Of the three species of fish which live in Afon Terrig (Salmo trutta, Cottus gobio and Nemacheilus barbatula), only S. trutta was infected by Cyathocephalus truncatus. The parasite has a clear annual cycle and hence a periodicity of occurrence in the brown trout.

No age resistance of fish to infection by C. truncatus is found. The increase of incidence with age among l+—3+ year-old fish and decrease in the older age groups are correlated with the variation in the food and feeding habits of trout with age.

An investigation of the factors necessary in the development of eggs of the mouse pinworm, A. tetraptera, outside the host, have shown that both oxygen and a suitable temperature (20–30°C.) are necessary. Development at high temperatures (37°C.) leads to a reduction in the number of infective eggs. It has also been demonstrated that there exists in the development of the infective eggs, a period of incubation during which the infective larvae within the eggs, though morphologically well-developed, are incapable of hatching.

A re-examination of the life-cycle of the mouse pinworm, Aspiculuris tetraptera has shown that the infective eggs hatch in the lower intestine or in the caecum of the host, to release the first stage larvae. After moulting, these larvae migrate into the crypts of the colon; those larvae which do not migrate into the crypts apparently pass out of the host with the faeces. The larvae in the crypts, presumably the third stage, later return to the lumen of the colon where they mature into adults. Males are mature by about the 20th day and females by the 23rd day. Viable eggs are released by the females 24 days after infection.

The details of the post-embryonic development of Echinorhynchus truttae in the intermediate host, Gammarus pulex, and the final host, Salmo truttae, are described.

The cystacanths of this species are sexually mature and are formed about 82 days after infection at room temperature, ca. 17°C.

Copulation takes place in the definitive host soon after the worms enter the pyloric region of the intestine. About 10 weeks after infection, mature eggs are passed out with the faeces of fish.

In the limited period available Mermis was found to be influenced by light, in activity, orientation and oviposition. The peak would appear to be in green light, with a peak at 540µ, corresponding closely to the β absorption band of Mermis oxyhaemoglobin.

Parasites in the alimentary canal of 123 birds from the Wildfowl Trust collection at Slimbridge, Gloucestershire, are listed. About ten thousand helminths belonging to 26 species were found. The nematodes Hystrichis tricolor and Porrocaecum crassum and the cestode Nematoparataenia southwelli are reported from Great Britain for the first time, and a large number of new host records are given.

A topographical distribution of Ostertagia spp. lesions in the abomasum of sheep has been observed under certain field conditions which is the reverse of that described by Sommerville (1953, 1954) and also seen by this author to be characteristic of a primary Ostertagia spp. infection.

This “reverse distribution” has been demonstrated experimentally under conditions in which a heavy primary Ostertagia infection has been acquired (in the field or artificially) and naturally eliminated; the “reverse distribution” is then characteristically acquired by a challenge infection.

Twenty species of helminth parasites were found in the alimentary canal of thirty mallard caught in the decoy at Slimbridge.

The planorbid snail Helicorbis coenosus (Benson) has been found experimentally to serve as the intermediate host of Gastrodiscoides hominis.

The cercaria of G. hominis is described. It is characterized by the presence of a pair of well-developed oral pouches, an alimentary tract having an oesophagus with a muscular bulb and two simple intestinal caeca, an excretory system with a cross-connection between the two main trunks and a reproductive system represented by rudimentary cell groups arranged in a manner similar to that found in the adult fluke.

The worm burden in grazing ewes was studied. Analysis of the results showed that correcting worm egg counts (e.p.g. fresh faeces) for variations in faecal D.M. content and in total faeces output is not of any great benefit unless precise information is available.

Data are also presented which suggest that under normal circumstances variations in the sub-clinical worm burden of adult grazing ewes are not related to any variations in food intake.

A new species of filarial worm, Paronchocerca mansoni, a cardiac parasite of Pica pica sericea (Gould) and Cyanopica cyana interposita Hartert, is reported from Peking, China. The thread-like adults of both species taper towards anterior and posterior ends. The cuticle exhibits superficial rugae, with multiple transverse striations underneath. The lipless mouth is surrounded by 4 paired submedian papillae and one lateral pair of amphids. The posterior end of the body is curved ventrad; the post-anal region is short. The male measures 17 mm. long and 0·14 mm. in diameter; the female, 35·8 mm. long by 0·23 mm. in diameter. In both sexes the anterior muscular oesophagus is short, the posterior glandular portion much longer. In the male, there are two pairs of pre-anal and one pair of post-anal papillae, likewise one pair of subterminal caudal papillae. There are neither caudal alae nor gubernaculum. The longer left spicule and the shorter right one have distinct species-characteristic morphological patterns. In the female, the vulva opens approximately 2·0 mm. from the anterior end. The microfilariae are sheathless; immature ones preponderate in heart blood; mature ones, present in heart blood and exclusively in circulating blood, measure 147 to 184 by 6·0 to 6·5 microns.

It has been shown that mice became infected after suckling from a female which has migrating Trichinella spiralis larvae within her body. The infections are usually of a very low order indicating that adult worms are not being picked up from the mother's faeces but that the infection is probably through the milk. It is also concluded that pre-natal infection of mice with T. spiralis occurs only rarely, if at all.

Groups of helminth-free weaner sheep were equilibrated on a ration containing almost no cobalt for 3 months. Cobalt supplementation was given orally thrice weekly to provide the minimum requirement for sheep. Infective larvae of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia spp. were given at two dosage levels to each of three groups, respectively, for 18 days, followed after a lapse of 30 days by small daily dosings of a mixed suspension of these helminths. Serum vitamin B12 was determined for a month before parasitization and for up to 12 weeks thereafter for every sheep. Sheep parasitized with higher numbers of T. colubriformis and Ostertagia spp. were significantly affected with loss of weight and in three cases, death, although there were no changes in the serum levels of vitamin B12 which remained fairly constantly at the lower end of the normal range during most of the parasitic stage. Sheep dying from these infestations showed terminal rises of serum vitamin B12 whilst those surviving showed falls in the last few weeks of the trial associated with significant decreases in liver cobalt levels. Morphological changes of some severity were found in the abomasum and intestines of the sheep receiving T. colubriformis and Ostertagia spp. and appeared to limit the uptake of vitamin B12. It is concluded that mild to moderate infestations of intestinal trichostrongyles and to a lesser degree Ostertagia spp. will interfere with the absorption of vitamin B12 and that these helminths may precipitate cobalt deficiency syndromes if cobalt is at or near the critical level in the feed.