244 THE ORIGIN OF THE NERVOUS SYSTEM 



effector portion is visibly differentiated. In the actinian 

 some degree of axial integration is undoubtedly present 

 though it may be slight, as Parker's experiments indicate. 



If the free-swimming sponge larva developed into a 

 free-swimming adult instead of becoming sessile, there 

 is no doubt that the axiation and physiological integra- 

 tion would progress instead of disappearing to a large 

 extent and that one factor of this progressive develop- 

 ment would be some sort of central nervous system. 

 In such a case the local excitation arcs consisting of 

 one or few cells would be more or less completely subor- 

 dinated to the great general arcs of the chief axis. In 

 short, the course of development of the excitation arc 

 and the structures which represent it must be different 

 in a form which remains motile throughout life from 

 that in a form which becomes sessile after an early 

 motile stage. In a free-living form nervous differen- 

 tiation may occur as early as, or even earlier than, the 

 differentiation of effectors, and this condition may be 

 just as primitive or even more so than in the sponge, 

 for since the nervous system is so intimately associated 

 with motility we should scarcely look for the most 

 primitive conditions in forms which are secondarily 

 sessile. 



Histologically, the condition in the sponges certainly 

 supports the views of Claus and Chun that muscles 

 and nerves originate independently and of course the 

 embryonic differentiation of muscles in complete inde- 

 pendence of nervous connections in the higher animals 

 may be regarded as pointing to the same conclusion, 

 but the origin and differentiation of muscles as of other 

 organs is unquestionably associated with physiological 



