578 



SIGNIFICANCE OF THE NERVOUS SYSTEM 



of the umbrella, lie in relation with a more. deeply placed network of 

 fibers in which a number of nerve cells are embedded. Fibers extend 

 from here to the reactive tissue in the innermost layers of the umbrella, 

 tentacles and manubrium. It appears, therefore, that these organisms 

 are already in possession of complete reflex circuits, each of which is 

 composed of a receptor, a sensory path, an intervening neuron 

 forming the center, and an efferent path with its effector. In fact, 

 this differentiation of the nervous elements seems to have progressed 

 quite far, because the sensory cells show certain individual differences 

 which lead us to suspect that some of them are set aside for the recep- 

 tion of mechanical impacts and rays of hght, while others seem to be 

 concerned with the position of the organism in space (static sense). 



A 



OjO o 



c 



• o o 



\. 



Fig. 287. — Dla.gram Illustrating the Evolution of the Nervous System. 

 A, Ordinary living cells; B, processes are sent out by some of them which (C) con- 

 nect with similar processes of more deeply placed nerve cells; D, the latter in turn form 

 connections with the muscle cells, thereby completing the path between the sensory cell 

 and the effector. 



It is, of course, quite probable that a more rudimentary arrangement 

 than this will in time be discovered in other forms; so far, however, 

 the one described is the most elementary with which we are acquainted. 



The Evolution of the Reflex System into a Reaction System. — While 

 a segmental arrangement of the tissues and organs is quite apparent 

 even in the highest animals, this condition may be studied most 

 advantageously in such forms as the vermes and crustacese. The 

 term segmentahsm really signifies that the bodies of these animals 

 are made up of a number of smaller units which are capable of leading 

 an independent existence. This is made possible by the fact that each 

 segment is equipped with a digestive, excretory, circulatory and nervous 

 system, so that a structural dissociation may be effected between 

 them without destroying or seriously impairing their life processes. 



As far as the nervous system is concerned, we find that the different 

 segments (Fig. 288) contain a centrally placed ganghon (G) from which 



