The influence of the density of snails on the incidence of larval trematodes

Samples of the snailVelacumantus australisfrom an area of 10 yards in diameter, were examined to determine if the incidence of larval trematodes varied with the density of the snails. The incidence in susceptible snails from areas where the density was less than 20 snails/sq. ft. was found to be significantly greater than in snails from areas where the density was higher than this. Further analysis showed that the variation in incidence, with density, was due almost entirely to the juvenile part of the population.