i6 



DEPTHS OF THE OCEAN 



furnish many varieties of this type. Sometimes the length axis is situated 

 in the division-plane of the cells, as, for instance, in Thalassiothrix 

 longissivia, one of the characteristic forms in colder seas ; at other times 

 division takes place across the elongated cell, as in the genus RJiir:osolenia, 

 of which there are many species (see Fig. 217). Hair-shaped cells of this 

 kind create a great deal of friction when horizontal, but would sink 

 rapidly when perpendicular, if it were not for the fact that they are 

 either slightly curved, or else their terminal faces are sloping ; so that 



Fig. 2\^.~Coscixodisc.us rex (-'V')- 



Fig. 216. — Pelagic Diatoms of the 

 ribbon-type {-^?^). 



Chain of Navicula vanhoffeni, the cells con- 

 nected by a band of mucilage ; b, part of a 

 chain of Frasrilai-ia oceaiiica. 



the resistance of the water soon restores them to an almost horizontal 

 position, and they sink slowly in long spiral sweeps. 



(4) The Branching Type. — The surface of the cell is enlarged by 

 various kinds of hair-shaped or lamelliform outgrowths. To this type 

 belongs the genus CJicetoceras with its numerous species (see Fig. 218).. 





j5^«fe«> 



Fig. 217.— Pelagic Diatom of the hair-type, Rhizosolenia hebetata-semispina. 

 a, Entire cell (^oo) ; b, end of a cell (^f «). 



Every cell has four long setiform outgrowths, and the cells are besides 

 nearly always associated in chains, so that these setae radiate in every 

 direction. When the chain is straight and stiff it is frequently furnished 

 with special terminal setae, which are stiffer than the others, and act as 

 a sort of steering apparatus. 



In addition to the actual outgrowths from the cell many 

 diatoms can secrete long filaments of mucilage from special 



