122 STUDIES IN INSECT LIFE, ETC. 



every twenty-four hours, and that is involved 

 in a still bigger fortnightly rhythm corresponding 

 with the full and the new moon ; for about half- 

 way between these two phases the tide rises more 

 slowly and to a lower height ; and again, just 

 as there is a half-daily and a half -monthly rhythm, 

 so we have a half-yearly rhythm in the vernal 

 and autumnal equinoxes. So regular are these 

 rhythms that the tide is calculated years in 

 advance, for all parts of the world, and navi- 

 gators rely trustfully on these calculations, which 

 are not found wanting. 



This rhythmical change has impressed itself 

 upon many marine organisms. As Mr. Johnstone 

 reminds us, to keep cockles healthy in aquaria 

 under artificial conditions one must run the water 

 off the tank at least once a day so as to simulate a 

 low tide. Convoluta a small and lowly worm 

 which lives on the sand and burrows beneath it 

 when the tide is ebbing off the beach, kept in a 

 laboratory in vessels of sea-water, periodically 

 burrows under the artificial sand at the bottom of 

 the vessel when the real tide is normally going 

 out. The phosphorescence of the surface organ- 

 isms which we have noticed above only comes 

 into play at best some time after sunset. If 



