A biological test to estimate the numbers of infective metacercariae of F. hepatica on pastures is described. The test is shown to be successful provided a snail site can be determined. Results from samples collected from field outbreaks are described and the possible applications of the test are discussed.

1. Biomphalaria p. nairobiensis, B. p. ruppelli and B. s. tanganyicensis have been found to be moderately susceptible to S. rodhaini from Kenya. A. glabratus from Puerto Rico is partially susceptible to the parasite. However, B. s. sudanica, B. dlexandrina, A. glabratus (Puerto Rico, albino strain) are refractory to infection with the same parasite from Kenya.

2. The present results together with those of some previous investigators suggest the existence of geographical strains of S. rodhaini.

The material comprises three complete male worms in good condition and some male fragments; there are no females.

The worms are small, cylindrical and greyish yellow in colour. The body is partially covered with spines.

The material comprises two immature females found in a baboon in Rhodesia. So far as I can gather from the available literature, they appear to be the first record of the genus Gigantorhynchus Hamann, 1892, emend. Yamaguti, 1963 in monkeys; moreover, they are a new locality record of this genus in Africa, since all the hitherto known species occur only in South America (Brazil, Venezuela and Peru).

The number of testes characteristic for males of Schistosoma mansoni has been described by various authors. Manson-Bahr & Fairley (1920) studying material from Egypt, reported that the males had 8 testes. Price (1929), Faust (1930), Neveu-Lemaire (1936) and Berghe (1939) reported that the number of testes varied from 8–9. Da Silva (1908), Lutz (1919) and Pinto & Almeida (1948) working on Brazilian material, gave the number of testes as 7, about 8 and C-9 respectively. Leiper (1918) studying material from Egypt reported that the number of testes varied from 7–9.

The ultrastructure of the metasomal body wall of the acanthocephalan Pomphorhynchus laevis has been studied.

The cuticle was seen to be pierced by numerous pores leading into tubular channels of the striped layer which appeared to have connections with the endoplasmic reticulum deeper in the body wall. The inner edge of the body wall was perforated by a number of channels which, it was thought, might form the final link in a chain of ramifying channels leading from the exterior of the parasite to the pseudocoel. The syncytial cytoplasm of the body wall was thought to be supported by the network of strands forming the felt layer.

The adult of Clinostomum tilapiae n.sp. obtained from the buccal cavity and oesophagus of Bubulcus ibis 7½ days after experimental infection with metacercariae is described.

The metacercaria of C. tilapiae n.sp. from Tilapia zilli, T. heudeloti and T. galilaea is described.

Anatomical differences between metacercariae and adults of the species are listed.

Some years ago I suggested that a medical malacologist must not only learn to think like a population of snails but that he must also try to see those snails through the eye-spots of populations of trematode miracidia. For the past ten years I have been attempting to perform this unlikely combined feat. As time passes my tendency has been to concentrate on the second part of the contortion, partly because as a helminthologist it is natural to take a worm's-eye view of life and partly because it is very easy to become absorbed in the biology of the snails for their own sake. We should remember that if molluscs were not essential hosts to some parasites of man and domestic animals there would be no justification for a symposium on medical malacology.

The experimental infection of young cattle egrets with metacercarial cysts of Clinostomum tilapiae Ukoli, 1966 from Tilapia spp. is described.

A table showing the duration of infection, the number of parasites recovered, the percentage of infection and the location of the parasites within the host is constructed and the results discussed.

A graph of mean length and breadth of body, oral and ventral suckers, cirrus pouch, ovary and anterior testis, is plotted against the duration of infection in days. Most of these dimensions are found to attain a peak two days after infection. It is also found that while the ovary increases in size with age, the cirrus pouch remains constant, so that in mature adults, both are about equal in size,

It is shown, by plotting length of organs against length of body and by expressing mean measurements of certain dimensions as percentages of body length, that apart from the testes, all the other organs in both the metacercariae and the adults maintain a constant relationship both with one another and with body length as they grow older.

The metacercariae of Euclinostomum helerostomum (Rud., 1809) from Tilapia zilli and T. heudeloti and immature adults recovered from the oesophagus of the African darter and the long-tailed shag, three days eighteen hours after experimental infection with metacercaria are described.

It was found that the number of diverticula from each intestinal caecum varies within species, hence Jaiswal's (1957) three species, E. bhagavantami, E. channai and E. heptacecum are proposed as synonyms of E. heterostomum.

The process of attack and penetration of the snail hosts by miracidia is an important stage in the life history of most digenetic trematodes. This paper is concerned with the mechanism of penetration, and further development within the snail, of Schistosoma mansoni as revealed in serial sections.

p. 108—line 12 from top for “obnormal” read “abnormal”