I] 



THE HABITAT OF CONVOLUTA 



19 



C. roscoffensis adjusts its working days to the rhyth- 

 mic changes of its environment. How remarkable is 

 the rhythmic movement up and down we shall pre- 

 sently discover. 



.501 S N S 



I I 



Fig. 7. Habitats of C. roscoffensis and G. paradoxa shown in relation 

 with the rise and fall of the tides during a lunar period. 

 S = spring tides. N=neap tides. Kosc. zone = habitat of 

 C. roscoffensis. Farad, zone = habitat of C. paradoxa. The 

 position of colonies of C. roscoffensis is just below the high-water 

 level of the slackest tides. The habitat of C. paradoxa is un- 

 covered at low water except during the slacker neap tides. 



Leaving the C. roscoftensis zone and passing the 

 rank, brown sea-weeds left high and dry by the tides, 

 the observer paddles into the shallow water, or, if the 

 tide is a big one, walks almost dry shod and sees the 

 long, yellow bands of another sea- weed (Ascophyllum) 

 swaying beneath the water of the pools or lying 



22 



