SEA-FISHERIES LABORATORY. 285 



special conditions have permitted of a quite exceptional 

 development this year. In the latter case, however, it is 

 curious that, considering all the plankton investigation 

 that has been carried on at Port Erin and oh the 

 Lancashire coast during the last 20 years, the species has 

 hitherto escaped notice. It appears to be present only 

 very rarely at Plymouth and elsewhere in the English 

 Channel (Blue-book, Cd. 3837, p. 228, 236, &c). 



CONCLUSIONS. 



We have expressed our opinions freely, both on 

 general questions and on matters of detail, where they 

 occurred to us in the course of writing the preceding 

 pages, but it may be convenient to have summarised here 

 the main conclusions at which we have arrived. 



1. It is clear that many of the great seasonal 

 variations in the plankton are not due to changes in the 

 sea-water such as are recognised in hydrographic 

 observations, but are caused simply by the normal 

 sequence of stages in the life-histories of organisms 

 throughout the year. No amount of " 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. 



2. Three factors, at least, seem to us to require 

 recognition as contributing to the constitution of the 

 plankton from day to day throughout the year : — 



(1) The sequence and periodicity of the stages in 

 the normal life history of the organisms ; 



(2) Irregularities introduced by the inter-action 

 of the organisms, as when one group serves as the 

 food, or enemy, of another; 



