600 GENERAL PHYSIOLOGY 



transverse position of the long axis the flagelluni is stimulated to 

 contract more strongly upon the side upon which the stimulus 

 falls than upon the opposite side, it makes stronger strokes toward 

 the former than toward the latter side, and the result is that the 

 anterior part of the body is turned away from the source of the 

 stimulus (Fig. 248). Exactly the same relations exist here as 

 in a boat moved by a single oar. The. bow of the boat also 

 turns toward the opposite side when the boat is propelled more 

 strongly upon one side than the other. The unequal strength of 

 the flagellar stroke in the two directions continues, and the anterior 

 part of the body is turned constantly more away from the source 

 of the stimulus, until the body has placed its long axis in the 

 direction of the incident stimulus (Fig. 248, d). Then both sides 

 of the flagelluni become equally stimulated and the protist swims 

 in a straight line, so long as the stimulus continues. Thus, 

 negative chemotaxis, phototaxis, etc., appear in uniflagellated 



Fio. 248. — Scheme of axial orientation of a ftagellate-infusorian cell, resulting from an excitation 

 of contraction upon tlie right side. The side toward which the concavity of the flagelluni is 

 directed is the stimulated side. The arrows indicate the direction of movement. 



Bacteria and Flagdlata as a necessary result of a unilateral excita- 

 tion of contraction in the flagellum. 



It is now very easy to imagine the relations as regards axial 

 direction in forms possessing two flagella, such as the flagellate 

 infusorian Polytoma (Fig. 249). If two flagella are present upon 

 the anterior end of the cell, the latter corresponds to a boat that 

 is moved by two oars at the bow. If the strokes of the two 

 oars are equal, the boat moves in a straight line. It is the 

 same with the flagellated cell. If one oar moves more strongly, 

 the bow of the boat is turned toward the opposite side. The 

 same will occur with the cell possessing two flagella, when a con- 

 tractile stimulus acts upon one side, causing one flagellum to beat 

 more strongly than the other (Fig. 249, a, h, c). The anterior end 

 of the cell must then be turned away from the source of the 

 stimulus, until the long axis is turned in the direction of the 

 latter, with the posterior end nearer it. In this direction the two 

 flagella are equally stimulated (Fig. 249, d), and as a result the 



