in rats are enhanced by splenectomy, the peak being higher but the time from infection to the peak not altering. The parasitaemia produced by both piroplasms results in a temporary anaemia and reticulocytosis. The anaemia is not prolonged which does not suggest autoimmune reactions. Both piroplasms preferentially invade reticulocytes as compared with mature erythrocytes although this is apparently reversed at the onset of reticulocytosis.

An explanation of the mechanism of the parasitaemia of the piroplasms in rats and mice is suggested.

This paper is a report of work carried out in part for a thesis submitted for the degree of Ph.D. in the University of London. The work was aided by an S.R.C. research studentship. I would like to thank Dr F. E. G. Cox for his supervision and also for performing the splenectomies.

Mice infected with Babesia rodhaini can be cured with a single injection of Diampron and are immune to challenge with the same species. Mice infected with B. microti recover naturally from their infections and are also immune to challenge. The immunity extends to the heterologous species in both infections. In some cases parasites persist at a low level after recovery but in others the immunity is of a true sterile type. Immunity once established is unaffected by splenectomy in mice which have recovered from B. rodhaini and challenged with either B. rodhaini or B. microti.

We wish to express our thanks to May and Baker Ltd. who kindly provided us with the Diampron used in these experiments.