Worms, Platyhelminthes a h b f c g d e Figure 72 Generalized digenean life cycle (a) Adult in definitive host, undergoes sexual reproduction and produces eggs; (b) eggs released with feces of host; (c) miracidium hatches from egg and penetrates first intermediate host (usually a mollusc); (d) miracidium develops into sporocyst containing numerous redia; (e) redia leave sporocyst and produce numerous cercariae; (f) cercaria escapes from first host and finds second intermediate host; (g) cercaria sheds tail, encysts, and develops into metacercaria; (h) second intermediate host containing metacercaria is eaten by definitive host a f g b e c d Figure 73 Generalized life cycle of aquatic cestode (a) Adult in definitive host, undergoes sexual reproduction, and produces eggs; (b) eggs released with faeces of host; (c) coracidium larva hatches from egg and penetrates first intermediate host (usually a copepod); (d) coracidium develops into procercoid; (e) first host eaten by second intermediate host; (f) procercoid develops into plerocercoid; (g) second host eaten by definitive host (usually a piscivorous mammal) 463 range of habitats invaded and life history strategies employed Most non-neodermatans undergo direct development such that the zygote develops directly into a form resembling a young adult As discussed by Ruppert (1978), however, there are several notable exceptions to this pattern One species of catenulid develops from a zygote into a pelagic larval stage known as a Luther’s larva (Figure 74, Plate 5); the larva is vermiform in shape, ciliated throughout, but in addition possesses several anterior rings of elongate cilia that aid in swimming The zygotes of some polyclads develop into freeswimming, ciliated larval stages bearing arms These larvae are generally referred to as Goăttes larvae (Figure 75, Plate 5) or Muăllers larvae (Figure 76, Plate 5) depending on whether they possess four or seven to ten arms, respectively This variation in developmental pattern is seen among individuals of some polyclad species Goăttes larvae are found only in members of the polyclad suborder Acotylea; Muăllers larvae are known from members of both suborders of polyclads There does not seem to be a phylogenetic pattern to the distribution of these larval stages among the polyclads For example, the polyclad genus Stylochus includes species that develop into Goăttes larvae, species that develop into Muăllers larvae, and species that undergo direct development In general, the monogenean life cycle consists of two stages, the adult and a free-swimming larva, which is usually ciliated, known as the oncomiracidium This larva usually possesses one or two pairs of ocelli, and is conspicuous in its possession of a posterior attachment structure known as a haptor, which is usually armed with hooklets (uncinuli) and sometimes also with hamuli (Figure 77, Plate 5) The oncomiracidium differs from the ciliated larvae of other neodermatans in that its cilia are often restricted to three distinct regions When the oncomiracidium hatches from an egg, which is sometimes anchored to the substrate, it swims or creeps to find its vertebrate host An oncomiracidial stage is mostly lacking from monogeneans such as the gyrodactylideans, which are unusually in that they are generally viviparous and undergo sequential polyembryony A generalized oviparous monogenean life cycle is shown in Figure 71 Prior to developing into the adult form, aspidogastreans pass through a larval stage called a cotylocidium (Figure 78, Plate 5), which is often but not always ciliated This larval stage is free-swimming in at least some species However, the life cycles of many species are not yet fully understood In some cases it appears that the cotylocidium hatches from the egg and parasitizes the same host as the parent individual from which it was released In other cases it appears that the cotylocidium hatches from the egg and moves into the water column to find a new host In yet other cases the cotylocidium never hatches from the egg; rather, the egg is ingested by the molluscan host In some species the life cycle is direct, involving only a single host In other species, the life cycle may involve a second host which may be infected by ingesting infected molluscs Digeneans undergo complex development in which certain life cycle stages are able to produce multiple copies of new generations before becoming sexually reproductive adults Consequently the term ‘‘larva’’ is used loosely when discussing life cycle stages in the Digenea The digeneans pass through a series of larval stages that are either parasitic or free-living,