THE REFLEX ARC 247 



developed on the apical body surface than elsewhere. 

 Other polar specialization is not very great, conse- 

 quently we need not expect to find any very marked 

 difference between general axial arcs and others, except 

 perhaps in the elongated manubria of certain species 

 and in the tentacles, and in these organs we do find 

 evidence of more or less definite axial arcs (see p. 112). 



It has been generally maintained that some degree 

 of structural axiation exists in the nervous system of 

 the sea anemones and corals and probably in that of 

 the hydroids. According to this view, there is a more 

 or less distinct aggregation or a greater development of 

 nervous tissue in the apical region in relation to the 

 mouth and tentacles. Wolff's (1903) figures of the 

 general plan of the nervous system of the hydroid and 

 the coral polyp indicate a considerable degree of axiation 

 and centralization, but these figures, while based on the 

 data of observation, are admittedly diagrammatic. If 

 any degree of axial localization of nervous tissue and of 

 receptors does exist in these sessile coelenterates, and 

 the facts permit little doubt on that point, some degree 

 of functional axiation undoubtedly exists in the excita- 

 tion arcs, even though experimental stimulation not 

 quantitatively controlled may show conduction occurring 

 in all directions. 



Many facts indicate that in these forms the receptor 

 function is most highly developed in the apical regions, 

 probably in the tentacles and about the mouth, and 

 these regions are normally most exposed to excitation. 

 In the tentacles the excitation arcs are as already 

 noted, predominantly basipetal in both structure and 

 function (pp. 112, 203), and there can be no doubt that 



