MARINE BIOLOGICAL STATION AT PORT ERIN. 65 



(1). The sequence and periodicity of stages in the life- 

 history of the organisms ; 



(2). Irregularities due to the inter- action of organisms, 

 as when one group serves as the food of another ; 



(3). Periodic changes and abnormalities of either time or 

 abundance caused by the nature of the sea-water 

 or by weather conditions which may either deter- 

 mine or prevent the normal 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 develop- 

 ment 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 hydro - 

 graphic conditions, are all examples that may be classed in 

 the third category. 



:< Two or all of these factors may, however, be at 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." 



It would be very difficult to summarise all the plankton 

 results obtained during the past thirteen years — when some 



