MARINE BIOLOGICAL STATION AT PORT ERIN. 67 
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 at work 
together, and so the explanation of any particular change 
Fig. 18. Protozoa and Diatoms (highly magnified). 
1-3, Dinoflagellates; 5, Noctiluca ; 8-11, Diatoms. 
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. 
The results of the hauls obtained on April 9th and 
10th in Port Erin Bay are good examples of a local 
