Gigantism and Variation in Peringia ulvœ Pennant 1777, caused by Infection with Larval Trematodes

In 1933 an examination of Peringia ulvœ for larval trematodes was under-taken at Plymouth (Rothschild, 1936). No actual measurements were made at the time, but there appeared to be a relation between the species of trematode and the size of the individual infected snails. Thus the commonest species, Cercaria oocysta Lebour, 1907 (14% infection), and C. ubiquita Lebour, 1907 (4% infection), were found in 75% of the largest snails, while the rarer species, such as C. ephemera Lebour, 1907 (non Nitsch) (2% infection), and C. pirum Lebour, 1907 (0–1% infection), occurred most frequently in medium sized individual