18 PLANT-ANIMALS [CH. 



reach of the shock of the waves. At every falling 

 tide, as the receding waters lay bare the zone, C. ros- 

 coffensis rises to the surface of the sand and becomes 

 a land plant-animal, or rather, a sedentary denizen 

 of the filmy rivers which have their sources in 

 the sand flooded by water when the tide is full. 

 Where the springs of drainage-water reach the sur- 

 face and become rivulets cutting seaward courses, is 

 the upper limit of the C. roscoffensis zone. Thus the 

 colonies are so situated on the beach that they are 

 bathed continuously in running water and receive the 

 maximum of light-exposure during low water at all 

 tidal periods. Records kept during a lunar month 

 show that the time of exposure during low tides is 

 very fairly constant. The time during which C. 

 roscoffensis lies on the surface is, on the average, five 

 and a half hours, and ranges from four and a half to 

 six hours. Twice during twenty-four hours the ros- 

 coffensis zone is submerged and the animals live a life 

 of darkness underground : twice the zone is uncovered 

 and the animals are free to rise to the surface of the 

 sand (Fig. 7). By fixing its station and adjusting its 

 habits, C. roscoffensis succeeds to a remarkable degree 

 in simplifying its environmental conditions. In that 

 station, periods of inundation succeed periods of ex- 

 posure at fairly regular intervals, and, by synchronising 

 its rhythmic movements up to the surface and down 

 below the surface with the movements of the tides, 



