INDEPENDENT EFFECTORS 61 



mal as a whole can no longer be called forth by prodding 

 it. Nevertheless, such stimulation will induce a sharply 

 denned ring of constriction to arise from the spot stimu- 

 lated and pass slowly round the column. As this experi- 

 ment succeeds even after the sea-anemone is deeply and 

 fully anesthetized, the contraction of the muscle must be 

 ascribed to direct stimulation. Hence the circular muscle 

 of the column in Metridium, though partly under nervous 

 control, is certainly open to direct stimulation, and from 

 this standpoint represents an independent effector 

 (Parker, 1916 a). 



A fuller instance of this condition is seen in the acon- 

 tial muscles of the same sea-anemone, Metridium. This 

 sea-anemone, in common with many other closely related 

 forms, carries within its body great numbers of delicate 

 thread-like filaments, the acontia. These acontia, which 

 may be as much as four or five centimeters long, are 

 attached by one end to the mesenteries in the interior of 

 the sea-anemone's body. Ordinarily the acontia are more 

 or less coiled up within the digestive cavity of the animal. 

 When for any reason the sea-anemone is made to con- 

 tract vigorously and the water contained within its cen- 

 tral cavity is thus driven out, the acontia stream with the 

 current of water in great profusion out of the mouth and 

 out of the lateral pores, or cinclides, by which the diges- 

 tive cavity is put into direct communication with the sur- 

 rounding seawater. Thus the contracting sea-anemone 

 throws out over its body a protecting system of filaments, 

 for these living threads are armed with the most vigor- 

 ous nettling cells of any in this animal and will effectively 

 sting an unwary intruder. As the sea-anemone gradu- 

 ally expands again by taking water into its body, the 

 acontia are slowly drawn back and eventually returned 



