356 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Phyto-plankton. — If, as is customary, we group the 

 Dinoflagellata (with, on the whole, a holophytic method of 

 nutrition) along with the Diatoms, the phyto-plankton so 

 constituted may be contrasted with the zoo-plankton in 

 several respects. The Dinoflagellata have usually one 

 well-marked peak or maximum in the year, and that lies 

 somewhere between the extremes of April and August. 

 Consequently the Dinoflagellates may be regarded as the 

 characteristically summer phyto-plankton. The Diatoms, 

 on the other hand, show two distinct maxima in the year 

 — in spring and in late autumn, with usually a summer 

 depression between. It thus seems that the conditions 

 prevailing in summer (June, July and August) are 

 unfavourable to a profuse development of Diatoms. 



Winter is also unfavourable, and as a rule the period 

 November to March shows the greatest reduction in 

 phyto-plankton. 



These results agree, on the whole, with those 

 obtained by Lohmann in the Baltic, but his autumnal 

 maximum of phyto-plankton seems to be greater than 

 ours. The Dinoflagellate Ceratmm trijjos, according to 

 Lohmann, begins in June, reaches its maximum in 

 August and remains until October or November. With 

 us this species is usually present all the year round, 

 reaches a maximum in April to June and, with a few 

 depressions, remains fairly high until August. 



The spring Diatom rise began later and ended later 

 this year than in the previous two years, and it is 

 interesting to notice that our hydrographic observations 

 in the Irish Sea show that both the temperatures and 

 salinities were lower than usual this spring. 



Zoo-plankton. — Changes in the volume of oar zoo- 

 plankton are almost entirely due to Copepoda, of which 

 fche most abundant forms in our district are Oithona, 



