28() TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



(3) Periodic changes and abnormalities of either 

 time or abundance caused by the character of the st;i- 

 water or by weather conditions which may either 

 determine or prevent the norma] or permit of an 

 abnormal development of certain species. 

 The appearance of swarms of Balanoid Nauplii, followed 

 after an interval by the " Cypris " stage, is an example 

 that comes under the first head. The disappearance of 

 Diatoms when used as food by the increasing swarms of 

 Copepoda and other Crustacea, both larval and adult, and 

 of the Copepoda in turn when eaten by the developing 

 post-larval fish, are changes falling under the second head. 

 The great increase in the number of Diatoms in spring 

 when the physical condition of the sea-water has become 

 favourable, the enormous development of Dinoflagellates 

 which may take place suddenly in autumn under unusual 

 weather conditions, the almost total suppression of a group 

 such as the Medusae in some localities in an unusually 

 stormy summer, and the immigration of a species or a 

 group of species from the open ocean or from a 

 neighbouring sea-area as the result of variations in the 

 hydrographic conditions, are all examples that may be 

 classed in the third category. 



Two, or all of these factors may, however, be ai 

 work together, and so the explanation of any particular 

 change may be a very complicated problem. The 

 increased development of a group, or the immigration of a 

 species, may so disturb the balance of nature as to be 

 followed by unusual changes in other groups. 



3. Lists compiled from our results and curves drawn 

 from these lists show that, as a consequence of the 

 above factors, certain groups and certain prominent 

 species differ from one another greatly in their relative 

 abundance throughout the months of the year. 



