A study of electrophoretic forms of a selection of enzymes has been made in strains of subspecies of the rodent malaria parasites Plasmodium berghei and P. vinckei. Considerable electrophoretic variation of these enzymes occurred among the strains studied. On the basis of the results it was possible to identify groups of parasites within each species. While generally substantiating the classical taxonomy the identification of groups provided further information on the relationships between the subspecies.

Oogenesis and fertilization in the filarial worm Dipetalonema viteae have been described. The female gametes are developed from a syncytium situated at the posterior tip of the ovary. The syncytium grows along the length of the ovary as an anucleate rachis carrying the germ cells with it. In the germinal region of the ovary the rachis is a central axial structure, but in the growth region it becomes very branched. The female gametes retain their attachment with the rachis until they have become mature primary oocytes; they acquire a surface coat whilst in the growth zone of the ovary. At fertilization the successful spermatozoon produces numerous ribosomes. Residual spermatozoa are apparently destroyed by the oocyte. The fertilized oocyte divides meiotically to yield an ovum and two polar bodies. In species destined to be unsheathed in the blood the surface coat and the original oolemma separate from the surface of the ovum. A new oolemma is produced, presumably under the stimulus of fertilization.

Crosses have been made between two lines of Plasmodium berghei yoelii differing in drug-sensitivity and enzyme-type. The two lines used were line A, which is pyrimethamine-resistant and contains an electrophoretic form of glucose phosphate isomerase termed GPI-1, and line C, which is pyrimethamine-sensitive and contains enzyme-form GPI-2.Equal numbers of blood forms of lines A and C were mixed and injected intravenously into a mouse. Mosquitoes were fed immediately on the mixture, and the resulting sporozoites used to infect further rodents. After treating these animals with pyrimethamine, drug-resistant parasites characterized by GPI-2 could be detected in the remaining infections. Controls showed that these parasites were recombinant forms, which had arisen by cross-fertilization of gametes in the mosquitoes, and not by mutation or ‘synpholia’.By cloning the products of crosses between lines A and C by dilution, parasite lines of two recombinant classes (resistant GPI-2 and. sensitive GPI-1) were isolated, together with lines of parental characteristics.

Lobatostoma manteri sp.nov. is described. It differs from other species of this genus in the number of marginal alveoli (usually 56–62), the location of the testis near the posterior end and the large size of the cirrus pouch. Mature worms occur in the intestine of the fish Trachinotus blochi. Eggs containing fully developed larvae are laid. The eggs are eaten by snails and hatch in the stomach. Larvae have an oral sucker, pharynx, simple caecum, ventro-terminal acetabulum, two dorsal excretory bladder cells in front of the acetabulum, and a caudal appendage. They migrate into the digestive gland and differentiate to pre-adults with fully developed genital organs and the full number of alveoli on the adhesive disk; young spermatozoa and egg cells develop but do not mature. Pre-adults have a minimum number of 8500 sensory papillae on the surface. The worms are usually found in a cavity formed by enlargement of the main duct and one or more (?) side ducts of the digestive gland near the stomach in Cerithium moniliferum, or in the stomach and main ducts of the digestive gland of Peristernia australiensis. They may creep from the ducts into the stomach and back into the ducts. Fish become infected by eating snails. Worms from fish die soon after transfer into sea water but can be kept alive for up to 13 days in frog's Ringer solution or dilute sea water (1:5), in which they lay eggs containing larvae infective to snails. Worms from snails remain alive in sea water, dilute sea water, frog's Ringer or Tyrode solution. Eggs of worms from single infections have the haploid chromosome number of 7; there is normally no self-fertilization and development does not reach the blastula stage. The life-cycle of Lobatostoma manteri is the simplest two-host cycle of trematodes known. Reasons are given why it must be considered the most primitive one, of a type from which digenean life-cycles have evolved.

The development of the epidermis of the oncomiracidium larva of Entobdella soleae was studied in embryos dissected from the egg and processed for electron microscopy. At first (7- to 8-day embryos at 15 °C) the embryo is covered with a nucleated primary epidermis of flattened cells which are closely associated with the viteUine cells and may take up nutrients from them. This layer is either replaced by or develops into the secondary epidermis which consists of ciliated cellular regions joined by an apparently syncytial interciliary cytoplasmic layer. Both ciliated cells and interciliary cytoplasm are at first nucleated but later both lose their nuclei. There may be a turnover of ciliated epidermal cells at the surface of young embryos. Between days 16–20 a discontinuous presumptive adult epidermis appears beneath the ciliated cells which has connexions to cell bodies lying in the parenchyma. This layer, apparently fuses with the syncytial (?) interciliary regions which have by this time lost their nuclei. After shedding of the ciliated cells, the presumptive adult epidermis spreads out to form a continuous syncytial covering to the post-larva. The ciliated epidermal cells apparently lose their nuclei between days 20 and hatching. This may be associated with the ‘programmed life’ of the ciliated cells. Details of the food reserves and morphology of the ciliated cells are discussed in connexion with the energetics of these cells.

An account is given of the structure and mode of action of the opisthaptor of the monogenean Pseudobenedenia nototheniae Johnston, 1931, from the skin of the fish Notothenia neglecta, caught in December 1969 at a depth of 40 m offSigny Island (60° 43′ S., 45° 36′ W.), South Orkney Islands. The opisthaptor has a roughly hexagonal central loculus surrounded by a septum which is continuous with six radiating septa. It is armed with 14 marginal hooklets and three pairs of sclerites which function as anchors.The opisthaptor obtains adhesion through gripping the host's skin with the muscular rim, and through close contact by the marginal membrane. These form a water-tight seal, the surface area of which is about one-quarter of that of the ventral surface of the opisthaptor. The seal enables the opisthaptor to function as an underwater sucker. Contraction of muscles associated with the opisthaptor, in particular the extrinsic muscles, can generate a pressure difference across the wall of the opisthaptor some 50 times greater than that required to support the weight of the parasite, thus suggesting that reserves are available to resist dislodging forces such as increased water flow. The instability and tendency to slide of the sucker is counteracted by anchorage from the hooklets and sclerites. The structure and mode of action of the opisthaptor of P. nototheniae are discussed in relation to studies made by other authors on similar adhesive organs.

A comparison was made between 14-day-old populations of Nematodirus battus from single infections and from those which developed concurrently with Haemonchus contortus from the day of infection and from days 5 and 10 after infection. The numbers of Nematodirus fell with decreasing length of exposure to Haemonchus and were negatively correlated with the numbers of Haemonchus. The percentage of the populations of Nematodirus in the adult stage and the mean worm lengths fell with increasing length of development with Haemonchus, the lowest values being found in infections which developed concurrently with the abomasal worm for the full 14 days (days 0–14). Mean Nematodirus worm lengths were negatively related to abomasal pH and Na+ levels in populations which were concurrent with Haemonchus for the last 9 days (days 5–14) and for the full 14 days of development studied. Nematodirus egg capacity also fell with exposure to Haemonchus, but was lower in the worm populations exposed for the last 9 days than in those exposed for 14 days. Egg capacity was related to abomasal electrolyte levels in populations which had developed concurrently with Haemonchus for the last 9 days and for the full 14 days. Significant differences between the population parameters of Nematodirus from the proximal and middle third of the small intestine were only found in the single Nematodirus infections and in those which were concurrent with Haemonchus for the last 4 days (days 10–14). The differences in the populations of Nematodirus were related to the probable changes in alimentary conditions affected by the different periods of concurrent infections of Haemonchus. It was suggested that the alimentary changes may have caused the loss of adult Nematodirus and have influenced the growth, development and reproductive capacity of the intestinal worm by acting upon the fourthand fifth-stage larvae.

The functional organization of the tail musculature of two species of cercariae has been studied by examination of serial sections in the light and transmission electron microscopes. A three-dimensional reconstruction of the tail has been established. The fine structure of the muscle cells has been investigated and the complexity of organization is related to the requirement for a fast contracting system in the cercarial tail.

Possible relationships between the numbers and development and growth of Haemonchus contortus and Nematodirus battus during concurrent infections and the levels of abomasal pH and Na+ were investigated in 2- to 3-month-old and 8-month-old lambs. The percentage of Haemonchus in the fourth larval stage was found to be positively related to the levels of abomasal electrolytes, while the mean lengths of the worms in the total male and female populations and in the fifth-stage populations were negatively related to these parameters. The numbers of Nematodirus and the mean lengths of worms in the total and fifth-stage populations were negatively related and the percentage of worms in the fourth-stage positively related to abomasal pH and Na+ levels. It is suggested that the changes in abomasal physiology caused by massive infections of Haemonchus directly affect the growth and development of this abomasal parasite. It is also suggested that the changes in the abomasal contents entering the proximal part of the small intestine during such infections of Haemonchus alter the intestinal environment and that this alteration directly affects the course of development of concurrent infections of Nematodirus.

Experimentally we have demonstrated what Lee (1969, 1971 a, b) postulated on the basis of morphological considerations. The female heterakid is capable of transmitting H. meleagridis from the bird to the caecal worm egg without the intervention of the male except for fertilization. If the male is otherwise involved in the transmission of H. meleagridis, it seems unlikely that its role is of appreciable significance in the life-cycle of the histomonad as we know it now. However, if as believed by many, the histomonad was originally a parasite of a nematode that may not have been an obligatory parasite, the role of the male worm may have been considerably more important to the survival of the histomonad.

When the eggs of the skin parasitic monogenean Entobdella soleae are subjected in the laboratory to alternating periods of light and darkness (LD 12:12, LD 6:18 or LD 18:6) at constant temperature (selected from the range 12–16 °C), hatching is rhythmical, most of the oncomiracidia emerging during the first 4 h of the period of illumination on each successive day.

Rhythmicity in response to alternating 12 h periods of light and darkness persists in a wide range of lighting conditions including natural daylight and in dim blue light, the latter being similar in intensity (65 nW/cm2) and in spectral quality to light at the depths where the host Solea solea lives.

The hatching rhythm persists with a periodicity of about 24 h in constant darkness and in constant light, provided that the eggs have been exposed previously to LD 12:12 until hatching begins. This indicates that there is a strong endogenous component to the rhythm.

Eggs laid and maintained in total darkness develop and hatch, but the oncomiracidia may emerge at any time during each 24 h period and there is little evidence of rhythmicity.

When alternate 12 h periods of light and darkness are reversed after the commencement of hatching, the hatching pattern also reverses after 24 h, so that emergence of larvae occurs at the beginning of the new illumination period.

Mechanical disturbance plays little or no part as a hatching stimulus.

Larvae, which had emerged from their eggs at some time between 20 and 80 min previously, responded to host skin, suggesting that an initial uninfective period is of short duration or absent.

Hatching soon after dawn may have high survival value for E. soleae in view of the host's activity pattern; soles are active at night and partly bury themselves in the sediment (where the eggs of the parasite lie) at dawn, and the fish is inactive and a stationary target during the day.

Perforated plastic cylinders (hair curlers) were implanted subcutaneously in rabbits for the collection of tissue fluid. Using this technique, a study was made of the chemical constituents of tissue fluid and serum in rabbits infected with Trypanosoma (Trypanozoon) brucei, as compared with uninfected, control animals. Changes were caused by the infection in the concentrations of proteins, electrolytes, urea, creatinine, cholesterol and pyruvate, and in the activities of the enzymes aspartate transaminase, creatine phosphokinase and alkaline phosphatase.Antitrypanosomal antibody, detectable by the indirect fluorescent antibody technique, was shown to penetrate the tissue fluid and to reach a titre of about one-fifth of that in the serum. Renal failure, probably resulting from allergic reactions, was shown to be a contributory cause of death in rabbits infected with T. (T.) brucei.

Mouse peritoneal macrophage cells suspended in a TC 199 calf serum medium and cultured in Leighton tubes, were infected with Trypanosoma (Schizotrypanum) cruzi culture forms in order to study the development of this parasite in vitro. Three cycles of development were thought to occur:1. Epimastigotes or trypomastigotes were taken up by macrophages and transformed into amastigotes. These multiplied by binary fission and ruptured the infected cell after 4‐5 days. Released amastigotes were taken up by uninfected macrophages and the cycle was repeated.2. A small proportion of intracellular amastigotes developed into ‘ovoid forms’ which transformed progressively into promastigotes, epimastigotes and trypomastigotes.3. Some amastigotes became ‘round forms’ from which sphaeromastigotes developed. These transformed directly into trypomastigotes without the formation of epimastigotes.

The effect of oxyclozanide (2,2′-dihydroxy-3,3′,5,5′,6-pentachlorobenzanilide) on Hymenolepis microstoma in the bile duct of mice, and H. diminuta in the small intestine of mice and rats was measured.Oxyclozanide at doses as low as 4mg/kg removed 13-day-old H. diminuta and caused no obvious harmful effect to the rat host up to the maximum level (256 mg/kg) tested. Worms were displaced and degenerating within 1 h. Results in mice were more difficult to assess because of the immune response, but similar total amounts of oxyclozanide caused destrobilation and loss of 7-day-old H. diminuta.Oxyclozanide was less effective against 21-day-old H. microstoma attached in the bile duct. Approximately half the strobila was lost following dosing at 5 mg/kg and progressively greater amounts as the dose level was increased. At 50 mg/kg worm loss commenced but even at 150 mg/kg 25 % of worms survived. The time taken to regrow to the original adult size varied but was complete within 7–9 days at levels up to 25 mg/kg. Double dosing at 5-day intervals did not enhance the effect of a single dose. The apparent existence of a sensitivity gradient down the strobila in H. microstoma is discussed.

Mesophryxus, a new genus of phryxid isopod is described. The only species, M. ventralis, is a parasite of Harpiliopsis beaupresi (Audouin) to which it is attached on the ventral aspect of the sixth abdominal segment, behind the fifth pair of pleopods. Mesophryxus is most closely related to the genus Anomophryxus Shiino, from which it differs in the arrangement of appendages on the abdomen.

The adults of Solenopotes tarandi (Mjöberg, 1915) are redescribed and illustrated, and its allied species of Solenopotes are compared. The nymphs of this species are for the first time described and illustrated. Ectoparasite infestation of the barren ground caribou (Rangifer tarandus granti Allen) is also discussed.

The kinetics of uracil, thymine, 5-bromouracil, hypoxanthine, guanine and adenine uptake by Hymenolepis diminuta were studied using shortterm (2 min) incubations. The effects of other purines and pyrimidines on the uptake of these compounds was determined. The data suggest that the purine-pyrimidine transport system of H. diminuta contains at least three distinct transport loci and that two of these loci contain multiple substrate binding sites. The three loci are characterized as follows: (1) A thymine-uracil locus which is responsible for the absorption of thymine and uracil via allosteric mechanisms and, therefore, apparently contains two binding sites for these pyrimidines. Adenine and hypoxanthine are non-productively bound to this locus; purine and 6-methyluracil are also bound to this locus, but it is unknown if binding is productive. (2) A locus which transports hypoxanthine (denoted as hypoxanthine locus no. 1), guanine, and adenine. The latter purine is apparently transported by an allosteric mechanism and, therefore, this locus apparently contains two adenine binding sites. Uracil and 5-bromouracil also bind to this locus, however, the nature of binding is uncertain. (3) A second hypoxanthine locus (hypoxanthine locus no. 2) can also be demonstrated which nonproductively binds adenine. Purine is also bound to this locus, but it is unknown if this binding is productive.

The adult of P. crucibulum is redescribed to form a basis for comparison with the cercaria and metacercaria. Particular emphasis is given to the structure and function of the rhynchus, digestive system and reproductive system. Some features, notably prepharyngeal glands and spematophores, have not previously been recorded in this species.

The sporocyst and cercaria which were found in Mytilus edulis are described for the first time. Out of 6907 mussels examined from Borth Rocks, Cardigan Bay, 0·26% were infected with P. crucibulum and 0·06 % with P. squamatus. The cercaria of the latter is redescribed on a comparative basis with that of P. crucibulum, to avoid confusion. The two cercariae are clearly distinguished by three features – flame-cell formula, shape of the excretory vesicle, and structure of the tail stem (this being bilobed in P. crucibulum and trilobed in P. squamatus). The significance of the two types of tail in securing attachment to the second intermediate host is discussed.

Metacercariae of P. crucibulum were obtained experimentally in ‘O’ group specimens of Scophthalmus maximus. Three other species of fish, namely Limanda limanda, Onus mustelus and Gobius minutus were also successfully infected. The metacercariae did not develop in the following fish: S. rhombus, Pleuronectes platessa, Cottus bubalis and Blennius pholis. Possible mechanisms which might account for host specificity within Pleuronectidae and Scophthalmus are suggested.

The development of the metacercaria in the connective tissues and musculature of S. maximus was followed over a period of 10 months. Within 1 month of infection the hyaline cyst of parasitic origin breaks down, enabling the parasite to feed directly on the host tissues. This feature is thought to account for the advanced development of the metacercaria, which 1 month after infection could clearly be identified with the adult on the basis of comparative morphology, and at 2 months resembles the adult in every respect except the development of the vitellaria. Egg production only occurs within the intestine of Conger conger, the definitive host.

Rhipicephalus appendiculatus ticks, infected with Theileria parva, were fed on rabbits and removed at daily intervals for 9 days. Half of the ticks removed were dissected and their salivary glands processed and examined for parasites. The remaining ticks which had been removed were ground up and aliquots of the supernatant fluid thus obtained were injected into East Coast fever susceptible cattle. Cattle injected with supernatant fluids from ticks fed for 4–9 days became infected, and this could be correlated with the histological detection of mature parasites in the corresponding ticks. The most highly infective supernatant fluids were prepared from 5-day-fed ticks. In a second experiment it was shown that female ticks of a variety of weights contained infective particles of T. parva.