Site specificity and distribution of 5 species offreshwater acanthocephalans Acanthocephalus clavula, A. lucii, A. anguillae, Pomphorhynchus laevis and Echinorhynchus trultae and I marine species, E. gadi, were studied in the intestines of several species of fish, especially Anguilla anguilla. When data are presented in the conventional manner with results from all fish and all parasites in a sample combined, each species exhibited a preference for a particular region of the intestine which altered in some species in relation to host identity and sex and abundance of parasite. Each species could nevertheless inhabit most of the length of the intestine and there was a considerable degree of overlap in most cases between the distributions of species in the same host, but the mean position of each species differed significantly from that of any other and the species appeared to exhibit site segregation and resource partitioning along the length of the intestine. In individual fish, fidelity of site location and distribution is poor and may bear little or no relationship to the position and distribution of the parasites in the population as a whole, and it is concluded that the concepts of site specificity and segregation are most applicable at the population level. Mixed species infections are very rarely encountered in Britain since most freshwater localities contain, or are almost completely dominated by, only a single species of acanthocephalan. The results are discussed in relation to current ideas on the role of interspecific competition in determining site segregation and community structure of parasites. It appears that site specificity in fish acanthocephala is not related to the probability of mating, or site segregation to interspecific competition past or present. Acanthocephalan communities do not exist as such; the normal situation is chance assemblages, with vacant niches, little organization, few or no interactions and little fidelity between habitats.

Chronic quartan malarial infection has been established in the common marmoset (Callithrix jacchius). Plasniodium brasilianunm from a douroucouli monkey (Aotus trivirgatus) was used to infect splenectomized twin animals, passed to an intact animal, and then to 4 other intact adults, 2 pairs of twins. In 2 of the 4 latter animals there was continuing patency with parasitaemias of ≤0·5% parasitized erythrocytes for 30 weeks. The other 2 had lower initial levels of paraaitaemia; in 1 of these parasitaemias remained low or subpatent. All marmosets developed lymphocytosis. One animal became ill 30 weeks after infection with anaemia, weight loss and mild proteinurea, the other 3 remained well. Histological examination showed minor changes in the kidneys; spleens of infected animals showed marked follicular hyperplasia and phagocytosis of pigment. The livers showed sinusoidal hyper cellularity and pigment deposition and in splenectomized animals, a marked lymphoid follicular hyperplasia in the portal tracts.

Spermatogenesis and sperm maturation in Nematospiroides dubius were studied using electron microscopy. The testis is telegonic and germ cells in the zones of mitosis, growth and meiosis are connected by a central anucleate mass of cytoplasm, the rachis. The early part of spermatogenesis is dominated by the synthesis and growth of membrane-bound vesicles called membranous organelles, which originate from RER-associated Golgi bodies. Following meiosis the spermatids separate from the rachis and their chroinatin, which is no longer bounded by a nuclear envelope, condenses into an arrow-head shape and is extruded to form a tail-like structure. After insemination spermatozoa undergo a profound change called activation. The cytoplasmic region which was previously long and cylindrical becomes spherical and the membranous organelles which lined its perimeter fuse with the plasma membrane and become confined to the posterior hemisphere of the sperm, close to the nuclear tail. The anterior half of the sperm is devoid of organelles but contains many filaments organized into clumps and chains; this region being responsible for amoeboid locomotion of the sperm.

Plastic ear tags impregnated with acaricides (synthetic pyrethroids; fenvalerate, 10% (w/w); fenpropathrin, 10% (w/w); fiuvalinate, 10% (w/w); and permethrin, 8% (w/w)) were evaluated for the control of Rhipicephalus appendiculatus infesting cattle in the laboratory and field in Kenya. These controlled release devices gave excellent control of R. appendiculatus for over 20 weeks on cattle in the laboratory where they were housed mostly indoors. Fluvalinate-impregnated tags gave consistently the most rapid control of ticks, but all of the tags normally prevented feeding of the ticks on the ears of cattle. Under field conditions in the Trans-Mara Division of Kenya, excellent control of R. appendiculatus naturally infested cattle was obtained by fluvalinate-impregnated tags followed by fenpropathrin. Less impressive control was obtained by fenvalerate- and permethrin-impregnated tags, but this control was more effective than immersion of cattle in toxaphene. In contrast to the laboratory experiments, the effect of the tags on R. appendiculatus infestation waned by 58 days after application. The potential of synthetic pyrethroid-impregnated tags for the control of ticks infesting cattle in Africa was demonstrated, but a more prolonged release of the active ingredients at an effective level in the field and, in some cases, a more robust tag design will have to be achieved before tick control with impregnated tags becomes a practical proposition.

The effect of host resistance on the feeding and development of Rhipicephalus appendiculatus, the African brown ear tick, was investigated. In this 3-host species the percentages of larvae and nymphs engorging on resistant cattle, and also of those subsequently moulting, were significantly less than those of ticks feeding on susceptible rabbits (i.e. without previous tick-feeding experience). The mean weights of engorged larvae, nymphs and females were significantly reduced on resistant hosts. Fewer nymphs and females engorged, even when fed on susceptible hosts, if their previous feed had been on a resistant host, and the engorged weight was also less. If only one feed during the life-cycle was on a resistant host the final egg production was reduced by a factor of 13· 2· and l· for the larval, nymphal and adult feed respectively. The egg production of a tick population with all 3 stages feeding only on resistant hosts could be reduced by more than 98% if these effects were combined.

Formation of the tegument and alimentary tract in developing Fellodistomum fellis cercariae has been examined by electron microscopy. Morphogenesis of the tegument is a two-stage event: (1) ectodermal cells at the germ ball periphery coalesce beneath a closely apposed and transitory primitive epithelium of sporocyst origin, forming a syncytial boundary of nucleated tegument. The primitive epithelium is then lost and the nuclei in the surface layer of the early tegument degenerate; (2) cells in the mesenchyme of the cercarial embryo differentiate as secretory cells and connect with the anucleate layer to form the true tegument. The foregut develops as an apical invagination of early nucleated tegument, with the primordial digestive cells differentiating behind the oesophagus. When fully formed, the surface anucleate layer of oesophagus tegument extends posteriorly over the apposed surfaces of the developing digestive cells, thus isolating them from each other and from the gut lumen. An apical cavity develops in each of the digestive cells and later communicates with the gut lumen through a pore in the overlying caecal tegument. Lamellated extensions of digestive cell cytoplasm project into the cavity, and development of GER and a Golgi apparatus in the cell mark the onset of digestive secretion.

An ultrastructural study of numerous serial sections of Plasmodium chabaudi trophozoites at various growth stages, followed by 3-dimensional reconstruction, allowed us to describe more precisely the internalization process of the erythrocytic stroma, both in space and in time. Two endocytotic processes are apparent. (1) Pinocytosis — as soon as the merozoite has become a young trophozoite (ring stage), small double membrane-bound vesicles can be seen budding off around the whole periphery of the parasite. After the inner membrane of the vesicle has disappeared, the contents alter and a pigment crystal appears. (2) Cytostomal system — this phenomenon coexists with, and eventually replaces, pinocytosis. It consists of invagination of the membrane of the parasitophorous vacuole and of the plasmalemma, through a typical cytostome, in order to form a cytostomal vacuole. This extends into a long tube, the cytostomal tube, which eventually digitates. When the tube reaches its maximal size, the cytostome disappears and the tube remains open to the erythrocytic stroma by a simple aperture. A new cytostome can form elsewhere on the parasite surface and another tube can extend. Two or three such tubes can coexist in a trophozoite although only one cytostome is functional at one time. At the end of the tubes vesicles bud off, the contents of which become modified as described previously. The residual product of haemoglobin degradation is the malarial pigment.

Over half of all living species of plants and animals are parasitic, which by definition involves intimate association with and unfavourable impact on hosts (Price, 1980). This paper will only consider parasites whose ‘unfavourable impact’ adversely affects the birth and/or mortality rates of their hosts (Anderson, 1978). Most organisms are potential hosts and must deal with the problem of parasitism. The probability of parasitic infection of a host is influenced by both environmental and genetic factors. Traditionally it was assumed that a host was either resistant or susceptible to a particular parasite and therefore the interaction between a parasite and potential host had only two possible outcomes: either the resistant host rebuffed the parasitic attack and remained uninfected or the parasite successfully invaded and significantly reduced the reproductive success of the susceptible host. This approach, however, ignored the intraspecific genetic variation present within both host and parasite populations (Wakelin, 1978). Since the outcome is determined by the interaction of a finite set of host genes and parasite genes, genetic variation in host susceptibility and parasite infectivity (Richards, 1976; Wakelin, 1978) suggests that more than two outcomes are possible. Variation in host and parasite genomes does not begin and end at the susceptibility/infectivity loci. Other genes may also influence the outcome of host–parasite interactions by altering the life-history patterns of hosts and parasites, and lead to a variety of outcomes.

Interest in the long-term low temperature preservation of malaria parasites was initiated by Coggeshall's (1939) demonstration that the intra-erythrocytic stages could survive storage at −76°C for 70 days. Numerous studies have subsequently been published in which a variety of techniques have been used. In addition, several reviews have included sections on the cryopreservation of malaria, the most recent being by James (1980), Leef, Hollingdale, & Beaudoin (1981), Nguyen-Dinh, Chemangey-Masaba, Spencer, Campbell, Chin & Collins (1981) and a WHO memorandum (1981). Most of these reports and reviews have dealt with the problems of cryopreserving malaria parasites from a parasitological viewpoint. In an attempt to provide a cryobiologically orientated review of value both to cryobiologists and practicing parasitologists the published technical reports have been summarized in chronological order in Tables 1 and 2, and the important cryobiological parameters are discussed.