EVOLUTION OF THE VERTEBRATE 193 



the manubrium near the mouth, and in a continuous medu- 

 soid series the continuous manubrial tube or gut would 

 be surrounded by all the successive nerve-rings. 



In the evolution of the Annelid type we can thus account 

 for the circum-pharyngeal nerve-collar, and would conclude 

 that this alone remains to represent the ancestral nerve- 

 rings in complete circle. Dorsally on the nerve-collar 

 terminal compression has massed together several ganglia 

 to form a primitive brain, while ventrally a smaller ganglionic 

 mass obtains as a " sub-brain." Posteriorly to this the 

 nerve-rings are only represented by one ganglionic mass 

 for each segment, plus the nerves given off laterally, and the 

 linking up of these ventral ganglia has given the ventral cord. 



In the primitive Vertebrate the same state of affairs 

 could, we may say, have possibly resulted. But here terminal 

 compression massed together a greater number of ganglia 

 to form a dorsal brain, the ancestral nerve-rings altogether 

 failing to surround the alimentary tract as rings or collars, 

 but being represented by the right and left cranial nerves. 

 Posteriorly to the brain the nerve-rings were represented 

 by a ganglion mass with lateral nerves in each segment, 

 but these developed from the first dorsally. The next figure 

 illustrates our suggestions. 



It seems clear that terminal compression was the chief 

 factor at work in the evolution of the Vertebrate brain ; 

 the developing evolving type habitually presenting its anterior 

 segments towards sources of attraction as it moved to them 

 in the face of water resistance. That the compressed brain- 

 mass is bilaterally symmetrical, as is also the spinal cord, 

 is a matter to be dealt with in the chapter on Symmetry, 

 being essentially connected with the demand by attraction- 

 sources for undeviating movement-response, or for the 

 correction of deviation. But it may be noted here that 

 such a demand would call for the representation in the 

 central nervous tract of two ancestral ganglionic chains, 

 such as are roughly drawn in Fig. 62, I. Further, the call 

 for a motor and sensory tract on each side of the body 

 would be inevitable, and this inclines one to suppose that 

 the vertebrate brain and cord represent, so to speak, four 

 ancestral ganglionic chains which have developed in olose 

 lateral continuity, and in a dorsal position. 



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