THE EVOLUTION OF NEUROMERES. 



49 



method alone applied to the vertebrates can ever give us a true picture of their 

 ancestral condition. That can only be obtained from the arthropods where the 

 highly specialized condition seen in the vertebrates has its origin. 



V. Neuromeres and Metamerism. 



Metamerism of the body and the subdivision of nerve cords into blocks or 

 neuromeres are characters that were probably slowly evolved in bilateral animals; 

 not inherited, even in a rudimentary form, from coelenterate ancestors. 



The evolution of neuromeres probably began in the trochozoa. They are 

 well developed in the annelids and in the arthropods, especially in the ab- 

 dominal regions. In the higher arthropods, the clear-cut distinction between 

 adjacent neuromeres of the head is greatly obscured by their fusion into larger 

 groups, and by the segregation of their constituents into new groups, according 



Fig. 41 — Model of the forebrain region of an embryo scorpion, stage G, Fig. i8. 



to their function. In vertebrates, the post-cephalic part of the neuron, which 

 has been more recently acquired, and which is not represented in arthropods, 

 is never divided into distinct neuromeres, and probably never was so divided. 



Even in the arthropods and annelids, it is doubtful whether there is any such 

 thing as a neuromere, complete in itself and devoted to a single body joint or 

 metamere. There are certainly none in Limulus, or in the scorpion, and the 

 lower down we go, as for example into the phyllopods, the less sharply defined the 

 neuromeres become; that is, the ganglionic masses are more diffuse, and the pe- 

 ripheral nerves more numerous, and not so strictly segmental in their origin or dis- 

 tribution (Branchipus) . 



In Limulus and scorpion, where there appears to be such an exact and exclu- 

 sive association of the body segment with its neuromere and nerves, there is no 

 such exclusive association in fact, because many motor neuromeres and the cen- 

 tral ends of many sensory fibers are located in some neuromere anterior to the one 

 where the nerve fibers leave the cord to reach their peripheral terminals. That 



