i] THE HABITAT OF CONVOLUTA 15 



On a peaceful beach, in quiet times, when storms 

 and tourists are absent, the colonial patches of C. 

 roscoffensis keep their respective outlines with sur- 

 prising constancy. Day after day the several patches 

 may be recognised, waxing in size with the spring tides, 

 waning with the neap or slack tides (Fig. 1) : larger, 

 also, on any day soon after the tide has receded 

 from their borders; smaller, just before the rising 

 tide invades them. At certain times, the multitude of 

 individuals which make up a patch may be seen lying 

 lethargic and motionless, bathed in the sunlight and 

 the film-like stream of drainage sea- water which oozes 

 from the sand and flows over them seaward. On days 

 of bright sunshine, in particular, the animals lie very 

 still ; on duller days, a constant gliding too and fro of 

 these minute films of living matter is to be observed 

 within the confines of a colony. It is on such occasions 

 that the observer must tread softly, for C. roscoffensis 

 is so sensitive to vibration that his heavy, approach- 

 ing tread may send it to earth with lightning speed. 

 How quickly the animals may make their descent 

 from the surface may be judged from the illustration 

 (Fig. 6) which depicts two photographs of a colony, 

 the second taken at an interval of five minutes after 

 the first. Three gentle taps on the sand, after the first 

 photograph was taken, served as the signal for retreat. 

 At that signal, the army, many millions strong, vanished 

 with amazing swiftness and took cover underground. 

 Lest the words "many millions" should seem to 



