46 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



early period in the evolution of arthropods the sensory elements become more and 

 more predominant in the neural nerves, and the motor elements in the haemal ones, 

 this condition being most strongly marked at the anterior end, and diminishing; 

 gradually in a caudal direction. 



Factors that Modify the Arrangement of Peripheral Nerves. — The more 

 important factors that modify the primitive segmental arrangement of peripheral 

 nerves are as follows: a. the location, isolation, and size of the peripheral terminals;; 

 b. the elimination of other terminals; c. the organic union of similar terminals^ 

 belonging to different metameres; d. the relative age of the metamere in which they 

 belong. 



Fig. 38. — Brain of a young Limulus about three 

 inches long; neural surface. 



Fig. 39. — Same; haemal surface. 



a. The segregation of like nerve fibers into peripheral nerves, or into nerve 

 tracts in the central nervous system, is determined by the time and place of origin 

 of the peripheral terminals. 



Wherever there are highly specialized organs, morphologically isolated, the 

 associated nerve fibers and nerve cells show a similar isolation or segregation, the 

 growth of each correlated part keeping pace, in the main, with the growth of 

 the other. The primary sensory organs are superficial in position and lie in the 

 ectoderm, close to the lateral margins of the neuron. The motor ones are deeper, 

 more lateral or hsemal in position. The corresponding nerves have, in the main, 

 similar relative positions, and these factors have controlled from the outset the 



