376 



THE MAINTENANCE OF THE INDIVIDUAL 



Connecting neurons tie up these with the effector neurons which stimu- 

 late the muscles to contract, or the glands to secrete. The dorsally 



placed vertebrate nervous sys- 



a few cells of -tfic^ 

 nerve. neLof 



N 



\adde-r system systsra of odultrnid^ 

 of a myriapod ■ *^'" ' 



tern is considered the most 

 highly developed type of all. 

 Here centralized function is 

 found at the anterior end of 

 the body in the so-called brain. 

 In the animal series all animals 

 except those built on the radial 

 plan show a very distinct 

 centralization of sense organs 

 (receptors) at the anterior end. 

 The organs of sight, hearing, 

 taste, and smell are found in 

 a relatively small area on the 

 head close to the brain. It is 

 easy to see how evolutionary 

 development has brought this 

 about, since it is the anterior 

 end which is constantly ex- 

 ploring for the rest of the 

 Upon the success of this exploratory ability rests the suc- 



Three types of nervous systems. What 

 are the general Hkenesses and diflerences ? 

 Which would be called the highest type and 



why i' 



animal 



cess of the animal in its struggle for existence. 



Tropisms, Reflexes, and Native Behaviors 



The term reflex action has been given to the response which comes 

 from the stimulation of a single reflex arc, a receptor with its neuron 

 leading inward to an effector neuron which in turn causes movement 

 through the effector muscles or glands. In most if not all cases, how- 

 ever, there is more than a single series of neurons engaged in the 

 action of the reflex arc. There is always a direct response in the 

 reflex. The response is quite predictable and results in movement 

 of a relatively small part of the animal's body. A tropism, on the 

 other hand, may be considered as a steady response to a continued 

 stimulus. As one writer well puts it, the tropism is "a steady under- 

 lying bias in behavior brought about by a constant stimulus." The 

 tropism affects the organism as a whole, the reflex directly affects 

 only a small portion of it. The activities of all animals, but espe- 

 cially the lower forms, are a continual series of reflexes and tropisms. 



