THE BIONOMICS OF CuNVOLUTA ROSCOFFKNSIS. 413 



as to take the animal out of the reach of the disturbing 

 influence. For example, when the tide comes in roughly, 

 the patches disappear before the surface water has reached 

 them. When the tide flows with the gentle quietness of a 

 still summer's day, it may surprise Convoluta at the 

 surface of the sand, where they may remain till a ripple gives 

 them the signal to disappear. As the waters withdraw, 

 Convoluta begins to rise, and the patches may be re- 

 defined when the water's edge is but a few yards away. 



4-30 a.m. 



J p.m. 11-20 p.m. 5-20 a.m. 



Text-fig, 4. — The two sketches show the daily variation in the 

 amount of Convoluta exposed or correlated below the surface 

 of the sand according as to whether the high water is at about 

 noon and niidnigiit (top figure), or is at about 5 a.m. and 5 p.m. 

 The tidal curve is coloured red, and the siiadiiig indicates the 

 amount of Convoluta above or below the sand, and its rise and 

 fall in relation to the rise and fall of tide. 



The reaction, in short, is automatic; and the Convoluta are 

 just, and only just, beyond the region of agitation. 



Diurnal Variation (Text-fig. 4). — The colonies of Con- 

 voluta appear at low tide during the day; at night, on 

 the other hand, they do not rise to the surface. During 

 moonlight nights a small patch may here and there be seen. 



If a patch be observed at intervals throughout the day, 

 it is found to undergo a sequence of changes in size. On the 

 following day the variation is found to be repeated. As the 



