82 



GENERAL PHYSIOLOGY 



commonly but inappropriately termed. Of the vacuoles two kinds 

 may be distinguished. Some collect only occasionally in the 

 protoplasm in a place where a substance lies that attracts water. 

 Others are permanent structures, and are present frequently in 

 such great numbers that the mass of the protoplasm is small 

 in proportion to them and merely forms thin walls for them ; 

 the protoplasm then presents a frothy appearance, as, e.g., in many 

 plant-cells (Fig. 24, a) and Eadiolaria (Fig. 24, &). Among the 

 constant vacuoles that serve as cell-organoids there are the so- 

 called contractile or pulsating vacuoles, drops of liquid that dis- 



FIG. 24. a, Plant-cell from a stamen-hair of Tradescantia. (After Strasburger.) b,Thalassicolla 

 nucleata, a radiolarian cell, c, Paramaicium aurelia, a ciliate-infusorian cell, which cc 

 within the protoplasm at each end a pulsating vacuole. 



appear and appear again at the same spot, usually rhythmically, 

 while the liquid rhythmically mixes with the protoplasm and 

 again accumulates. Many of these pulsating vacuoles have special 

 efferent canals and a constant wall, as is the case in many uni- 

 cellular free-living organisms, especially the ciliate Infusoria (Fig. 

 24 c\ 



In addition to such constant elements, in many cells solid 

 constituents are met with that are present as such only tempo- 

 rarily. Here belong especially the food-bodies that are found in 

 cells that nourish themselves by taking in solid food-constituents. 

 Unicellular naked organisms, such as Amceba, white blood-cells, 



