CHAP. XXXVIII PLANKTON-FORMATION 157 



(especially in Alpine lakes), Fragilaria, Asterionella, and others.^ The 

 genera Rhizosolenia and Attheya, in fresh water are represented respec- 

 tively by a few and by one species, which are widely distributed ; the 

 remaining species of these genera are marine. Some are sohtary, but 

 many live combined in chains of various kinds. They are all true 

 plankton-organisms which are incapable of forming masses swimming at 

 the surface of the water. Some are enveloped in mucilage. 



4. Peridineae (Dinoflagellata) occur especially in salt water. They 

 are found in greater quantities, but in smaller numbers of species in tem- 

 perate seas ; while the individuals are fewer, but the species more numerous 

 and diversified in warmer seas - ; the genera Ceratium and Peridinium 

 are particularly common. The numerous forms (geographical races) of 

 Ceratium tripos play the greatest part in the sea ; while C. hirundinella 

 occurs in large quantities in fresh water. They are provided with two 

 flagella and are motile. Some marine forms are luminiferous, and in the 

 autumn, when they are most numerous in the North Sea, Skager-Rak, and 

 western Baltic, cause the sea to be phosphorescent. 



Nearly all plankton-organisms are included in the four groups named 

 above. But both in the sea and fresh water there are other species 

 of Chrysomonadineae (Phaeocystis, Dinobryon, and others). 



Only in the sea are found Silicoflagellata, Coccolithophoridae, and 

 other Flagellata. The Coccolithophoridae are present in vast numbers, 

 but, on account of their minuteness, escape through the dredging-net, 

 so that their true abundance has not been appreciated until recently.-' 

 Phaeocystis Poucheti is a flagellate which seems to be common to the 

 coasts of Norway, the Faroe Isles, and Iceland, and was discovered 

 by Pouchet in huge quantities by the Lofoden Isles. The protococcaceous 

 Halosphaera viridis, which has the form of a sphere one millimetre in 

 diameter, commonly occurs in the temperate and warmer parts of the 

 Atlantic Ocean at the surface, or down to a depth of 200 metres, and 

 has been found at the depth of 2,200 metres. At considerable depths 

 (100-300 metres below the surface) of the tropical Atlantic Ocean, Halo- 

 sphaera and the diatomaceous Planktoniella represent a kind of 

 shade-flora. 



In fresh water there live many Chlorophyceae, among which some 

 can be described as true plankton-organisms ; among these may be 

 mentioned Sphaerocystis Schroeteri, Dictyosphacrium, Oocystis, Botryo- 

 coccus, and Golenkinia radiata.* Occasionally Desmidiaceae, or Scene- 

 desmus, Pediastrum, and other forms are intermingled with plankton 

 in fresh water. 



ADAPTATIONS AND DISTRIBUTION 



The power of flotation shown by plankton-organisms has recently 

 been investigated by Wolfgang Ostwald.-^ Power of flotation constitutes 

 the main difference between plankton and all other communities of 

 organisms. This flotation merely denotes a tendency to sink exceedingly 

 slowly. If a body is to sink in a liquid, its weight must exceed that of 

 an equal volume of the hquid. The rate of sinking depends partly on 



' (Schroter und) Kirchner, 1896. ' Schiitt, 1893. ' See Lohmann, 190J. 



* See Chodat, 1898 ; (Schroter und) Kirchner, loc. cit. 

 ' See Wesenberg-Lund, 1908. 



