218 Marine Microbiology 



and fragile than Cyclococcolithus, and unable to build internal 

 resting spores. Rare when oxygen is below 90 per cent of satura- 

 tion. Examples: Thomcosphaera heimii (Lohm), Corisphaeni 

 fagei (Bernard, etc.) 



3. Small Naked Flagellates 



In majority, less than 13 /x long. Various species, belonging 

 chiefly to Volvocales and Chrysomonads (five other orders 

 are concerned but are not common). They are often colorless, but 

 I have seen, in the Mediterranean at 1,000 to 2,500 m depth, 

 several cells with colored plastids. Rarely more than 12 per cent 

 of the total volume: numbers of cells are sometimes large (300 

 to 1,000/ml), but their size is small. 



Naked flagellates are much more abundant in low salinities 

 (Gibraltar, Atlantic current off Algeria) and ten times rarer 

 at the highest salinities ( 38.6 to 39%c ) of the oriental Mediterran- 

 ean. 



4. Myxophyceae 



The prevailing form everywhere is a small Nostoc, of 4 

 to 6 /A, near N. phnctonicwn Poretzky from Carelian lakes: the 

 cysts in the Indian Ocean are quite similar to those off Monaco 

 and Algeria, often entirely lacking, but when present, sometimes 

 very numerous ( 400 to 12,000 elements/ml ) . There is no evidence 

 of upwelling of bottom mud: regions of upwelling, such as 

 Senegal and Greece, are almost devoid of Myxophyceae. The 

 genera Dactyliococcopsis and Microcystis are uncommon: gen- 

 erally half of the small flagellate value by volume (5 to 6% of 

 total). 



5. Dinophyceae 



Very numerous (with Exuviella, Ceratium and Gymnodin- 

 ium) only in the uppermost photic layer (0-25 m in the Medi- 

 tenanean). Generally subordinate in tlie deep sea, with exception 

 of some stations off Lybia, where Exuviella were swarming at 

 500 and 800 m depth. At mean aphotic value, Dinoflagellates do 

 not make up a volume equal to the Myxophyceae. 



It is possible to calculate some habitual characteristic quanti- 

 tative relations between the above named groups of unicellular 

 organisms. Each region of sea shows relations rather different 



