PARAMECIUM 



73 



comes in contact with a solid object, assuming it to be swimming at full 

 speed forward, the force of the contact produces a negative response. 

 The animal backs up, pivots upon its posterior end, and swims forward 

 again. If it again strikes the object, it exhibits the same negative reac- 

 tion, which is repeated until on swimming forward the animal passes the 

 obstacle (Fig. 21). Though it is called an avoiding reaction, which 

 would imply a consciousness on the part of the animal, all that it is doing 



is simply giving a series of negative 



responses. The term ' 'avoiding reac- 

 tion" thus gives rise to impHcations 

 which are unwarranted, a fact usually 

 indicated by inclosing the words in 



Fig. 22. Fig. 23. 



Fig. 22. — A Paramecium shown pivoting upon its posterior end and sampling the water 

 before starting off in a direction; which may be determined by the result of such sampling. 

 {From Jennings, "Behavior of the Lower Organisms," by the courtesy of Colum.hia University 

 Press.) The figures indicate successive positions and the arrows show the direction of 

 movement. 



Fig. 2.3. — Diagram illustrating the method of trial and error in Paramecium. {Modified 

 from Borradaile and Potts, " The Invertebrata," after Kiihn, by the courtesy of The Macmillan 

 Company.) At the center is shown a soluble substance {C) which is dissolving and diffusing 

 into the surrounding water. The circles represent zones of equal concentration, the zone 

 0, in which protozoans, which might bo paramecia, are gathered, being the zone of optimum 

 concentration. The irregular line shows the path of a Paramecium which enters the area 

 involved and after repeated stopping, sampling, and change in direction, comes to rest in 

 the zone of optimum concentration at x. 



quotation marks. A slowly moving organism like an ameba would not 

 exhibit such a response. 



Paramecia also show what is known as the method of "trial and 

 error,'' which implies a series of experiments on the part of the animal. 

 A Paramecium is constantly taking water into its oral groove with suffi- 

 cient force to draw it from a little distance in front and to produce 

 a cone of movement in the water. Thus it "samples" the water just 

 ahead (Fig. 22). If the water is too hot or too cold or if it contains an 

 injurious chemical substance, the animal gives an avoiding reaction. 

 This may be repeated again and again. While paramecia seem to be 

 swimming aimlessly through the water in all directions, the repetition 

 of these avoiding reactions sooner or later brings them into that part 

 of their environment which is most favorable (Fig. 23). The result 



