1. Aedes togoi infected with B. pahangi were allo'ved to engorge on mice, to probe (but not to feed) on live and on dead mice, and to engorge on blood and on honey from cotton-wool pledgets. The number of worms in the mosquitoes after feeding or probing was compared with the number in unfed controls.

2. These experiments suggest that increased l?aemocoelic pressure as a result of the insect's gut filling with blood does not trigger the escape of nematodes from the mouthparts.

3. Host factors such as moisture, warmth, and chemical stimuli in the blood do not appear to be of any real significance.

4. The bending back of the labium, as the fascicle is driven into the skin, seems to be highly important. Some factor (or factors) associated with the movement of the mouthparts appears to be directly and chiefly responsible for the egress of infective filariae from the mouthparts of the mosquito blood-feeding on its host.

The nature and histochemistry of the metacercarial cyst wall of five species of trematodes of different ecological habitats was investigated. Both structurally and chemically, the cysts of Posthodiplostomum sp. and Posthodiplostomoides leoncnsis are similar. They differ from the metacercarial cysts of Parorchis acanthus, Posthodiplostomum nanum and Clinostomum tilapiae

Four groups of 8 rabbits were infected with a range of doses of N. bath's larvae. The percentage recoveries of worms from groups receiving 100 and 1000 larvae were significantly greater than those from groups receiving 10000 and 50000 larvae. It is suggested that the worm population density is dependent on the degree of host response stimulated by the initial larval dose.

Experience of a chemically abbreviated patent infection of N. batlus gave significant protection to a group of 8 rabbits against establishment of worms from subsequent challenge compared with inexperienced controls.

Rabbits aged 5 months and 11 weeks were as susceptible to the establishment of worms as rabbits aged 7½ weeks; but age resistance vas manifested by the greater proportion of worms which developed to the fifth stage in the worm populations of the youngest group than in the other 2 groups.

The dosing of rabbits with 300 000 or 500 000 larvae of Nematodirus battus produced. pathogenic effects in some rabbits similar to nematodiriasis in lambs. The clinical symptoms observed were diarrhoea, listlessness, anorexia and weight loss. Fourth-stage larvae were responsible for the onset of diarrhoea about one week after infection but immature fifth-stage worms and adult parasites may also have been pathogenic. Several of the clinically affected rabbits died, the others usually recovered during the fourth week of infection. There was no direct relationship between the occurrence of clinical symptoms and the size of the worm populations recovered at death or slaughter.

Comparison of two groups of weaned and unweaned rabbits showed that suckled rabbits were less susceptible to the establishment of N. battus infections and possibly to the effects of parasitism.

Brown and rainbow trout from Hanningf?eld Reservoir, Essex and rainbow trout from associated raceways were found to be infected with the metacercariae of Cotyluriis erraticus throughout 1968. Both o brown and rainbow trout of age 2+ introduced into the reservoir in the spring became heavily infected with C. erraticus during their first summer in the reservoir. Rainbow trout were more heavily infected than brown trout. Infection increased with age in rainbow trout but this was not apparent in brown trout. The metacercariae are almost completely restricted to the pericardial cavity of the trout. The life-cycle of C. erraticus in Hanningfield possibly involves gulls as final hosts and the gastropod Valvata piscinalis as first intermediate host. C. erraticus has probably been introduced into the reservoir in infected trout from outside sources, or by infected avian final hosts.

Perch, ruffe, brown trout and ten-spined stickleback from Hanningfield Reservoir, Essex were found to be infected with Bunodera luciopercae in 1968–1969. Only perch were heavily infected. B. luciopercae showed an annual cycle of occurrence and maturation in perch. Mean intensity of infection with B. luciopercae increased to a maximum in perch 15–19.9 cms in length but decreased in larger and older fish. There was a significant positive association between the presence of B. luciopercae and that of the cestode Proteocephalus percae in perch, with both parasites occurring together more often than expected from chance alone.

During the Glasgow University Iceland Expedition in 1972 freshwater snails and fish in southern Iceland were sampled for larval trematodes. This survey was prompted by the lack of literature concerning larval trematodes there. Indeed, the apparent absence of liver fluke Fasciola hepatica (L.1758) from Iceland, (Palsson, pers. comm.) despite the presence of its molluscan and definitive hosts and the low summer temperatures, might lead to the supposition that climatic conditions are too harsh to permit completion of many trematode life-cycles. A number of adult trematodes have been reported from migratory birds in Iceland (Brinkmann 1956), although these parasites could have been acquired elsewhere. However, Crepidostomum farionis (Müller, 1784) from the gut of trout and char in Iceland (Brinkmann 1956) presumably completes its life cycle in freshwater there, although this has yet to be demonstrated.

Two species of caryophyllids, L. longicollis sp. nov. and L. indicus (Moghe, 1926) were found in the fresh water cat fish, Clarias batrachus (L) (Clariidae) in India. The adult of L. longicollis is described in detail. It differs from all other species of the genus in having a receptaculum seminis.

The life cycle of Echinostoma ivaniosi n.sp., found in the rectum of domestic fowl (Gallus gallus domesticus) has been worked out in detail. The eggs hatch in 7–8 days in tap water at room temperature (24-28°C). Body of the miracidium covered with 18 epidermal plates arranged in four tiers of 6, 6, 4, 2 from the anterior to the posterior end. Miracidia develop into sporocysts in the snails, Lymnaea luteola f typica. There are two redial generations and the first cercariae are liberated 25–30 days post-infection with miracidia, or a little later. 37 collar spines of the cercaria are arranged in the pattern-5(3+2)+6+15(8+7)+6+5(3+2). Caudal finfold present, cuticle spinose, paraoesophageal gland cells present, penetration glands two pairs and excretory system with 18 pairs of flame cells in triplets. Metacercaria develops into adult in domestic fowl and the eggs are recovered from the faeces of the host, 12–14 days post-infection. The adult is compared with other 37-spined echinostomes whose life cycles are known or which show characteristic differences from the present species and the creation of a new species is justified.

In a previous hybridisation study it was found that six African schistosome species could be distinguished by their egg morphology and snail infectivity, and that their hybrids were also distinct. The present paper describes complementary studies on the behaviour of the same parasites in rodents.

Statistically significant differences were found between the behaviour of each pair of sibling species with regard to some of the following characters: percentage of worm recovery; rate of egg production; percentage distribution of eggs in the liver, small and large intestines. This is further evidence that these forms are distinct species.

There were also significant differences between the hybrids and their “parental” species, and between the behaviour of the parasites in mice and hamsters. The distinct behaviour of the hybrids raises the possibility of genetically modifying these parasites in order to attenuate their pathogenicity with a view to using hybrids as heterologous immunising agents.

The behaviour of six species of African schistosomes and their hybrids has been compared qualitatively and quantitatively in baboons, with a view to finding a schistosome of high infectivity but low pathogenicity to the baboon for use in heterologous immunity experiments. This study, which includes the first comparative study of the pathogenicity of these six species in primates, is complementary to earlier studies in rodents in which these schistosomes and their hybrids were compared in terms of egg morphology, snail infectivity, and infectivity and pathogenicity. In rodents, statistically significant differences were detected in the behaviour of each pair of sibling species with regard to percentage worm recovery, rate of egg production and percentage distribution of the eggs in the tissues, and the hybrids also differed significantly from their “parental” species.

Even greater differences were found in baboons. For example, although all the strains studied produced mature egg-laying adults in rodents, no worms, eggs, or histopathological changes resulted from exposure of baboons to S. rodhaini or S. bovis. With regard to prepatent period, S. mansoni was the shortest (5–6 weeks), S. haematobium the longest (12–14 weeks), while S. mattheei and S. intercalatum occupied intermediate positions (7–9 weeks). The hybrids all matured in 7–9 weeks except the S. bovis/S. mattheei hybrid (11–16 weeks). S. haematobium similarly had a much longer maturation time in rodents. Complete or partial “self-cure” occurred in 3 baboons but was not observed in rodents. The percentage worm recovery of S. mansoni was higher than for the S. mansoni/S. rodhaini hybrids while with the S. haematobium group of species, worm recovery was highest with S. mattheei followed by S. intercalatum and S. haematobium, the same order as in rodents. With the S. haematobium-group hybrids, worm recovery was generally high except with the S. bovis/S. haematobiuun hybrid.