62 PLANT-ANIMALS [CH. 



the sand or the containing vessels send them down, 

 only to reappear when the tapping* has ceased. But, 

 as we have seen, a colony may disappear before the 

 tide has mounted high enough to disturb it. The 

 eyes of C. roscoffensis mere pigment spots are too 

 rudimentary to allow us to suppose that it sees the 

 water coming in and so takes warning and descends 

 betimes. Indeed a simple experiment suffices to 

 demonstrate, not only that this is not the case, but 

 also that whole colonies of C. roscoffensis may descend 

 beneath the sand in the total absence of an apparent 

 external stimulus. 



Thus, if a batch of animals from a roscoffensis patch 

 is scooped up with sand and water by means of a cup 

 and taken into the laboratory, the shaking to which, of 

 necessity, the specimens are subjected in the process 

 causes their swift descent. By the time the cup is 

 brought indoors, not a trace of green may be visible 

 in it ; but, in the calm of the laboratory, the animals 

 reascend once more and lie as a thick, dark green 

 scum upon the surface of the sand. They remain in 

 this state for hours, then suddenly disappear. 



Wondering at this swift retreat, and as we wonder, 

 staring through the laboratory window at the shore 

 a stone's throw away, we note first vaguely and then 

 with quickening curiosity that the rising tide is just 

 about to flood the roscoffensis zone. Curious, this 

 descent of the green scum of animals in the cup ! 



