CHAPTER III. 



EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



I. MEANING OF THE TERM BRAIN. 



In the vertebrates, the term "brain" vaguely signifies the specialized an- 

 terior end of the neuron. In the invertebrates, the term may be even more vague, 

 in that it is often used to signify only that part assumed to lie originally in front 

 of the oesophagus, that is, the supra-oesophageal ganglion. Or the term may 

 signify that ganglion, plus a varying number of post-oral neuromeres. 



The lack of precise definition in both cases is significant, and justifies the use 

 of the term, as we shall use it here, namely, to signify a varying number of neuro- 

 meres consolidated in the region of the primitive mouth. 



The number of neuromeres thus set apart, their specialization, and the 

 intimacy of their union, gradually increases throughout the arthropod-vertebrate 

 series, and furnishes an impressive picture of persistent, progressive specializa- 

 tion. 



In the arthropods, there are many oscillations in the total number, and in 

 the grouping, of the brain neuromeres. The primary causes of their union are 

 too complex to be analyzed, except in the broadest way; but we may readily recog- 

 nize a steady progression toward a definitely organized collection of neuromeres 

 that it is entirely proper to call a brain in the vertebrate sense, for it contains 

 approximately the same total number of neuromeres as the vertebrate brain; and 

 it is divided into similar groups of neuromeres, each of which is associated with 

 nerves, sense organs, and other structures similar to those in vertebrates. 



The evolution of the brain cannot be effectively studied apart from the body 

 regions to which it belongs, for each moulds the other and reflects the other's 

 changes. The events that created the vertebrate brain, and whose influence is 

 still effective in moulding its form and function, are to be found in the arthropods. 

 There, all the initial phases in the successive incorporation of one region of the 

 trunk after another into a more complex "head," and of one part of the cord after 

 another into a more and more complex "brain," have taken place, and probably 

 nearly all the more important steps in the process are there crystallized into 

 recognizable form. 



The five groups of neuromeres included in the first fifteen or twenty that 

 make up the vertebrate brain may be definitely identified with the corresponding 

 divisions of the arthropod brain. We cannot hope to identify more than that 



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