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 
