THE ARRANGEMENT OF PERIPHERAL NERVES. 47 



segregation of motor components in a haemal direction, and the sensory ones in a 

 neural direction, both as regards their location in the peripheral nerves and in 

 the central nervous system. With the invagination of the nerve cords, these con- 

 ditions were still further exaggerated by the union of the neural crests in the median 

 dorsal line, and by the position of the mesoblastic somites. (Fig. 137.) 



The wide separation of the neural and haemal nerves, as for example in the 

 thorax of Limulus and the scorpion, is due on the one hand to the location and spe- 

 cialization of the appendages, coxal sense organs and ganglia, and on the other to the 

 location of the more peripheral trunk muscles and sense organs. It no doubt 

 had its origin at a very early period in the evolution of metameres. 



b. Elimination. — In the arthropods there is a progressive elimination from 

 the anterior metameres of the motor, nutritive, cardiac, and respiratory organs, 

 leaving little but the leg and jaw muscles, and the primary sense organs, such as 

 the eyes, olfactory, gustatory, auditory, and tactile organs. The nerve elements 

 associated with those organs disappear with them. Those that remain increase 

 in volume and independence with their corresponding peripheral terminals, while 

 their central terminals tend to completely monopolize their appropriate neuro- 

 meres. In this way the primitive character of the segmental nerves may be lost or 

 greatly modified. This is the case in the procephalon, where the only peripheral 

 nerves that remain belong to the eyes and olfactory organs, all other peripheral 

 elements having been eliminated, if they ever existed there. 



c. Union. — Where organs belonging to different metameres perform the same 

 function their nerves tend to unite, forming a common bundle, or nerve, or tract. 

 Such compound nerves, consisting of the united branches of separate segmental 

 nerves, may themselves simulate independent segmental nerves, and greatly dis- 

 guise the original segmental arrangement.^ Examples of this mode of segregation 

 are seen in the segmental cardiacs, the hypobranchial, the intestinal (Figs. 57, 58), 

 and to a lesser degree, the gustatory nerves of Limulus. 



c. Historic Factor. — If we attempt to homologize the nerves in one part of 

 the head with those in another, or with those in the trunk, we meet with insuper- 

 able difhculties because, as we have seen, each group of metameres has a history 

 of its own that is different from that of all the others, and this history is reflected 

 in the structure of its nerves and neuromeres. The attempt to homologize the 

 structures in the head with those in the trunk or tail, except in the most general 

 way, is an illogical and hopeless undertaking, for the caudal metameres belong 

 to later generations that came into existence under new conditions and were 

 provided with different organs from those in the old. Except for a small number 

 of the most anterior ones, the trunk and caudal metameres of vertebrates did not 

 exist in the arthropods. They arose with the vertebrate stock and never developed 

 any organs comparable with the cephalic appendages, jaws, gill arches, or visual 

 organs. Hence it is clear that there can be no exact homology between the head 

 metameres of an arachnid or a vertebrate and a trunk metamere of the same 

 animal. For that reason, therefore, we may not consider the cranial nerves, or 



