[Illustration: Fig. 16. Bacillaria paradoxa]
This Bacillaria paradoxa (fig. 16) differs from the preceding 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 Naviculæ 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 individuals 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 increases in the polar regions with the depth, and approaches the standard, which is probably universal, near the bed of the ocean.
Nothing can exceed the vividness of colour or massiveness of the endochrome or soft internal matter of the floating diatoms, that matter which diminishes their specific gravity and makes the plant buoyant which otherwise would be weighed down by its silicious coat. At those periods in which the structural and reproductive phenomena proceed most vigorously, their position in depth must be fluctuating; hence they approach and vanish from the surface. Their growth is perfected by the heat and light which penetrates the sea in calm weather.