ELEMENTARY VITAL PHENOMENA 231 



and in the surrounding water must always take place, i.e., 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, 

 i.e., 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. 



In this manner the Dcsmidiacece move themselves. The crescent- 

 shaped Clostcrium (Fig. 93), which we have already become 

 acquainted with in considering the Brownian molecular movement, 

 secretes a slimy substance at each end of its unicellular body. 

 While it thus clings to the bottom with one end, the other end 

 floats freely in the water, so that the whole body is directed 



