Electron-microscopical, histological and histochemical observations were carried out on the body wall of the daughter sporocyst stage of Schistosoma mattheei and S. bovis. Electron microscopy revealed that the body wall consists of a continuous outer layer of cytoplasm, collagen fibres, fibroblasts and a layer of somatic cells which are apparently continuous with the cytoplasmic layer. This layer forms numerous microvilli at the surface.

Pronounced alkaline phosphatase activity was found in the sporocyst body wall, but no evidence of esterases was found in the parasite although esterase activity was readily demonstrated in the snail tissue that the schistosome parasitized.

It is concluded that the passage of substances such as glucose across the surface of the sporocyst is an active process probably mediated by enzymes and that little, if any, lipid metabolism occurs in the sporocyst stage.

1. O. gutturosa infections are of widespread occurrence in South-East England. 58% of 1583 cervical ligaments from milk cattle slaughtered in an abattoir at Reading, Berkshire, showed the presence of adult worms. A more detailed study of 50 cattle from a small abattoir near St. Albans in Hertfordshire showed 42 (84%) infected.

2. The adult worms were found either in the connective tissue adjoining the nuchal ligament or between the spleen and rumen. There was no evidence of an increase in the parasite density with age of the cattle suggesting a strong immunity to superinfection.

3. Irrespective of the situation of the adult worms the microfilariae of O. gutturosa appear to have a directional mechanism and were always concentrated in the skin around the umbilicus. This is an adaptation to transmission by Simulium ornatum which bites preferentially in the umbilical region.

4. There were no marked pathological changes associated with the parasites and no evidence of clinical disease attributable to infection with O. gutturosa.

1. Observations have been made on the behaviour of S. ornatum in relation to its role as a vector of O. gutturosa, a common parasite of cattle in Britain.

2. Most of the S. ornatum biting cattle alight on the ventral surface of the animal and it has been shown that this behaviour is affected by the degree of illumination.

3. The umbilicus is the preferred site for a blood meal by S. ornatum and it is this region where the microfilarial density is the highest.

4. The biting behaviour of S. ornatum is affected by the arrangement of the hair in relation to the skin surface. The umbilicus is preferred because here the hairs are so arranged that the flies readily gain access to the skin. The teats are bitten usually when fly activity is very heavy and then the umbilicus is covered with feeding S. ornatum.

5. The mean number of O. gutturosa microfilariae ingested by S. ornatum, but not the quantity of blood, increases with increased time spent feeding on an infected animal.

6. There is no causal relationship between the loss of blood per anum during and after feeding and the number of microfilariae ingested by S. ornatum.

7. Few microfilariae of O. gutturosa are lost per anum by S. ornatum during feeding despite the loss of blood by this route from the majority of flies.

8. The infective larvae of O. gutturosa are deposited by S. ornatum in the skin of the umbilicus of the cow from where they migrate to the neck or gastro-splenic region. This suggests that the worms possess an inherent directional mechanism determining their final site of development and that the point of inoculation of the infective larvae may have less importance in determining the sites of final maturation of Onchocerca species than has hitherto been believed.

A comparison of the characteristics of two strains of S. haematobium in experimental infections in hamsters is made. A limited study of these strains is also made in Mastomys.

Growth changes in male and female adult worms are compared, as are changes in the uterine egg counts.

The quantitative distribution of eggs in the tissues of the hamster is recorded. The approximate mean daily egg output per female worm for each strain is calculated at specified time intervals, and for the entire period of observations.

The recorded characteristics of the two strains of parasite are compared with those described for other strains of S. haematobium.

It is considered that the hamster provides a useful model for such studies provided that standardised infection procedures are employed.

It is considered that differences in host characteristics and susceptibility of individual animals account for the apparent variability recorded with regard to growth of parasites and egg depositions.

Infective third-stage larvae of Dracunculus medinensis in cyclops have been maintained at 37°C in normal saline containing from 0·05 to 1·0% of hydrochloric acid. At acid levels normally met with in the human stomach 50% mortality of emerging larvae occurred in 150 minutes, suggesting that the time taken for larvae to pass through the stomach may be of importance in determining whether an infection is successful or not.