48 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



cranial neuromeres, or cephalic sense organs, as modifications of those in the 

 trunk, or vice versa, without conveying an entirely false impression of their real 

 history and meaning. 



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In the lower arthropods, the peripheral nerves are generally arranged through- 

 out the whole body, in typical segmental fashion. In the higher arachnids, due 

 to the operation of the above described factors, this clear cut metamerism declines, 

 or is greatly obscured. The broad distinction between cranial and spinal nerves 

 becomes clearly established, and the extensive elimination of motor elements, as 

 well as the local segregation of sensory and motor components of different nerves 



Fig. 40. — Models of the brain of a young scorpion, just hatched. A, Hffimal surface; B, neural surface. 



into compound nerves having a similar function and distribution, has given to the 

 entire system the same structure and general arrangement of parts seen in the 

 vertebrates. 



In Limulus, for example, this process of specialization has produced the 

 highly characteristic olfactory, pineal eye, and lateral eye nerves, as well as the 

 compound system of gustatory, branchio-thoracic (hypoglossal), cardiac, and 

 intestinal nerves. These nerves are already so complex and highly modified 

 that the original segmental arrangement is now exceedingly difficult or impossible 

 accurately to determine. 



The same conditions, but in a still more exaggerated form, are seen in verte- 

 brates, and in part justifies the revolt of certain American neurologists against the 

 apparently hopeless task of determining the segmental value of vertebrate cranial 

 nerves and their relation to the dorsal or ventral roots of spinal nerves. They 

 have laid great stress on the analysis of nerves into their functional components; 

 but in perfecting a highly artificial system, they have neglected the deeper mor- 

 phological problems involved in their more primitive segmental arrangement. It 

 is clear that both the old and the new method must be retained. But neither 



