38 LIFE IN THE SEA [OH. 



relationships of the two series of events are so complex 

 that the organic changes are far less susceptible of 

 prediction than the physical ones. Nevertheless we 

 can easily show that very many changes in the nature 

 and abundance of life in the sea, and in the habits of 

 animals recur again and again with great regularity. 

 Let us take the case of the influence of the tides. 

 Nothing impresses one so much as the way in which 

 a fisherman regulates his practice according to the 

 tidal rhythms, so much so that he will associate the 

 latter with all sorts of variable occurrences with which 

 they can have nothing to do. The dependence, of 

 littoral animals on the ebb and flow of the tides is 

 absolute, for it is only when the foreshore is covered 

 with water that they can feed or move about. Many 

 animals such as the. cockle and mussel close their 

 shells when the tide ebbs off from the foreshore, and 

 others, such as the lug worm, burrow into the sand. 

 Convoluta, which is a little flat-worm living on the 

 sands in some parts of the coasts of France, also 

 burrows beneath the surface when the water leaves 

 it. These habits are easily explained, for a shell-fish, 

 such as a mussel, would become dried up if it were 

 to keep its shell open during the whole period of 

 ebb tide on a hot summer day, and the worms would 

 also become parched up by the heat of the sun. So 

 also with the spawning of Convoluta, which takes 

 place only once a fortnight at a certain phase of the 



