which must locate and penetrate the body wall of the snail host. After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst. The mother sporocyst undergoes a round of internal asexual reproduction, giving rise to either rediae (sing. redia) or daughter sporocysts. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl. cercariae). Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish). The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host. Two-host life cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost). In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the metacercaria, which is usually encysted in a cyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host's gut in response to a variety of physical and chemical signals, such as gut pH levels, digestive enzymes, temperature, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats. Evolution The evolutionary origins of the Digenea have been debated for some time, but there appears general agreement that the proto-digenean was a parasite of a mollusc, possibly of the mantle cavity. Evidence for this comes from the ubiquity of molluscs as first intermediate hosts for digeneans, and the fact that most aspidogastreans (the sister group to the Digenea) also have mollusc associations. It is thought that the early trematodes (the collective name for digeneans and aspidogastreans) likely evolved from rhabdocoel turbellarians that colonised the open