Samples of Peringia ulvae were collected and measured on eight occasions, over a period of two years from an isolated pool in the saltings of the River Tamar. Growth conditions in this pool are at a maximum. Spawning occurs in the autumn and the greatest number of small specimens are found in the sieve (1 mm. mesh) in February. Maximum growth takes place during April-July. After ten months of age,.uninfected snails which have reached a length of 5¾ mm. grow only 1–1¼ mm., and gradually die off from 17 months of age onwards. Infected snails display gigantism and variation and attain dimensions of 9–10 mm. They are probably no older than uninfected specimens measuring 6¾ mm. long.

The infection rate is high and shows certain seasonal fluctuations, no doubt linked with the migrations of the wading birds which are the principal final hosts of the trematodes.