66 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
It is clear that some 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 Hchinoderm larve at a time of 
year when they are not produced, nor of Crab Megalopas 
when they do not naturally oceur. 
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 prevent the normal or permit 
of an abnormal development of certain species. 
The appearance of swarms of Balanoid Nauplu, 
followed after an interval by the ‘“‘ Cypris” stage, is an 
example that comes under the first head. The disappear- 
ance 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 
‘otal suppression of a group such as the Meduse in some 
