PLANKTON 267 



the abundant species of Chcetoceras (Plate XIX, Fig. 2), 

 jointed filaments with groups of delicate curved hairs and 

 spines projecting at their sides, and although species differ 

 somewhat in their times of appearance, the genus as a whole 

 is characteristic of late April and early May. After Chce- 

 toceras comes the equally large and important genus Rhizo- 

 solenia (Plate XIX, Fig. 1), long, slender, needle-Hke forms 

 of a dark brown colour when present in mass. In the Irish 

 Sea we have three most abundant species which follow in 

 this order — Rhizosolenia semispina in late May, R. shrubsolii 

 in June, and R. stolterfothi in late Jime and early July. When 

 any one of these kinds of Diatoms is present in abundance, 

 it may discolour the sea, and give a characteristic appearance 

 to a plankton gathering in a glass vessel. Coscinodiscus 

 and Biddulphia give a yellowish brown tint and a granular 

 appearance. Chcetoceras colours the water pale green, and 

 when the numerous filaments sink to the bottom they 

 adhere together in fluffy masses like cotton-wool. Rhizo- 

 solenia in mass has a dark greenish brown colour and a very 

 characteristic silky appearance. 



Then, again, some species of Chcetoceras and Rhizosolenia 

 help to constitute the second (autumnal) maximum in 

 September and October, and Biddulphia sinensis makes its 

 appearance in quantity in November. 



There are many other genera and species of Diatoms which 

 appear in the plankton during the year, all, no doubt, with 

 their special characters and requirements. I have only 

 taken, as examples, the few that are most abundant in the 

 Irish Sea, and are probably the most important as food for 

 animals in the plankton. 



There are thus many problems of the plankton connected 

 with the determination of the causes of all these seasonal 

 variations I have referred to — ^first the sudden awakening of 

 microscopic plant-life in early spring, when the water is 

 still at its coldest, and when in the course of a few days the 

 upper layers of the sea may become so filled with Diatoms 



