Himasthla quissetensis and Lepocreadium setiferoides: emergence patterns from their molluscan host, Nassarius obsoletus.

The timing of cercarial release from the intermediate host Nassarius obsoletus, was determined for two species of larval trematodes, Himasthla quissetensis and Lepocreadium setiferoides. In a light-dark schedule the cercariae of H. quissetensis emerged in the first few hours of darkness, while emergence of L. setiferoides was predominantly diurnal. Evidence from both species suggests endogenous control of release under constant conditions. Larvae of L. setiferoides, which possess pigmented eyespots, were photonegative in a light gradient; H. quissetensis larvae, which lack eyespots, showed no phototactic response.