PLANKTON 235 



times been obtained in one haul of a small tow-net in May. 

 Chcetoceras (Plate XIX, Fig. 2) is characterized by the long 

 slender cm'ved spines which project in groups from the ends 

 of the cells. 



Rhizosolenia — another large genus, some of the species 

 (Plate XIX, Fig. 1) of which are very abundant in our seas in 

 early summer and late autumn, reaching the maximum 

 usually in June, when up to 180 millions have been taken in 

 one haul of the tow-net. 



Coscinodiscus appears as discoid and drum-shaped forms 

 in which the siliceous frustrules are marked with concentric 

 and other geometric curves so as to form elaborate patterns. 

 It is a winter and early spring form. (Seen as discs on Plate 

 XVII, Fig. 2.) 



Biddulphia is also a common winter and spring form and 

 has square or oblong cells with spines at the corners and 

 bright yeUow contents . In addition to the common European 

 species, B. mohilienis (? B. regia), a more elongated form, 

 B. sinensis (Plate XVII, Fig. 2), has appeared of late years 

 and is now abundant. It is supposed to have come from far 

 eastern seas, and, according to Ostenfeld, to have been 

 found first in the North Sea near the Elbe in 1903, and to 

 have spread from there to the Irish Sea, the English Channel 

 and up the coasts of Denmark to Norway. 



DinoflageUata or Peridiniales are minute unicellular 

 organisms which are usually regarded as Protozoa, but have 

 been claimed by some as plants. They may be very abun- 

 dant on occasions and are of great importance as the food of 

 some of the larger organisms of the plankton and even of 

 small fishes. Two genera are very abundant in our seas : 

 Ceratium (Plate XIV, Fig. 2), which is said to be the chief 

 food of the sardine at times on the coasts of France and 

 Portugal, and Peridinium, which is sometimes so abundant 

 as to discolour the sea. 



Noctiluca scintillans (Plate XIV, Fig. 1); a globular gelatin- 

 ous Protozoon, related to the Dinoflagellates, which gives rise 



