850 TEXTBOOK OF ZOOLOGY 



have phototropism, response to light; thermotropism, response to 

 heat ; chemotropism, response to chemicals ; thigmotropism, response 

 to contact; geotropism, response to gravity. 



Reflex Behavior 



Tropistic behavior is the dominant type of behavior in the organism 

 without a well-differentiated nervous system. The protozoa are with- 

 out a nervous system, and though a few of them have threadlike struc- 

 tures running through their bodies in such a way as to suggest a cor- 

 relating mechanism of some sort, it seems that, in general, this sensi- 

 tivity and conductivity are properties of the protoplasm itself. In 

 such animals as Hydra and the jellyfish, as well as in all higher ani- 

 mals, we have to do with activities promoted by nervous tissue. In 

 such animals, we no longer call their behavior tropistic but speak of 

 their simplest reactions as reflexes. In principle there is little dif- 

 ference between a tropism and a reflex except in the relative com- 

 plexity of the mechanism involved. The reflex is the dominant type of 

 behavior in the segmented animals which have a simple ganglionic 

 type of nervous system. Also according to Coghill reflexes represent 

 secondary patternings of behavior in the developing vertebrates. The 

 sensitivity and conductivity involved in the production of a reflex are 

 the same qualities of protoplasm as are concerned in tropisms, but 

 they are enhanced by the special differentiation of nervous tissue in 

 these particular directions. 



The human being, as weU as other animals, exhibits reflexes. They 

 are sometimes called physiological reflexes and are reactions, such as 

 the wink of the eyelids which follows the entrance of a grain of sand 

 into the eye or which follows the sudden approach of an object to the 

 eye. Other reflexes are the knee jerk, contraction of the pupil of the 

 eye (pupillary reflex) when a strong light is flashed into the eye, and 

 the general start of the whole body when a sudden and unexpected 

 noise occurs. 



These reflexes have two characteristics which are important. First, 

 they are brief in duration, consisting, in general, of a single contrac- 

 tion or relaxation pattern of a group of muscles. Second, they are 

 predictable ; a particular stimulus always, in the normal subject, pro- 

 duces the reaction. If the reaction (such as the knee jerk or the pupil- 

 lary reflex) does not follow the appropriate stimulus (the blow on the 



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