THE GENERAL PHYSIOLOGY OF THE PROTOZOA 203 



to gases may be considered as equivalent to a sense of smell in 

 higher organisms (osmotaxis). 



It has been shown by many experiments that a given species is 

 attracted towards certain chemical substances, repelled by others. 

 Thus, Paramecium is attracted towards weak acids, but repelled by 

 them in greater concentration. If a drop of acid of suitable strength 

 is placed in the midst of a number of Paramecia distributed evenly 

 in the water under a cover-slip on a slide, they tend to gather round 

 the drop. As the drop diffuses in the surrounding water, the 

 Paramecia arrange themselves in a ring in the region of optimum 

 concentration. If, however, the drop of fluid employed is of a 

 strength which represents the optimum of chemotaxis for the species, 



FIG. 83. Diagram showing the course taken by a Paramecium which has entered 

 a drop of fluid to which it is positively chemotactic. The forward movements 

 of the Paramecium are indicated by arrows ; its backward movements by 

 dotted lines ; the outline of the drop of fluid by a circle. Each time the 

 Paramecium, in its forward movement, reaches the confines of the drop, it 

 comes into contact with fluid which is less positively chemotactic than the drop 

 into which it has entered ; it then shrinks backward (avoiding reaction), after 

 which it moves forward again with the same result every time it reaches the 

 edge of the drop. After Lang (10). 



the Paramecia gather within it, and in such a case the position taken 

 up by each Paramecium depends on the avoiding reaction made by 

 it when it comes in contact with a less attractive medium. Thus, 

 if a Paramecium, swimming in a straight line, enters a drop of fluid 

 which is positively chemotactic to it, when it has crossed the drop 

 to its opposite boundary it comes to the region where it meets with 

 fluid which is less chemotactic to it ; it then shrinks back with an 

 avoiding movement ; after a time it again moves forward, and comes 

 again into the negatively chemotactic region, with the same result 

 as before. Thus its movements are as if caught in a trap (Fig. 83), 

 in which it is held by the automatic movements called forth by the 

 difference between the more and the less chemotactic fluids, until 



