In some notes on the Trematodes of Northumbria published in 1905 a few remarks were made on a larval Trematode inhabiting the liver of the common periwinkle Littorina littorea. The liver in two per cent. of the periwinkles from Budle Bay was full of rediæ containing cercariæ more or less developed, the latter agreeing in every way with an encysted Echinostomum larva which inhabits mussels, cockles and other bivalve mollusks in the same locality. So close was the resemblance that I had no hesitation in declaring them to be the same worm in different stages, but hoped for an opportunity of demonstrating this by experiment. In October 1908 through the courtesy of Professor Meek I had the opportunity of conducting some feeding experiments in the Dove Marine Laboratory, Cullercoats, which have given satisfactory results, and although it is not possible to state absolutely that the forms are identical yet the evidence is so strong that I think I am justified in regarding the young worm in the periwinkle as an earlier larval form of the encysted worm in the foot of the mussel and cockle.