50 The Animal Mind 



more complex than that of some other members of the type 

 Protozoa. There is a large group of single-celled animals 

 called Ciliata, from the fact that their bodies are covered with 

 little hairlike protoplasmic filaments or cilia which serve as 

 organs of locomotion by acting like tiny oars. A common 

 representative of the group is Paramecium. The structure of 

 this animal is distinctly more specialized than that of Amoeba. 

 Not only are the cilia modified locomotory structures, but 

 there is a definite region for food -taking. A groove extends 

 obliquely down one side of the body, terminating at its 

 lower end in a mouth. The cilia along this oral groove 

 beat with especial vigor and create currents which sweep 

 food particles to the mouth. Paramecium swims rapidly 

 through the water with a spiral motion of its body, due 

 to the facts that the aboral cilia beat more strongly than 

 the rest, and that the animal compensates for the turning 

 thus occasioned by turning on its long axis. Its reactions 

 to stimulation Jennings has shown to be only two in 

 number. First, there is a very definite avoiding or negative 

 reaction. This is given in response to decided mechanical 

 stimulation at the anterior end, as when the animal 

 swims rapidly against an obstacle, and also in response 

 to chemical stimulation, to strong ultra-violet rays (167), and 

 to temperatures above or below a certain middle region called 

 in this case, as in analogous cases with other animals, the 

 optimum. For Paramecium it lies between 24 and 28 C. 

 The negative reaction consists, according to Jennings, of the 

 following process : the animal darts backward, reversing the 

 beat of its cilia, turns toward the aboral side (that opposite to 

 the oral groove) by increasing the beat of the oral cilia and 

 lessening the compensating rotation, and continues on a 

 forward course that is now at an angle with its former line of 

 motion. If this new course carries it clear of the stimulus, 



