Two species of chondracanthid copepods Acanthochondria platycephali Heegaard, 1940 and Pterochondria alatalongicollis Heegaard, 1940 are described. The latter was described by Heegaard as Acanthochondria platycephali f. alata-longicollis. However, it is placed in a new genus, based on the studies of type specimen and new material. A discussion on the other species of Acanthocondria parasitic on the flathead is given. It is suggested that A. bulbosus Heegaard, 1943, is synonymous with A. tchangi Y¨, 1935, and A. diastema Kabata, 1965, with Pterochondria alatalongicollis.

1. The in vivo growth of Schistocephalus solidus plerocercoids was investigated by surgical implantation of worms into the body cavity of uninfected, recipient sticklebacks. 2. Burdens of one, two and five worms were used. 3. Results show a reduction in parasite growth with both increasing size and increasing numbers of parasites. 4. Reasons for the decrease in growth rate of multiple burdens are discussed and reference is made to the significance of burden size in completion of the life-cycle under natural conditions.

Two strains each of Eimeria acervulina and Eimeria mivati were examined for their ability to develop in the chorioallantoic membrane (CAM) of chicken embryos and the caeca of chickens. It was found that between 100 and 1000 times more sporozoites of E. acervulina were needed to produce infections in the caeca and the embryo CAM respectively. The reproduction of both species in infections of chickens was similar and a high degree of cross-protection occurred.

It was found that oocysts of E. mivati differed in size and shape depending upon whether they were produced from infections of embryos or chickens. When maintained by passage in chickens they assumed size and shape characteristics of E. acervulina.

These findings, with the known similarity of the life-cycle stages, pathogenicity and pathology, indicate that E. mivati is not sufficiently different from E. acervulina to be a distinct species. It is suggested that it is best referred to as E. acervulina var. mivati.

The results of experimental infections of Trichuris muris in wild field mice (Apodemus sylvaticus) and laboratory-bred wild house mice (Mus musculus) showed that the parasite elicited an immune response similar to that previously described in strains of laboratory mice. Experiments in laboratory mice showed that the parasite was able to become sexually mature only when small single infections or repeated low-level infections were given. A survey of a population of 43 wild house mice naturally infected with T. muris showed that the pattern of small worm burdens in the majority of mice was consistent with a situation of repeated low-level infection, except in the case of six female mice which harboured larger mature worm burdens. It is suggested that in these mice pregnancy and/or lactation may have suppressed the immune response, allowing the accumulation of a worm burden in excess of the threshold for worm expulsion.

The ultrastructure of the reproductive system of mature (54-day-old), immature (32-day-old) and females from unisexual infections of Schistosoma mansoni is described in detail. The uterus is tegumentary in structure but the vitelline duct and oviduct are complex and possess cilia as well as lamellae on their luminal surfaces. The characteristics of the cells forming the walls of the ducts suggests that they may have a digestive function. The posterior portion of the oviduct of the adult worm contains sperm which become enveloped by lamellae. The vitelline cells of the adult contain vitelline droplets, much lipid and little glycogen. A second type of body derived from endoplasmic whorls is also present. Mehlis's gland contains only one type of gland cell and these cells pass through the ootype wall and open into its lumen. The female from unisexual infections has an incompletely developed Mehlis's gland, an ovary in which the Golgi complexes do not produce typical cortical granules and has vitelline cells which remain immature. The oviduct, ootype and uterus are well developed in contrast to the vitelline duct. A comparison with young, but not inseminated worms, suggests that the presence of sperm in the oviduct is not the major stimulus which induces maturation of the female worm.

Surface aminopeptidase (APase) in the acanthocephalan, Moniliformis dubius, was studied using leucylleucine, leucylglycylglycine, glycylglycine, alanylalanine, tri-alanine and tetra-alanine as substrates. In the presence of intact worms all peptides, except glycylglycine, were hydrolysed liberating amino acids and/or peptide subunits into the incubation medium. In 5 min incubations with 5mM leucylleucine, 93% of the liberated leucine was absorbed by the worms indicating a kinetic advantage for absorption of the leucine. In media with leucylleucine plus 10 mM methionine as an inhibitor of leucine uptake, the liberated leucine was not absorbed by the worms, but accumulated in the incubation medium. The inhibitory effects of leucylleucine and alanylalanine (or products of hydrolysis) on the uptake of 0·1 mM [14C]leucine in 2 min incubations were examined. Inhibition of [14C]leucine uptake by these peptides was non-linear up to 5 mM inhibitor concentration. Concentrations of either peptide greater than 5 mM did not produce further inhibition. A significant portion of [14C]leucine transport was not inhibited by these peptides. Inclusion of 0·5 mM Pb2+, an APase inhibitor, blocked the inhibition of [14C]leucine uptake by leucylleucine indicating that the inhibition is caused by the liberated leucine alone, and that interaction of the intact dipeptide and the leucine transport system is negligible. It is concluded that the surface membrane of adult worms has APase which can hydrolyse peptides in the ambient medium. The spatial arrangement of the APase and the leucine transport system is such that the hydrolysis of a peptide confers a kinetic advantage for absorption of the liberated amino acids. Encysted or mechanically excysted cystacanth larvae from the haemocoele of the cockroach showed no APase activity. Activation of the APase required a 30 min treatment in lipase, [Na+]taurocholate or other surface active agents. Since the APase in adult worms was unaffected by washing, the data strongly suggest that this enzyme is of parasitic origin.

Biochemical studies on the non-specific phosphomonoesterases have demonstrated the presence of acid phosphomonoesterase with maximum activity at pH 4·0 in Gastrodiscus aegyptiacus (enzyme I) and at pH 4·5 in the case of Fasdolopsis buski (enzyme II). The Km for ρ-nitrophenyl phosphate hydrolysis was 0·66 mM for enzyme I and 1·1 mM for enzyme II. Different concentrations of fluoride, arsenate, tartrate, tartaric acid, cysteine and copper brought about inhibition of both enzymes and magnesium, iodoaeetate, iodoacetamide and EDTA had no influence on either enzyme activity. Cobalt activated both enzymes while zinc inhibited enzyme I and strongly stimulated enzyme II.

The histology and ultrastructure of the salivary gland in Dermacentor andersoni are presented with particular emphasis on those aspects relating to fluid secretion. We suggest that the group III acinus contributes most of the fluid portion of the saliva (i.e. water and small molecules) and that the main cell-type involved is what we name the ‘water-cell’. The granule-cells possibly secrete the cement by which the tick secures its mouthparts to the host, and the ‘vacuolar cell’ possibly produces a protein-rich secretion. The function of the group I acinus remains obscure.

Groups of mice were exposed to Schistosoma mansoni cercariae in a large volume of water. A factorial design was used to investigate the effects of cercarial concentration and length of exposure on infection rates and worm burdens. Low exposure times and cercarial concentrations gave no infections, but prolonged exposure to high concentrations gave high infection rates and worm burdens. Prolonged exposure to low concentrations gave low worm burdens and moderate infection rates: short exposure to high concentrations gave moderate infection rates and worm burdens. Statistical analysis showed that both infection rates and mean worm burdens were significantly related to the two main factors, but the relationship was complicated in each case by curvilinear effects and by a significant interaction between the main factors. Field cercarial concentrations are generally low on St Lucia, a fact suggesting that brief human water contacts, such as fording and filling domestic water containers, play little part in transmission. Prolonged contact during swimming, bathing and washing clothes may produce significant infection rates accompanied by low worm burdens. However, on the rare occasions when high cercarial concentrations occur, even the briefest exposure may produce significant infection rates and moderate worm burdens.