The life history of P. avitus has been experimentally traced. The cercariae occur in the marine snails, Urosalpinx cinereus and Thais lapillus. Adults have been obtained from the cloaca of the common tern, Sterna hirundo, and the roseate tern, S. dougalli, after feeding the young birds with encysted larvae. It has been shown that a specific secondary intermediate host is not essential for the completion of the life history; only a means of transference is necessary. Additional morphological differences between P. avitus and P. acanthus are described.