Circatidal (12.4 h) rhythms were first observed in the intertidal acoel worm Symsagittifera (formerly Convoluta) roscoffensis . These worms appear bright green because they contain a photosynthesising alga. Therefore, at low tide, the worms migrate to the sand surface to be exposed to sunlight. As the tide advances up the beach they disappear into the sand once more so as not to wash away. This behaviour is anticipatory and driven by a circatidal biological clock. In 1903 two scientists, Gamble and Keeble collected the animals from the shore and put them into laboratory tanks. They noted: ‘After a spell of insolation, colonies sink below the surface [of the sand], and after a certain sojourn in darkness they return to the surface. These movements synchronise with the covering and uncovering of the Convoluta zone by the tides’. The worms’ innate ability to anticipate the tidal cycle enabled predictive downward migrations into the sand even when removed from the shore.