The tegumentary ultrastructure of Leucochloridiomorpha constantiae metacercariae and adults has been described. A filamentous glycocalyx invests the tegument of metacercariae and large numbers of biconcave, disk-shaped secretion vesicles are found in its outer zone, cytoplasmic bridges and cytons. Mitochondria within the outer tegumentary zone of metacercariae are restricted to its basal half. The transformation of metacercariae to adults involves the loss of the glycocalyx, a decrease in the number of disk-shaped secretion vesicles, a more uniform distribution of mitochondria through the outer tegumentary zone, an apparent degeneration of cytons, a thinning of the fibrous basal lamina and a decrease in the number of visible cytoplasmic bridges traversing it. The significance of these events for a metacercaria which does not encyst within the molluscan host is discussed. In addition, crystalline, spine-like inclusions in the outer tegumentary zone of adult L. constantiae are described and their possible function discussed.

In lactating rats examined 3 weeks post infection, adult Nippostrongylus brasiliensis were found in the duodenum close to the pylorus. Worms migrated from the predilection site, shown by Brambell (1965) and Alphey (1970) to be approximately 25% down the intestine, during the second week of infection. A similar migration took place in neonatally infected rats.

It was suggested that the migration was an active response to immunological damage. It could be prevented by early betamethasone treatment but only modified by treatment given late in infection.

A substituted azauracil derivative has been shown to have marked anticoccidial activity. Chickens given large challenge doses of Eimeria tenella were protected from mortality by a dietary level of 0·25 ppm drug, while oocyst output was controlled by 10 ppm. Under conditions of continuous medication using 15 ppm of the azauracil derivative the development of E. tenella was arrested at the multinucleate first generation schizont stage; in the majority of parasites merozoite differentiation was inhibited. Experiments using restricted medication indicated that, with E. tenella in the chick, the drug was coccidiocidal rather than coccidiostatic, and that treatment for as little as one day could control the infection. Medication started during the second asexual cycle again resulted in inhibition at the multinucleate schizont stage just before merozoite differentiation. Mortality in birds inoculated with E. brunetti was controlled by 1 ppm azauracil in the diet and oocyst output by 2.5 ppm; unlike the situation with E. tenella, the drug had no influence on the first two asexual cycles of reproduction. There was some inhibition of the third schizogony cycle, but the main effect of the drug in the case of E. brunetti was found to be against sexual reproduction. Effects similar to those noted in vivo were seen with E. tenella and E. brunetti growing in tissue culture. Occasional schizonts of E. tenella under the influence of drug were able to produce a peripheral array of merozoites surrounding a multinucleate residual mass. In contrast to the situation in vivo, with drawal of drug from tissue cultures infected with E. tenella allowed the further development of some inhibited parasites. E. necatrix responded to the azauracil derivative in a manner similar to E. tenella.

Previously it has been shown that eggs of the skin-parasitic monogenean Entobdella soleae, maintained free of host contamination at constant temperature (12 °C) and exposed to alternating 12 h periods of light and darkness, hatch during the first few hours of each period of illumination as a result of endogenous processes in the larva in conjunction with the illumination cycle. Washings, prepared by immersing the parasite's host (the common sole, Solea solea) for 1 h in seawater (just sufficient to cover the body), enhance this ‘morning’ hatching. Furthermore, the application of host washings during the latter half of the period of illumination or during the dark period also stimulates hatching. Experiments have shown that the hatching factor is present in sole skin mucus, is not destroyed by boiling for 5 min or by freezing and is produced by small soles measuring 4–5 cm in length as well as by larger soles (15–21 cm long).

When eggs are incubated at about 12°C in the absence of fish washings, hatching begins about 30 days after laying, but there is evidence that some of the larvae inside their shells are fully developed and capable of hatching in response to host hatching factor 1–3 days earlier.

When fish washings are added to eggs which range in age from 25 to more than 30 days (at 12 °C) many eggs are stimulated to hatch (in addition to ‘morning’ hatching as a result of endogenous rhythmical processes and the illumination cycle) and others fail to hatch. These remaining unhatched eggs, in the absence of further treatment with host hatching factor, will complete their development and subsequently will hatch during the ‘morning’ hours in response to the illumination cycle. Further contact with host hatching factor is likely to stimulate the hatching of some of these remaining eggs.

The host hatching factor is not specific to S. solea; washings from plaice, dab, halibut, whiting and ray induce hatching in E. soleae. In experiments in which oncomiracidia were offered scales from dab and sole, most of the attached larvae were found on sole skin, irrespective of whether the hatching stimulant had been provided by dab or by sole.

The relationship between the behaviour of the common sole and hatching phenomena in the skin parasite is discussed; hatching rhythms and the use of host hatching factors adapt the parasite to take advantage of most opportunities to infect the host.

It has been found by use of electrophoresis that the hepatopancreas of the snail Biomphalaria alexandrina (Ehrenberg) contains one phosphoglucoseisomerase. Snails which have secondary sporocysts of the trematode Schistosoma mansoni contain an additional phosphoglucoseisomerase which is also found in the adult trematode. It is supposed that the secondary sporocysts are secreting phosphoglucoseisomerase into the hepatopancreas of the snail in order to increase the transformation of glycogen to simpler carbohydrates, in which form the parasite is able to absorb the carbohydrates necessary for metabolism.

An outbreak of the Microsporidian parasite Thelohania contejeani is reported in a population of Austropotamobius pallipes in Northumberland, U.K. Since less than 10% of all crayfish caught were infected it is concluded that T. contejeani has no serious effect on the population studied. The mode of transmission of the parasite from host to host is discussed.

The host–parasite relationship and life-cycle of T. contejeani were studied using histological and electron microscope techniques. Spores were observed in some detail, and diplocarya, sporont and pansporoblast stages were identified within the general spore mass that invades host muscle tissue. Using serial sections the sporoblast ‘diplocaryon’ was shown to be a single nucleus with a deep cleft. T. contejeani was found to occur mainly in striated muscle, although it was found in cell bodies in the central nervous system.

The effect of immune inhibition on the pleomorphism of Trypanosoma brucei was studied by counting trypomastigotes and their granules on agar. A statistical analysis is presented. Busulphan 32 mg/kg, given 24 h before inoculation, caused an inhibition of the infection detectable at 72 h, but not thereafter. Immune inhibition caused by busulphan was associated with increased numbers both of agranular forms and forms containing lipid (type II) granules. After 96 h, treatment did not alter the proportion of agranular to granular forms. The remission was delayed 24 h in treated rats, the remaining trypomastigotes being more sluggish, more fragile and containing more numerous granules of increased size. We conclude that the remission is not produced by the immune reaction but by an innate process of dissolution of the ageing trypomastigotes.

Humoral encapsulation is an effective defence reaction against fungal pathogens. The development of injected spores in the haemocoele of Chironomus larvae may be completely (Aspergillus niger) or partially (Mucor hiemalis) prevented by this reaction. The encapsulation proceeds very rapidly; within 5 min of injection most of the spores are enclosed in a solid capsule. Disintegrating blood cells may participate in the formation of capsule substance, but this is not usually the case. Encapsulation also occurs within the cuticle against invading hyphae of Beauveria bassiana. Histochemical tests show that the capsule substance does not consist of polysaccharides; some tests for proteins and all tests for melanin were positive. The significance of melanin formation and the biochemistry of the phenoloxidase system in insects is discussed in detail. The data presented lead to the conclusion that humoral encapsulation is based upon an activation of phenoloxidases and that the capsule substance represents a polyphenol—protein complex. In contrast to current opinion, the formation of melanin in cellular as well as in humoral encapsulation is not considered to be a secondary process, independent of the actual encapsulation procedure. Instead, the authors interpret the presence of melanin as an indication of the activity of phenoloxidases which cause the formation of a capsule substance on the surface of the parasites.

Experiments to study the mortality rate of the acanthocephalan Pomphorhynchus laevis in goldfish are described. Infections with different population densities of parasite were given to fish on one occasion only and on several occasions with different time intervals between infection.

The parasites established and grew in the experimental hosts at rates comparable to those in natural hosts on all occasions. The proportion establishing did not decrease with increasing population size and bore no relationship to the presence of an existing infection. The greatest loss of parasites occurred during the period of establishment. Thereafter, mortality was gradual, and the proportion of the population lost was not related to either the density of infection or to the intervals between infections. No evidence of reduced growth rate was found at the higher population densities.

It is concluded that parasite mortality does not function as a feedback control and so is relatively unimportant in regulating the population size of fish acanthocephalans. Both field and laboratory evidence indicate that this is controlled by factors influencing recruitment. The equation of mortality and recruitment rates observed in many field populations must therefore be a matter of chance, and the implications of this with respect to the stability of fish-acanthocephalan systems is discussed.

The cysticercoid type of larval development, involving the retraction of the scolex and the associated development of a cyst, is of widespread occurrence within cyclophyllidean cestodes, but previously its possible adaptive value has been unexplained. Scolex retraction in Hymenolepis microstoma is an active, muscular process which takes only about a minute to complete. Following its retraction the scolex develops adult structures. The cyst enclosing the scolex also differentiates further, involving the secretion of three layers of collagen fibres and the development of a myelin-like layer adjacent to the retracted scolex. The cells within the cyst, including the muscle, become condensed or are autolysed causing a decrease in thickness of the cyst and an increase in the proportion of fibrous tissue. The collagenous part of the cyst provides mechanical protection for the scolex during grinding by the teeth on entry into the mouse host. The myelin-like layer insulates the scolex from the low pH conditions of the stomach. Values of lower than pH 3·0 are lethal to newly excysted worms. The myelin-like layer loses its insulatory capacity following exposure to bile salts.

Some effects of insemination on Moniliformis dubius were studied by means of a surgical technique involving the transplantation of worms of known age, sex and reproductive state from one male rat to another. Insemination resulting in the development of eggs was found to have occurred in female M. dubius aged 16 days. An individual male M. dubius was found to be capable of inseminating as many as 17 females of the same age. The duration of the patent period appeared to be related to the length of the period of contact between male and female worms at the start of the course of infection. The evidence indicated that the patent period of a female worm was curtailed in cases where the female was isolated from males sooner than 5 weeks after the start of the infection. Male worms estimated to be not more than 21 days old were found to be capable of inseminating females older than 100 days; in these cases, the patent period was brief. Evidence was obtained to suggest that insemination is followed by further growth of the somatic tissues of the female worms. These results apply to experimental infections, and caution should be exercised in applying them directly to natural infections.

Two major cytoplasmic inclusions, multilaminate vesicles and discoid granules, are present in the tegument of Schistosoma mansoni. These are produced at separate Golgi apparatuses in the tegument cell bodies and move up to the surface by a combination of diffusion and fluid flow. The discoid granules contain neutral mucopolysaccharide and are believed to break down to form the ground substance of the tegument. The multilaminate vesicles are rich in phospholipid and the contents, at least superficially, resemble unit membranes. The multilaminate vesicles are believed to contribute their contents to the multilaminate surface of the worm which appears to be continually replaced. These observations are related to current ideas on membrane turnover and the ability of the worm to acquire a disguise of host erythrocyte glycolipid.

The structure of the schistosome tegument can be changed by in vitro incubation of worms in medium containing chemicals selected for their ability to disrupt secretion in other cell systems. The observed structural changes support the hypothesis that the function of the discoid granules is to maintain the integrity of the tegument ground substance. They further support the idea that the multilaminate vesicles and/or their contents are incorporated into or on to the worm surface as part of a mechanism of surface turnover. Vesicle life appears to be similar to that estimated for other trematodes and in cestodes. In addition the worm surface appears to be replaced more rapidly than the worm acquires and loses its disguise of host antigens. It is suggested that the surface channels of the tegument may be a variable feature, and that transfer of fluids between environment and interstitial spaces may occur across the tegument.