Penetration of the liver-fluke, Fasciola hepatica into the snail, Limnaea truncatula.

Abstract

THE life-history of Fasciola hepatica has been recounted in nearly every text-book of zoology or of parasitology since it was elucidated by R. Leuckart (1881–82) and A. P. Thomas (1881–83). In spite of this and much original work by other investigators our knowledge of (a) the form which penetrates the snail host and (b) the manner of its penetration is misconceived. For example, in the modern account given by G. Lapage1 it is stated that once a snail has been found, the miracidium “applies the papilla at its broadest, anterior end to the soft skin of the snail and, spinning by means of its cilia on its long axis, it drives the papilla into the snail and penetrates the snail's body”. Other writers have introduced something between this sort of statement and the more correct idea, succinctly expressed by Faust2, that penetration “is accomplished by the secretion of digestive enzymes elaborated in the so-called ‘penetration glands’ which discharge the secretion at the anterior end of the miracidium”. It is difficult to prove that ‘digestive secretions’ are produced, or even to demonstrate the cytological effects produced by a penetrating larva which is smaller than some ciliated protozoa. It is here shown, for the first time by means of photomicrographs, that the miracidium creates a perforation in the snail's integument by the loosening, cytolysis and abstraction of epithelial cells, an action which appears to be chemical rather than mechanical and is probably the result of enzyme activity. It will be shown also that, because the miracidium loses its ciliated epithelium and is in other ways transformed before penetration is effected, it is an early sporocyst and not a miracidium which enters the snail.