70 THE SUBDIVISIONS OF THE BRAIN. 



which were mainly associated with the gills, heart, and viscera; they united with 

 the brain at a later period. 



The combined vagal and branchial neuromeres of arachnids form the hind- 

 brain, or medulla, of vertebrates. (Figs. 57-58.) 



In arachnids, all the branchial neuromeres do not form a part of the brain, 

 except possibly in certain specialized land forms; but they already show the begin- 

 ning of that functional segregation of sensory, branchial, cardiac, and motor 

 nerve fibers that is so characteristic of the hindbrain neuromeres of vertebrates. 

 (See cardiac and hypobranchial nerves). 



In arachnids (scorpion and Limulus) there are in all seven vagus and branchial 

 neuromeres, which is close to the estimated number in this region in vertebrates. 

 However, this number probably varies in vertebrates, as it does in arachnids, and 

 only an approximate numerical agreement and serial location is offered as having 

 significance. The segregation of the vagal and branchial nerves (in both verte- 

 brates and arachnids) into the groups mentioned above, is of much greater sig- 

 nificance than the agreement in the number of neuromeres. 



