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S. O. MAST AND K, S. LASHLEY 



actual contact with the particles. But in regard to the manner 

 of swimming no difference could be detected between those which 

 produce the cone and those which do not. 



One of the chief causes of the occasional production of the cone 

 under the above conditions, can readily be detected in observa- 

 tions with the binocular. The ordinary cover-glass mount in- 

 cludes a layer of water which is scarcely thicker than the width 

 of the spiral course taken by the paramecia. The animals con- 

 sequently strike against the slide or cover-glass at almost every 





Fig. 1 Diagram illustrating the movement of particles produced by Para- 

 mecium swimming freely in a dilute suspension of India ink: a composite of 

 many observations in which single particles were selected and kept in view as 

 the organism swam past them. The dots represent the positions of particles 

 when the Paramecium had the position represented by the continuous outline; 

 the arrows represent the direction and the extent of movement of the particles 

 during the time required by the Paramecium to move from the original position 

 to that represented by the broken outline. Note that the particles in front of 

 the paramecia do not move until the anterior end comes very near them. 



Fig. 2 The production of a feeding-cone due to retardation caused by con- 

 tact with the substratum. The ink spreads most rapidly along the substratum 

 and, when the edge of the cloud is brought into sharp focus, only those animals 

 are visible which are swimming near the bottom. These frequently come in con- 

 tact with the substratum and the cone results from the consequent mechanical 

 retardation of the animal. 



