THE BIONOiMIOS OP CONVOLUTA ROSCOFFENSIS. 423 



disappearance of tlie colonies when the sand is tapped or 

 when the tide flows over them. They suggest that light and 

 gravity provide the stimuli whereby these movements are 

 regulated. 



B. Observations of the Authors. — The Convoluta 

 zone is determined by the outflow of drainage tidal water. 

 The upper limit of the zone corresponds to the high-water 

 mark of lowest neap tides. This situation ensures the 

 maximum of light-exposure consistent with security from 

 desiccation. Convoluta is neither a sub-aerial nor a marine 

 animal. It lives in a film of water, and has migrated 

 from the sea shorewards to the highest level consistent with 

 aquatic life. The stations occupied by Convoluta are 

 remarkably constant. The constituent patches of a colony 

 may be recognised day after day for months together. 



Nevertheless diurnal and fortnightly variations in the size 

 of the colonies occur. 



The diurnal variations are tidal ; each patch reaches its 

 maximum size soon after its site is exposed. The size slowly 

 decreases till the tide comes in. When the tide comes 

 within a few feet, the patch disappears suddenly. At night, 

 the colonies do not ascend. 



In addition to its daily variations, Convoluta exhibits a 

 fortnightly lunar variation. The colonies, with their con- 

 stituent patches, increase to a maximun during spring tides, 

 decrease to a minimum during neap tides. 



The daily variations are due to the tonic effect of light; in 

 which tonic effect must be included the "after-effect" of 

 prolonged light-exposure. 



The fortnightly variations are due to periodicity of repro- 

 duction. The majority of animals of a mature colony dis- 

 charge their eggs in egg-capsules at the onset of the spring 

 tides. The capsules are laid beneath the sand. In most 

 cases the body of the animal is ruptured during the process 

 of laying. The hinder half remains in the sand, the head end 

 rises and joins the patch. Thus at neap tide the size of the 

 patches, and hence of the colony, is decreased. 



