88 THE ELEMENTAEY NERVOUS SYSTEM 



the column wall. On the larger mesenteries these muscles 

 are small and inconspicuous in comparison with the other 

 musculature of these organs, but on the very small mesen- 

 teries they are almost if not quite the only muscles 

 present. 



13. The longitudinal muscle of the acontium in Metrid- 

 inm is a delicate double band of fibers that extend length- 

 wise the filamentous acontium, one of which is attached 

 to the free edge of each mesentery. 



These thirteen sets of muscles constitute the effectors 

 through which the nervous system of the actinians finds 

 its only means of reacting on the surroundings, for the 

 other three systems of effectors, the mucous glands, the 

 cilia, and the nettle cells, are entirely free from nervous 

 control. 



