SOME PROBLEMS IN THE PHYSIOLOGY OF THE PXOTOZOA 309 



the inability of the protoplasm to hold objects above a certain 

 weight. 



In other cases where the shell material is deposited, such as the 

 lime or silicious shells of Sarcodina, the substance itself is built up by 

 a chemical process within the protoplasm, and the deposition may 

 take place periodically or continuously. 



The naked Amoeba might be considered a complex chemical com- 

 pound, endowed, like all compounds, with special properties in this 

 case with the power of motion, of irritability, of metabolism, and of 

 growth and reproduction. Like many chemical compounds it is, dur- 

 ing the living state, of unstable equilibrium, which involves a constant 

 change in chemical composition. These several properties with 

 which it is endowed, however, are not confined exclusively to the liv- 

 ing organisms, for many inorganic compounds possess one or more of 

 them. Thus, a drop of oil quite spontaneously assumes forms which 

 simulate different species of Amoeba, while a mixture of sugar or salt 

 and olive oil simulates not only the movements, but even the struc- 

 ture, of living protoplasm (Butschli). In these cases the motion is satis- 

 factorily explained by the laws of surface tension, although the move- 

 ments are almost as remarkable as those of a rhizopod. The motion 

 of the protoplasm of a plant-cell is explained as the resultant of the 

 chemical changes taking place within the body of the plant, and simi- 

 larly the motion of Amceba has recently been interpreted by Berthold, 

 Verworn, Butschli, and Rhumbler, as a series of responses to changes 

 in the chemical composition, with corresponding changes in density 

 within the organism. The movements which engender the phenom- 

 ena of phototaxis and thermotaxis, of chemotaxis and barotaxis, are also 

 duplicated by inorganic substances. Thus the impact of ether waves 

 upon various bodies brings about a rearrangement of molecules. 

 Affinity, in chemistry, is the elective property by which one substance 

 seeks another, and the mutual action is analogous to, if not the same 

 as, chemotaxis ; while the phenomena of cohesion and of adhesion 

 come under the head of barotaxis. In a similar way it can be shown 

 that irritability, or response to stimuli, has its analogue in the inorganic 

 world ; any compound substance in a state of unstable equilibrium, as 

 in explosive compounds, will react to stimuli of various kinds, while 

 even metabolism is simulated in inorganic objects, as shown in the 

 excellent illustration cited by Verworn ('94) of nitric acid in the pres- 

 ence of sulphurous anhydride. Growth, too, is not confined to organic 

 substances, as shown by the continual growth by accretion of various 

 crystals or solids, while growth by intussusception takes place when- 

 ever a solid becomes dissolved in a liquid. The stimuli necessary to 

 bring about protoplasmic reaction may be so delicate that we cannot 

 perceive them, and we are thus led to assign some mystic cause under 



