20 4 DIATOMACE&. PART n. 



ceding species in its motion ; each half of the row of 

 frustules moves in an opposite direction on each side of 

 a central stationary frustule, and the alternate motion is 

 so regular as to time, that if in advancing, the frustules 

 meet with an impediment, they wait till the proper time 

 comes for their retreat. The jerking motions of the 

 Naviculse are ascribed by Prof. W. Smith to forces acting 

 within the plants, originating in the vital operations of 

 growth, by which the surrounding water is drawn in at 

 one end of the frustule, and expelled at the other. 



Some species of diatoms are so universal that they are 

 found in every region of the globe ; others are local, but 

 the same species does not inhabit both fresh and salt 

 water, though some are found in brackish pools. The 

 ocean teems with them. Though invisible as indi- 

 viduals to the naked eye, the living masses of the 

 pelagic diatoms form coloured fringes on larger plants, 

 and cover stones or rocks in cushion-like tufts ; they 

 spread over the surface as delicate velvet, in filamental 

 strata on the sand, or mixed with the scum of living 

 or decayed vegetable matter floating on the surface 

 of the sea; and they exist in immense profusion in 

 the open ocean as free forms. The numbers in which 

 they exist in all latitudes, at all seasons, and at all 

 depths extending from an inch to the lowest limit to 

 which the most attenuated ray of light can penetrate, 

 or at which the pressure permits are immeasurably in 

 excess of what "we have been in the habit of assuming. 

 Temperature has little to do with the distribution of 

 diatoms in the tropics ; it decreases with the depth at a 

 tolerably fixed rate till it becomes stationary. It in- 

 creases in the polar regions with the depth, and ap- 

 proaches the standard, which is probably universal, near 

 the bed of the ocean. 



Nothing can exceed the vividness of colour or massive- 

 ness of the endochrome or soft internal matter of the 



