22 PROCEEDINGS OF THE 



" hydrographic " change in the water will determine the presence 

 of Echinoderm larvae at a time of year when they are not produced, 

 nor of Crab Megalopas when they do not naturally occur. 



Three factors, at least, contribute to the constitution of the 

 plankton from day to day throughout the year : — 



(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 determine or pre- 

 vent 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 development of Dinoflagellates which 

 may take place suddenly in autumn under unusual weather con- 

 ditions, 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 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. 



The results of the hauls obtained on April 9th and 10th in 

 Port Erin Bay are good examples of a local plankton mainly com- 

 ])osed of Diatoms. It is noticed in running the eye down the 

 groups that whereas the Diatoms occur in thousands extending up 

 to even 100,000, the Dinoflagellates are in hundreds, extending, at 

 most, to a thousand ; the Copepoda are in tens, rarely reaching a 

 hundred or two, while the fish-eggs are scattered units, such as 

 1 and 2. The general character of the hauls on April 9th is that 

 there are ten times as many Copepods as fish-eggs ; ten times as 

 many Dinoflagellates as Copepods, and ten times as many 

 Diatoms as Dinoflagellates, per species. On the following day, 

 April 10th, the proportions are somewhat the same ; and if we pick 

 out the largest numbers recorded in each of these groups, these 



