226 



ELEMENTARY BIOLOGY 



In the simple organisms the response, like the irritation, concerns 

 the whole cell \ that is, the whole organism. We cannot separate 

 the part of the animal's structure that is irritable in the sense in 



which a neuron is irritable, from the part 

 that is irritable in the sense in which a 

 muscle fiber is irritable. Nor can we 

 separate the perceiving part from the 

 contracting part, although of course we 

 may readily believe that in the complex 

 mixture that we c?^^ protoplasm there are 

 some contractile arrangements of mate- 

 rials and some irritable combinations. 



In the Vorticella (Fig. 90) and other 

 one-celled animals it is indeed possible to 

 distinguish a strand of highly contractile 

 substance. 



In the Hydra (Fig. 91) we can see 

 the beginning of separation between 

 irritable region and contractile region. 



264, Organs of touch. In our- 

 selves, as well as in the other higher 

 animals, the sense of touch is de- 

 pendent upon the presence of special 

 nerve endings in the skin, and their 

 connection, direct or indirect, with 

 other neurons (see Fig. 92). 



In some parts of the body the touch 

 organs are much closer together than 

 they are in others ; for example, they 

 are set very close together in the skin 

 of the tips of the fingers, and compara- 

 tively far apart on the back of the hand. 

 It seems that we perceive hot through the stimulation of certain end 

 organs in the skin, and cold through the stimulation of certain others. 



265. Organs of taste. On the upper surface of the tongue, 

 on the palate, and in other parts of the lining of the mouth 



Original 

 Path 



Fig. 89. "Trial and error" in 

 lowest animals 



When a simple animal, like Para- 

 mecium, runs into a region un- 

 favorable to its existence, the 

 stimulus causes a reversal of its 

 movements, with a change of di- 

 rection. On moving forward in the 

 new path, 2, it may again meet the 

 same obstacle. The same reaction 

 is repeated. After a number of 

 trials the animal is likely to find a 

 clear path. This behavior gives the 

 appearance of trying again after 

 each failure until success is attained 



