146 THE OCEAN 



or suspension in the ocean-waters with high 

 salinity, so that this distribution of silica- 

 secreting organisms is correlated with the law 

 of the minimum in agriculture. In the great 

 Southern Ocean, and in some other areas of 

 limited extent, the dead frustules of diatoms 

 accumulate on the bottom in such numbers 

 that the deposit is called diatom ooze. 



(2) Peridineans. — These are mobile uni- 

 cellular algae, or organisms which function 

 as algae, having a cell-wall of an organic sub- 

 stance like cellulose, which dissolves after 

 the death of the organism. Consequently the 

 remains of these organisms are never detected 

 in marine deposits. They occur in enormous 

 quantities, and in many varieties, while most 

 of them are brilliantly phosphorescent. Peri- 

 dinium depressum is a typical species. A 

 distinct difference is seen between the anterior 

 and posterior ends, and also between the dorsal 

 and ventral surfaces. There are character- 

 istic furrows on both front and back, and a 

 third furrow, known as the ring-furrow, encir- 

 cles the cell. One cilium projects through a 

 pore in the posterior furrow, and can be 

 withdrawn spirally into a sheath ; another 

 cilium lies in the ring-furrow. Gran states 

 that in Peridinium reproduction does not 

 take place by cell-division, but the cell- 

 contents are changed into one, two or four 



