ii] RHYTHMICAL CHANGE IN THE SEA 49 



invaded by the protozoan Noctiluca, and about this 

 time the water becomes brilliantly phosphorescent 

 because of the presence of the organism. It lasts 

 well on into the winter, when it disappears. During 

 the autumn, diatoms belonging to various species may 

 again become abundant, though this is not an in- 

 variable occurrence, and sometimes during the same 

 period peridinians belonging to the genus Ceratium 

 attain a maximum of abundance. 



This series of changes in the planktonic life of 

 the sea is of course characteristic of this particular 

 locality alone and the sequence may be modified 

 in other parts of the northern seas. But almost 

 everywhere the general scheme will be: (l)a relative 

 scarcity of all kinds of life in the first two months 

 of the year ; (2) the appearance of various species 

 of diatoms which attain a maximum of temporary 

 abundance, and then become much less in number ; 

 (3) a short scarcity of planktonic life during the 

 early summer months ; and (4) a relative abundance 

 of various species of protozoa and micro-crustacea 

 during the autumn. As the winter approaches all 

 forms of life again become scarce in the sea. 



Now associating these various changes with the 

 changes in the physical conditions of the sea we 

 notice that they are always to be regarded as de- 

 pendent on certain physical rhythms : the inflowing 



