. General physiology; an outline of the science of life. ELEMENTARY VITAL PHENOMENA 231 and in the surrounding water must always take place, , the liquid of the vacuole must always contain in solution the same number of molecules as the water. But it must be assumed that these are molecules of substances different from those in the water. If, therefore, we imagine some of the substances dissolved in the liquid of the vacuole to possess low specific gravity, we can under- stand how, upon the whole, the contents of the vacuole can be lighter than the water. K. Brandt ('95) has recently made
. General physiology; an outline of the science of life. ELEMENTARY VITAL PHENOMENA 231 and in the surrounding water must always take place, , the liquid of the vacuole must always contain in solution the same number of molecules as the water. But it must be assumed that these are molecules of substances different from those in the water. If, therefore, we imagine some of the substances dissolved in the liquid of the vacuole to possess low specific gravity, we can under- stand how, upon the whole, the contents of the vacuole can be lighter than the water. K. Brandt ('95) has recently made it very probable that it is the carbonic acid produced by the protoplasm that, dissolved in the liquid of the vacuole, lowers the specific gravity of the protoplasm below that of the sea-water. If the vacuole-layer is developed • to a sufficient extent, the specific gravity of the whole cell will be less than that of the sea-water, , the cell will float at the surface. If by the burstmg of the vacuoles the volume of the layer becomes diminished, or if in the cold, when the metabolism sinks to a minimum, the production of carbonic acid becomes greatly decreased, the radiolarians will sink again. e. Mcrvements hy Secretion Movements that come about through secretion by the cell are limited to a few groups of organisms, particularly the Algce, DesmidiacecB, and Oscillarim. The principle of this mode of motion is extremely simple. It consists simply in the cell lying upon the bottom and pressing out at a definite place upon its surface and in a definite direction a mass of secretion, usually of a slimy nature ; this sticks to the bottom, and the motile cell-body thereby thrusts. Fjq_ 93, Closteriura, a desmid, shoving itself along the bottom by a secretion of slime. The non- secreting end swings freely in the water. itself forward in a definite direction, just as a fisherman pushes his boat off the shore with a pole. If the secretion continues, the cell glides slowly along. I
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