Abstract

The overt oviposition rhythm ofLymnaea stagnalis should be regarded as an endogenous one. Despite considerable deviations of its period from 24 hrs, entrainment by 24 hr. external periodicities is possible. Possibly, entrainment is effectuated via a second — covert — internal rhythm. The fact that similar rhythms have not been established before, is probably connected with the nature of the phenomena to which previous studies have generally been limited